. Preface
. An Introduction to the Human Body
1. Overview
2. Structural Organization of the Human Body
3. Anatomical Terminology
4. Medical Imaging
. Body Tissues
1. Types of Tissues
2. Epithelial Tissue
3. Connective Tissue Supports and Protects
4. Muscle Tissue and Motion
5. Nervous Tissue Mediates Perception and Response
6. Tissue Injury and Aging
. Integumentary System
1. Layers of the Skin
2. Accessory Structures of the Skin
3. Diseases, Disorders, and Injuries of the Integumentary
System
. Bone
1. The Functions of the Skeletal System
2. Bone Classification
3. Bone Structure
4. Bone Formation and Development
o. Fractures: Bone Repair
. Axial Skeleton
1. Divisions of the Skeletal System
2. The Skull
3. The Vertebral Column
4. The Thoracic Cage
. Appendicular Skeleton
1. The Pectoral Girdle
2. Bones of the Upper Limb
3. The Pelvic Girdle and Pelvis
4. Bones of the Lower Limb
8. Articulations
1. Classification of Joints
2. Fibrous Joints
3. Cartilaginous Joints
4. Synovial Joints
5. Types of Body Movements
6. Anatomy of Selected Synovial Joints
9. Skeletal Muscle
1. Overview of Muscle Tissues
2. Skeletal Muscle
3. Types of Muscle Fibers
4. Exercise and Muscle Performance
5. Cardiac Muscle Tissue
6. Smooth Muscle
7. Development and Regeneration of Muscle Tissue
8. Interactions of Skeletal Muscles, Their Fascicle
Arrangement, and Their Lever Systems
9. Naming Skeletal Muscles
10. Axial Musculature
1. Muscles of the Head, Neck, and Back
2. Muscles of the Abdominal Wall and Thorax
11. Appendicular Musculature
1. Muscles of the Pectoral Girdle and Upper Limbs
2. Muscles of the Pelvic Girdle and Lower Limbs
12. Heart
1. Heart Anatomy
2. Cardiac Muscle and Electrical Activity
3. Cardiac Cycle
13. Blood Vessels
14.
15.
16.
17.
18.
19.
20.
21;
22.
1. Structure and Function of Blood Vessels
2. Circulatory Pathways
Blood
1. An Overview of Blood
2. Production of the Formed Elements
3. Erythrocytes
4. Leukocytes and Platelets
Nervous Tissue
1. Basic Structure and Function of the Nervous System
2. Nervous ‘Tissue
Central Nervous System
1. Anatomy of the CNS
2. Circulation and the Central Nervous System
Peripheral Nervous System
1. Nerves and ganglia
Senses
1. Sensory Perception
Autonomic Nervous System
1. Divisions of the ANS
2. Central Control
Respiratory System
2. The Lungs
Digestive System
1. Overview of the Digestive System
3. The Stomach
4. The Small and Large Intestines
5. Accessory Organs in Digestion: The Liver, Pancreas, and
Gallbladder
Lymphatic System
23. Urinary System
1.
2.
3:
Gross Anatomy of Urine Transport
Gross Anatomy of the Kidney
Microscopic Anatomy of the Kidney
24. Endocrine System
1;
. The Pituitary Gland and Hypothalamus
. The Thyroid Gland
. The Parathyroid Glands
. The Adrenal Glands
. The Pineal Gland
. Gonadal and Placental Hormones
. The Endocrine Pancreas
9.
CON MU BW NHN
An Overview of the Endocrine System
Organs with Secondary Endocrine Functions
25. Reproductive System
1.
2.
Male Anatomy
Female Anatomy
Preface
Human Anatomy is designed for a semester-long course taken by life
science and allied health students. The textbook is derived from OpenStax
Human Anatomy and Physiology, and its coverage and organization were
informed by hundreds of instructors who teach the course. Instructors can
customize the book, adapting it to the approach that works best in their
classroom. The artwork for this textbook is aimed focusing student learning
through a powerful blend of traditional depictions and instructional
innovations. Color is used sparingly, to emphasize the most important
aspects of any given illustration. Significant use of micrographs from the
University of Michigan complement the illustrations, and provide the
students with a meaningful alternate depiction of each concept. Finally,
enrichment elements provide relevance and deeper context for students,
particularly in the areas of health, disease, and information relevant to their
intended careers.
Welcome to Human Anatomy, a resource designed for a semester-long
course aimed at preparing undergraduate students for health-related
programs. This book is derived from Human Anatomy and Physiology by
OpenStax College. The source materials were created with several goals in
mind: accessibility, customization, and student engagement—helping
students reach high levels of academic scholarship. Instructors and students
alike will find that this textbook offers a thorough introduction to the
content in an accessible format.
About OpenStax College
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To broaden access and encourage community curation, Human Anatomy
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Cost
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editions.
Contents
01.
02.
03.
04.
05.
06.
07.
08.
09.
Preface
An Introduction to the Human Body
Body Tissues
Integumentary System
Bone
Axial Skeleton
Appendicular Skeleton
Articulations
Skeletal Muscle
10. Axial Musculature
11. Appendicular Musculature
12. Heart
13. Blood Vessels
14. Blood
15. Nervous Tissue
16. Central Nervous System
17. Peripheral Nervous System
18. Senses
19. Autonomic Nervous System
20. Respiratory System
21. Digestive System
22. Lymphatic System
23. Urinary System
24. Endocrine System
25. Reproductive System
About Our Team
Customization for One-Semester Human Anatomy
Marcos Gridi-Papp University of the Pacific
Senior Contributors
J. Gordon Betts Tyler Junior College
Peter Desaix University of North Carolina at Chapel Hill
Eddie Johnson Central Oregon Community College
Jody E. Johnson Arapahoe Community College
Oksana Korol Aims Community College
Dean Kruse Portland Community College
Brandon Poe Springfield Technical Community College
James A. Wise Hampton University
Mark Womble Youngstown State University
Kelly A. Young California State University, Long Beach
Advisor
Robin J. Heyden
Other Contributors
Kim Aquarius Institute; Triton College
Aaronson
Lopamudra Augusta Technical College
Agarwal
Gary Allen Dalhousie University
Robert i
‘allison McLennan Community College
Heather Southern Union State Community College
Armbruster
Timothy University of North Carolina Wilmington
Ballard
Matthew Eastern New Mexico University
Barlow
William Furman Universit
Blaker
Julie Bowers East Tennessee State University
Emily Florida Southern College
Bradshaw
Nishi Bryska University of North Carolina, Charlotte
Susan Caley
Opsal
Boyd
Campbell
Ann Caplea
Marnie
Chapman
Barbara
Christie-Pope
Kenneth
Crane
Maurice
Culver
Heather
Cushman
Noelle Cutter
Lynnette
Danzl-Tauer
Jane Davis
AnnMarie
DelliPizzi
Susan Dentel
Pamela
Dobbins
Illinois Valley Community College
Southwest College of Naturopathic Medicine and
Health Sciences
Walsh University
University of Alaska, Sitka
Cornell College
Texarkana College
Florida State College at Jacksonville
Tacoma Community College
Molloy College
Rock Valley College
Aurora University
Dominican College
Washtenaw Community College
Shelton State Community College
Patty Dolan
Sondra
Dubowsky
Peter
Dukehart
Ellen DuPré
Elizabeth
DuPriest
Pam Elf
Sharon
Ellerton
Carla Endres
Myriam
Feldman
Greg Fitch
Lynn Gargan
Michael
Giangrande
Chaya
Gopalan
Victor Greco
Susanna
Pacific Lutheran University
McLennan Community College
Three Rivers Community College
Central College
Warner Pacific College
University of Minnesota
Queensborough Community College
Utah State University - College of Eastern Utah:
San Juan Campus
Lake Washington Institute of Technology;
Cascadia Community College
Avila University
Tarant County College
Oakland Community College
St. Louis College of Pharmacy
Chattahoochee Technical College
Skagit Valley College
Heinze
Ann
Henninger
Dale Horeth
Michael
Hortsch
Rosemary
Hubbard
Mark Hubley
Branko
Jablanovic
Norman
Johnson
Mark
Jonasson
Jeff Keyte
William
Kleinelp
Leigh
Kleinert
Brenda Leady
John Lepri
Sarah Leupen
Wartburg College
Tidewater Community College
University of Michigan
Marymount University
Prince George's Community College
College of Lake County
University of Massachusetts Amherst
North Arkansas College
College of Saint Mary
Middlesex County College
Grand Rapids Community College
University of Toledo
University of North Carolina, Greensboro
University of Maryland, Baltimore County
Lihua Liang
Robert Mallet
Bruce Maring
Elisabeth
Martin
Natalie
Maxwell
Julie May
Debra
McLaughlin
Nicholas
Mitchell
Shobhana
Natarajan
Phillip
Nicotera
Mary Jane
Niles
Ikemefuna
Nwosu
Betsy Ott
Ivan Paul
Aaron Payette
Johns Hopkins University
University of North Texas Health Science Center
Daytona State College
College of Lake County
Carl Albert State College, Sallisaw
William Carey University
University of Maryland University College
St. Bonaventure University
Brookhaven College
St. Petersburg College
University of San Francisco
Parkland College; Lake Land College
Tyler Junior College
John Wood Community College
College of Southern Nevada
Scott Payne
Cameron
Perkins
David Pfeiffer
Thomas Pilat
Eileen
Preston
Mike Pyle
Robert
Rawding
Jason Schreer
Laird
Sheldahl
Brian
Shmaefsky
Douglas
Sizemore
Susan
Spencer
Cynthia
Standley
Robert
Sullivan
Kentucky Wesleyan College
South Georgia College
University of Alaska, Anchorage
Illinois Central College
Tarrant County College
Olivet Nazarene University
Gannon University
State University of New York at Potsdam
Mt. Hood Community College
Lone Star College System
Bevill State Community College
Mount Hood Community College
University of Arizona
Marist College
Eric Sun
Tom Swenson
Kathleen
Tallman
Rohinton
Tarapore
Elizabeth
Tattersall
Mark Thomas
Janis
Thompson
Rita Thrasher
David Van
Wylen
Lynn
Wandrey
Margaret
Weck
Kathleen
Weiss
Neil
Westergaard
David
Wortham
Middle Georgia State College
Ithaca College
Azusa Pacific University
University of Pennsylvania
Western Nevada College
University of Northern Colorado
Lorain County Community College
Pensacola State College
St. Olaf College
Mott Community College
St. Louis College of Pharmacy
George Fox University
Williston State College
West Georgia Technical College
Umesh Yadav
Tony Yates
Justin York
Cheri Zao
Elena
Zoubina
Shobhana
Natarajan
Special Thanks
University of Texas Medical Branch
Oklahoma Baptist University
Glendale Community College
North Idaho College
Bridgewater State University; Massasoit
Community College
Alcon Laboratories, Inc.
OpenStax College wishes to thank the Regents of University of Michigan
Medical School for the use of their extensive micrograph collection. Many
of the UM micrographs that appear in Human Anatomy and Physiology are
interactive WebScopes, which students can explore by zooming in and out.
We also wish to thank the Open Learning Initiative at Carnegie Mellon
University, with whom we shared and exchanged resources during the
development of Human Anatomy and Physiology.
Overview
By the end of this section, you will be able to:
¢ Compare and contrast anatomy and physiology, including their
specializations and methods of study
e Discuss the fundamental relationship between anatomy and physiology
Human anatomy is the scientific study of the body’s structures. Some of
these structures are very small and can only be observed and analyzed with
the assistance of a microscope. Other larger structures can readily be seen,
manipulated, measured, and weighed. The word “anatomy” comes from a
Greek root that means “to cut apart.” Human anatomy was first studied by
observing the exterior of the body and observing the wounds of soldiers and
other injuries. Later, physicians were allowed to dissect bodies of the dead
to augment their knowledge. When a body is dissected, its structures are cut
apart in order to observe their physical attributes and their relationships to
one another. Dissection is still used in medical schools, anatomy courses,
and in pathology labs. In order to observe structures in living people,
however, a number of imaging techniques have been developed. These
techniques allow clinicians to visualize structures inside the living body
such as a cancerous tumor or a fractured bone.
Like most scientific disciplines, anatomy has areas of specialization. Gross
anatomy is the study of the larger structures of the body, those visible
without the aid of magnification ([link]a). Macro- means “large,” thus,
gross anatomy is also referred to as macroscopic anatomy. In contrast,
micro- means “small,” and microscopic anatomy is the study of structures
that can be observed only with the use of a microscope or other
magnification devices ({link]b). Microscopic anatomy includes cytology,
the study of cells and histology, the study of tissues. As the technology of
microscopes has advanced, anatomists have been able to observe smaller
and smaller structures of the body, from slices of large structures like the
heart, to the three-dimensional structures of large molecules in the body.
Gross and Microscopic Anatomy
(a) Gross anatomy considers large structures such as
the brain. (b) Microscopic anatomy can deal with the
same structures, though at a different scale. This is a
micrograph of nerve cells from the brain. LM x 1600.
(credit a: “WriterHound”/Wikimedia Commons; credit
b: Micrograph provided by the Regents of University
of Michigan Medical School © 2012)
Anatomists take two general approaches to the study of the body’s
structures: regional and systemic. Regional anatomy is the study of the
interrelationships of all of the structures in a specific body region, such as
the abdomen. Studying regional anatomy helps us appreciate the
interrelationships of body structures, such as how muscles, nerves, blood
vessels, and other structures work together to serve a particular body region.
In contrast, systemic anatomy is the study of the structures that make up a
discrete body system—that is, a group of structures that work together to
perform a unique body function. For example, a systemic anatomical study
of the muscular system would consider all of the skeletal muscles of the
body.
Whereas anatomy is about structure, physiology is about function. Human
physiology is the scientific study of the chemistry and physics of the
structures of the body and the ways in which they work together to support
the functions of life. Much of the study of physiology centers on the body’s
tendency toward homeostasis. Homeostasis is the state of steady internal
conditions maintained by living things. The study of physiology certainly
includes observation, both with the naked eye and with microscopes, as
well as manipulations and measurements. However, current advances in
physiology usually depend on carefully designed laboratory experiments
that reveal the functions of the many structures and chemical compounds
that make up the human body.
Like anatomists, physiologists typically specialize in a particular branch of
physiology. For example, neurophysiology is the study of the brain, spinal
cord, and nerves and how these work together to perform functions as
complex and diverse as vision, movement, and thinking. Physiologists may
work from the organ level (exploring, for example, what different parts of
the brain do) to the molecular level (such as exploring how an
electrochemical signal travels along nerves).
Form is closely related to function in all living things. For example, the thin
flap of your eyelid can snap down to clear away dust particles and almost
instantaneously slide back up to allow you to see again. At the microscopic
level, the arrangement and function of the nerves and muscles that serve the
eyelid allow for its quick action and retreat. At a smaller level of analysis,
the function of these nerves and muscles likewise relies on the interactions
of specific molecules and ions. Even the three-dimensional structure of
certain molecules is essential to their function.
Your study of anatomy and physiology will make more sense if you
continually relate the form of the structures you are studying to their
function. In fact, it can be somewhat frustrating to attempt to study anatomy
without an understanding of the physiology that a body structure supports.
Imagine, for example, trying to appreciate the unique arrangement of the
bones of the human hand if you had no conception of the function of the
hand. Fortunately, your understanding of how the human hand manipulates
tools—from pens to cell phones—helps you appreciate the unique
alignment of the thumb in opposition to the four fingers, making your hand
a structure that allows you to pinch and grasp objects and type text
messages.
Chapter Review
Human anatomy is the scientific study of the body’s structures. In the past,
anatomy has primarily been studied via observing injuries, and later by the
dissection of anatomical structures of cadavers, but in the past century,
computer-assisted imaging techniques have allowed clinicians to look
inside the living body. Human physiology is the scientific study of the
chemistry and physics of the structures of the body. Physiology explains
how the structures of the body work together to maintain life. It is difficult
to study structure (anatomy) without knowledge of function (physiology).
The two disciplines are typically studied together because form and
function are closely related in all living things.
Review Questions
Exercise:
Problem:
Which of the following specialties might focus on studying all of the
structures of the ankle and foot?
a. microscopic anatomy
b. muscle anatomy
c. regional anatomy
d. systemic anatomy
Solution:
C
Exercise:
Problem:
A scientist wants to study how the body uses foods and fluids during a
marathon run. This scientist is most likely a(n)
a. exercise physiologist
b. microscopic anatomist
c. regional physiologist
d. systemic anatomist
Solution:
A
CRITICAL THINKING QUESTIONS
Exercise:
Problem:
Name at least three reasons to study anatomy and physiology.
Solution:
An understanding of anatomy and physiology is essential for any
career in the health professions. It can also help you make choices that
promote your health, respond appropriately to signs of illness, make
sense of health-related news, and help you in your roles as a parent,
spouse, partner, friend, colleague, and caregiver.
Exercise:
Problem:
For whom would an appreciation of the structural characteristics of the
human heart come more easily: an alien who lands on Earth, abducts a
human, and dissects his heart, or an anatomy and physiology student
performing a dissection of the heart on her very first day of class?
Why?
Solution:
A student would more readily appreciate the structures revealed in the
dissection. Even though the student has not yet studied the workings of
the heart and blood vessels in her class, she has experienced her heart
beating every moment of her life, has probably felt her pulse, and
likely has at least a basic understanding of the role of the heart in
pumping blood throughout her body. This understanding of the heart’s
function (physiology) would support her study of the heart’s form
(anatomy).
Glossary
anatomy
science that studies the form and composition of the body’s structures
gross anatomy
study of the larger structures of the body, typically with the unaided
eye; also referred to macroscopic anatomy
homeostasis
steady state of body systems that living organisms maintain
microscopic anatomy
study of very small structures of the body using magnification
physiology
science that studies the chemistry, biochemistry, and physics of the
body’s functions
regional anatomy
study of the structures that contribute to specific body regions
systemic anatomy
study of the structures that contribute to specific body systems
Structural Organization of the Human Body
By the end of this section, you will be able to:
¢ Describe the structure of the human body in terms of six levels of
organization
e List the eleven organ systems of the human body and identify at least
one organ and one major function of each
Before you begin to study the different structures and functions of the
human body, it is helpful to consider its basic architecture; that is, how its
smallest parts are assembled into larger structures. It is convenient to
consider the structures of the body in terms of fundamental levels of
organization that increase in complexity: subatomic particles, atoms,
molecules, organelles, cells, tissues, organs, organ systems, organisms and
biosphere ({link]).
Levels of Structural Organization of the Human Body
Oxygen atom
Chemical level
Atoms bond to form
molecules with three-
dimensional structures.
Jd Water molecule
Smooth muscle cell
Cellular level
A variety of molecules
combine to form the
fluid and organelles
of a body cell.
Organelle
Cell fluid
Organelle
Smooth muscle tissue
Tissue level
A community of similar
cells form a body tissue.
Bladder
Organ level
Two or more different tissues
combine to form an organ.
Skeletal
muscle
Urinary tract system
Kidney
Organ system level
Ureter Two or more organs work
closely together to perform
he functi \f .
Bladder the functions of a body system.
A, Ureth ra
1"
Organismal level
Many organ system work harmoniously
together to perform the functions of an J
independent organism. fo
The organization of the body often is discussed in terms of six
distinct levels of increasing complexity, from the smallest
chemical building blocks to a unique human organism.
The Levels of Organization
To study the chemical level of organization, scientists consider the simplest
building blocks of matter: subatomic particles, atoms and molecules. All
matter in the universe is composed of one or more unique pure substances
called elements, familiar examples of which are hydrogen, oxygen, carbon,
nitrogen, calcium, and iron. The smallest unit of any of these pure
substances (elements) is an atom. Atoms are made up of subatomic particles
such as the proton, electron and neutron. Two or more atoms combine to
form a molecule, such as the water molecules, proteins, and sugars found in
living things. Molecules are the chemical building blocks of all body
structures.
A cell is the smallest independently functioning unit of a living organism.
Even bacteria, which are extremely small, independently-living organisms,
have a cellular structure. Each bacterium is a single cell. All living
structures of human anatomy contain cells, and almost all functions of
human physiology are performed in cells or are initiated by cells.
A human cell typically consists of flexible membranes that enclose
cytoplasm, a water-based cellular fluid together with a variety of tiny
functioning units called organelles. In humans, as in all organisms, cells
perform all functions of life. A tissue is a group of many similar cells
(though sometimes composed of a few related types) that work together to
perform a specific function. An organ is an anatomically distinct structure
of the body composed of two or more tissue types. Each organ performs one
or more specific physiological functions. An organ system is a group of
organs that work together to perform major functions or meet physiological
needs of the body.
This book covers eleven distinct organ systems in the human body ([Link]
and [link]). Assigning organs to organ systems can be imprecise since
organs that “belong” to one system can also have functions integral to
another system. In fact, most organs contribute to more than one system.
Organ Systems of the Human Body
Integumentary System Skeletal System
¢ Encloses internal * Supports the body
body structures ¢ Enables movement
* Site of many (with muscular
sensory receptors system)
Muscular System Nervous System
¢ Enables movement * Detects and
(with skeletal system) processes sensory
¢ Helps maintain body information
temperature * Activates bodily
responses
Skeletal
muscles
Endocrine System Cardiovascular System
Pituitary
gland * Secretes hormones * Delivers oxygen
. ¢ Regulates bodily and nutrients to
Thyroid processes tissues
gland * Equalizes
temperature in
the body
Pancreas
Adrenal
glands
Blood
vessels
Ovaries
Organs that work together are grouped into organ
systems.
Organ Systems of the Human Body (continued)
Lymphatic System
¢ Returns fluid to
blood
¢ Defends against
pathogens
Digestive System
¢ Processes food for
use by the body
« Removes wastes
from undigested
food
Stomach
Liver
Gall
bladder
Large
intestine
Small
intestine
Male Reproductive
System
¢ Produces sex
hormones and
gametes
* Delivers gametes
to female
Epididymis
Respiratory System
* Removes carbon
dioxide from the
body
* Delivers oxygen
to blood
Nasal passage
Trachea
Lungs
Urinary System
* Controls water
balance in the
body
« Removes wastes
from blood and
Kidneys excretes them
Urinary
bladder
Female Reproductive
System
* Produces sex
Mammary hormones
glands and gametes
* Supports embryo/
fetus until birth
* Produces milk for
: infant
Ovaries
Organs that work together are grouped into organ
systems.
The organism level is the highest level of organization. An organism is a
living being that has a cellular structure and that can independently perform
all physiologic functions necessary for life. In multicellular organisms,
including humans, all cells, tissues, organs, and organ systems of the body
work together to maintain the life and health of the organism.
Chapter Review
Life processes of the human body are maintained at several levels of
structural organization. These include the chemical, cellular, tissue, organ,
organ system, and the organism level. Higher levels of organization are
built from lower levels. Therefore, molecules combine to form cells, cells
combine to form tissues, tissues combine to form organs, organs combine to
form organ systems, and organ systems combine to form organisms.
Review Questions
Exercise:
Problem:
The smallest independently functioning unit of an organism is a(n)
a. cell
b. molecule
c. organ
d. tissue
Solution:
A
Exercise:
Problem:
A collection of similar tissues that performs a specific function is an
a. organ
b. organelle
c. organism
d. organ system
Solution:
A
Exercise:
Problem:
The body system responsible for structural support and movement is
the
a. cardiovascular system
b. endocrine system
c. muscular system
d. skeletal system
Solution:
D
CRITICAL THINKING QUESTIONS
Exercise:
Problem:Name the six levels of organization of the human body.
Solution:
Chemical, cellular, tissue, organ, organ system, organism.
Exercise:
Problem:
The female ovaries and the male testes are a part of which body
system? Can these organs be members of more than one organ system?
Why or why not?
Solution:
The female ovaries and the male testes are parts of the reproductive
system. But they also secrete hormones, as does the endocrine system,
therefore ovaries and testes function within both the endocrine and
reproductive systems.
Glossary
cell
smallest independently functioning unit of all organisms; in animals, a
cell contains cytoplasm, composed of fluid and organelles
organ
functionally distinct structure composed of two or more types of
tissues
organ system
group of organs that work together to carry out a particular function
organism
living being that has a cellular structure and that can independently
perform all physiologic functions necessary for life
tissue
group of similar or closely related cells that act together to perform a
specific function
Anatomical Terminology
By the end of this section, you will be able to:
e Demonstrate the anatomical position
e Describe the human body using directional and regional terms
e Identify three planes most commonly used in the study of anatomy
e Distinguish between the posterior (dorsal) and the anterior (ventral)
body cavities, identifying their subdivisions and representative organs
found in each
e Describe serous membrane and explain its function
Anatomists and health care providers use terminology that can be
bewildering to the uninitiated. However, the purpose of this language is not
to confuse, but rather to increase precision and reduce medical errors. For
example, is a scar “above the wrist” located on the forearm two or three
inches away from the hand? Or is it at the base of the hand? Is it on the
palm-side or back-side? By using precise anatomical terminology, we
eliminate ambiguity. Anatomical terms derive from ancient Greek and Latin
words. Because these languages are no longer used in everyday
conversation, the meaning of their words does not change.
Anatomical terms are made up of roots, prefixes, and suffixes. The root of a
term often refers to an organ, tissue, or condition, whereas the prefix or
suffix often describes the root. For example, in the disorder hypertension,
the prefix “hyper-” means “high” or “over,” and the root word “tension”
refers to pressure, so the word “hypertension” refers to abnormally high
blood pressure.
Anatomical Position
To further increase precision, anatomists standardize the way in which they
view the body. Just as maps are normally oriented with north at the top, the
standard body “map,” or anatomical position, is that of the body standing
upright, with the feet at shoulder width and parallel, toes forward. The
upper limbs are held out to each side, and the palms of the hands face
forward as illustrated in [link]. Using this standard position reduces
confusion. It does not matter how the body being described is oriented, the
terms are used as if it is in anatomical position. For example, a scar in the
“anterior (front) carpal (wrist) region” would be present on the palm side of
the wrist. The term “anterior” would be used even if the hand were palm
down on a table.
Regions of the Human Body
Frons or forehead (frontal). Oculus or eye
(orbital or ocular)
Cranium
or skull
(cranial)
Bucca or cheek (buccal) Cephalon or head
Shoulder (cephalic)
(acromial)
Facies
or face
(facial)
Auris or ear (otic)
Nasus or nose (nasal)
Dorsum
or back
F Cervicis or neck
Oris or mouth (oral) Cervicis or neck (cervical)
Mentis or chin
(dorsal)
(mental) ; : Seaeun
Axilla or armpit (thoracic) (brachial)
(axillary) Mamma Olecranon
. or breast or back
bslhoral o (mammary) of elbow
arm (brachial) ‘abdanner Trunk (olecranal)
Antecubitis (abdominal) Lumbus
or front of elbow or loin
(antecubital) Umbilicus (lumbar) Upper
or navel limb
Antebrachium Sacrum
(umbilical)
or forearm
(antebrachial)
Carpus
or wrist
(carpal)
Pollex Pelvis
or thumb (pelvic) (manual)
Palma or
Inguen or groin Gluteus
palm (palmar) ite inal) 9g sbatiock
Digits (phalanges) (gluteal)
ig
or fingers (digital or
phalangeal) aie or thigh
Patella or (pubic) tone)
ineecap Femur Poplit
atellar) A opliteus or
P ) . Naty back of knee
Crus or (femoral) (popliteal)
leg (crural) Sura
or calf
Tarsus (sural)
or ankle
(tarsal) Calcaneus or
heel of foot
Digits (phalanges) (calacaneal)
or toes (digital or Pes or foot
phalangeal) (pedal) Planta or sole
~ of foot (plantar)
(a) Anterior view
Antebrachium
or forearm
(antebrachial)
(sacral)
Manus
or hand
(b) Posterior view
The human body is shown in anatomical position in an (a)
anterior view and a (b) posterior view. The regions of the body
are labeled in boldface.
A body that is lying down is described as either prone or supine. Prone
describes a face-down orientation, and supine describes a face up
orientation. These terms are sometimes used in describing the position of
the body during specific physical examinations or surgical procedures.
Regional Terms
The human body’s numerous regions have specific terms to help increase
precision (see [link]). Notice that the term “brachium” or “arm” is reserved
for the “upper arm” and “antebrachium” or “forearm” is used rather than
“lower arm.” Similarly, “femur” or “thigh” is correct, and “leg” or “crus” is
reserved for the portion of the lower limb between the knee and the ankle.
You will be able to describe the body’s regions using the terms from the
figure.
Directional Terms
Certain directional anatomical terms appear throughout this and any other
anatomy textbook ({link]). These terms are essential for describing the
relative locations of different body structures. For instance, an anatomist
might describe one band of tissue as “inferior to” another or a physician
might describe a tumor as “superficial to” a deeper body structure. Commit
these terms to memory to avoid confusion when you are studying or
describing the locations of particular body parts.
e Anterior (or ventral) Describes the front or direction toward the front
of the body. The toes are anterior to the foot.
¢ Posterior (or dorsal) Describes the back or direction toward the back
of the body. The popliteus is posterior to the patella.
e Superior (or cranial) describes a position above or higher than
another part of the body proper. The orbits are superior to the oris.
¢ Inferior (or caudal) describes a position below or lower than another
part of the body proper; near or toward the tail (in humans, the coccyx,
or lowest part of the spinal column). The pelvis is inferior to the
abdomen.
e Lateral describes the side or direction toward the side of the body. The
thumb (pollex) is lateral to the digits.
e Medial describes the middle or direction toward the middle of the
body. The hallux is the medial toe.
¢ Proximal describes a position in a limb that is nearer to the point of
attachment or the trunk of the body. The brachium is proximal to the
antebrachium.
¢ Distal describes a position in a limb that is farther from the point of
attachment or the trunk of the body. The crus is distal to the femur.
e Superficial describes a position closer to the surface of the body. The
skin is superficial to the bones.
¢ Deep describes a position farther from the surface of the body. The
brain is deep to the skull.
Directional Terms Applied to the Human Body
Superior
Cranial
-> Anterior or
ventral
Posterior +-7----
or dorsal
Caudal
Inferior
Paired directional terms are shown as applied to
the human body.
Body Planes
A section is a two-dimensional surface of a three-dimensional structure that
has been cut. Modern medical imaging devices enable clinicians to obtain
“virtual sections” of living bodies. We call these scans. Body sections and
scans can be correctly interpreted, however, only if the viewer understands
the plane along which the section was made. A plane is an imaginary two-
dimensional surface that passes through the body. There are three planes
commonly referred to in anatomy and medicine, as illustrated in [link].
e The sagittal plane is the plane that divides the body or an organ
vertically into right and left sides. If this vertical plane runs directly
down the middle of the body, it is called the midsagittal or median
plane. If it divides the body into unequal right and left sides, it is called
a parasagittal plane or less commonly a longitudinal section.
e The frontal plane is the plane that divides the body or an organ into an
anterior (front) portion and a posterior (rear) portion. The frontal plane
is often referred to as a coronal plane. (“Corona” is Latin for “crown.”)
e The transverse plane is the plane that divides the body or organ
horizontally into upper and lower portions. Transverse planes produce
images referred to as cross sections.
Planes of the Body
Frontal
(coronal)
plane
Transverse
The three planes most commonly used
in anatomical and medical imaging
are the sagittal, frontal (or coronal),
and transverse plane.
Body Cavities and Serous Membranes
The body maintains its internal organization by means of membranes,
sheaths, and other structures that separate compartments. The dorsal
(posterior) cavity and the ventral (anterior) cavity are the largest body
compartments ({link]). These cavities contain and protect delicate internal
organs, and the ventral cavity allows for significant changes in the size and
shape of the organs as they perform their functions. The lungs, heart,
stomach, and intestines, for example, can expand and contract without
distorting other tissues or disrupting the activity of nearby organs.
Dorsal and Ventral Body Cavities
Cranial
cavity
Vertebral
cavity
Thoracic cavity:
Superior
mediastinum
Pleural cavity
Pericardial
cavity within
the mediastinum
Diaphragm
Vertebral
cavity Ventral body
cavity
(both thoracic and
i i — 7 abdominopelvic
Abdominal cavity Abdomino- | cavities)
pelvic
cavity
Pelvic cavity
Lateral view Anterior view
The ventral cavity includes the thoracic and abdominopelvic
cavities and their subdivisions. The dorsal cavity includes the
cranial and spinal cavities.
Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities
The posterior (dorsal) and anterior (ventral) cavities are each subdivided
into smaller cavities. In the posterior (dorsal) cavity, the cranial cavity
houses the brain, and the spinal cavity (or vertebral cavity) encloses the
spinal cord. Just as the brain and spinal cord make up a continuous,
uninterrupted structure, the cranial and spinal cavities that house them are
also continuous. The brain and spinal cord are protected by the bones of the
skull and vertebral column and by cerebrospinal fluid, a colorless fluid
produced by the brain, which cushions the brain and spinal cord within the
posterior (dorsal) cavity.
The anterior (ventral) cavity has two main subdivisions: the thoracic cavity
and the abdominopelvic cavity (see [link]). The thoracic cavity is the more
superior subdivision of the anterior cavity, and it is enclosed by the rib cage.
The thoracic cavity contains the lungs and the heart, which is located in the
mediastinum. The diaphragm forms the floor of the thoracic cavity and
separates it from the more inferior abdominopelvic cavity. The
abdominopelvic cavity is the largest cavity in the body. Although no
membrane physically divides the abdominopelvic cavity, it can be useful to
distinguish between the abdominal cavity, the division that houses the
digestive organs, and the pelvic cavity, the division that houses the organs
of reproduction.
Abdominal Regions and Quadrants
To promote clear communication, for instance about the location of a
patient’s abdominal pain or a suspicious mass, health care providers
typically divide up the cavity into either nine regions or four quadrants
({link]).
Regions and Quadrants of the Peritoneal Cavity
(a) Abdominopelvic regions (b) Abdominopelvic quandrants
There are (a) nine abdominal regions and (b) four abdominal
quadrants in the peritoneal cavity.
The more detailed regional approach subdivides the cavity with one
horizontal line immediately inferior to the ribs and one immediately
superior to the pelvis, and two vertical lines drawn as if dropped from the
midpoint of each clavicle (collarbone). There are nine resulting regions.
The simpler quadrants approach, which is more commonly used in
medicine, subdivides the cavity with one horizontal and one vertical line
that intersect at the patient’s umbilicus (navel).
Membranes of the Anterior (Ventral) Body Cavity
A serous membrane (also referred to a serosa) is one of the thin
membranes that cover the walls and organs in the thoracic and
abdominopelvic cavities. The parietal layers of the membranes line the
walls of the body cavity (pariet- refers to a cavity wall). The visceral layer
of the membrane covers the organs (the viscera). Between the parietal and
visceral layers is a very thin, fluid-filled serous space, or cavity ([link]).
Serous Membrane
Visceral
pericardium
Pericardial
cavity
Parietal
pericardium
\/—
Air space
Balloon
Serous membrane lines the pericardial cavity
and reflects back to cover the heart—much the
same way that an underinflated balloon would
form two layers surrounding a fist.
There are three serous cavities and their associated membranes. The pleura
is the serous membrane that encloses the pleural cavity; the pleural cavity
surrounds the lungs. The pericardium is the serous membrane that encloses
the pericardial cavity; the pericardial cavity surrounds the heart. The
peritoneum is the serous membrane that encloses the peritoneal cavity; the
peritoneal cavity surrounds several organs in the abdominopelvic cavity.
The serous membranes form fluid-filled sacs, or cavities, that are meant to
cushion and reduce friction on internal organs when they move, such as
when the lungs inflate or the heart beats. Both the parietal and visceral
serosa secrete the thin, slippery serous fluid located within the serous
cavities. The pleural cavity reduces friction between the lungs and the body
wall. Likewise, the pericardial cavity reduces friction between the heart and
the wall of the pericardium. The peritoneal cavity reduces friction between
the abdominal and pelvic organs and the body wall. Therefore, serous
membranes provide additional protection to the viscera they enclose by
reducing friction that could lead to inflammation of the organs.
Chapter Review
Ancient Greek and Latin words are used to build anatomical terms. A
standard reference position for mapping the body’s structures is the normal
anatomical position. Regions of the body are identified using terms such as
“occipital” that are more precise than common words and phrases such as
“the back of the head.” Directional terms such as anterior and posterior are
essential for accurately describing the relative locations of body structures.
Images of the body’s interior commonly align along one of three planes: the
sagittal, frontal, or transverse. The body’s organs are organized in one of
two main cavities—dorsal (also referred to posterior) and ventral (also
referred to anterior)—which are further sub-divided according to the
structures present in each area. The serous membranes have two layers—
parietal and visceral—surrounding a fluid filled space. Serous membranes
cover the lungs (pleural serosa), heart (pericardial serosa), and some
abdominopelvic organs (peritoneal serosa).
Review Chapter
Exercise:
Problem:
What is the position of the body when it is in the “normal anatomical
position?”
a. The person is prone with upper limbs, including palms, touching
sides and lower limbs touching at sides.
b. The person is standing facing the observer, with upper limbs
extended out at a ninety-degree angle from the torso and lower
limbs in a wide stance with feet pointing laterally
c. The person is supine with upper limbs, including palms, touching
sides and lower limbs touching at sides.
d. None of the above
Solution:
D
Exercise:
Problem:
To make a banana split, you halve a banana into two long, thin, right
and left sides along the
a. coronal plane
b. longitudinal plane
c. midsagittal plane
d. transverse plane
Solution:
C
Exercise:
Problem: The lumbar region is
a. inferior to the gluteal region
b. inferior to the umbilical region
c. superior to the cervical region
d. superior to the popliteal region
Solution:
D
Exercise:
Problem: The heart is within the
a. Cranial cavity
b. mediastinum
c. posterior (dorsal) cavity
d. All of the above
Solution:
B
Critical Thinking Question
Exercise:
Problem:
In which direction would an MRI scanner move to produce sequential
images of the body in the frontal plane, and in which direction would
an MRI scanner move to produce sequential images of the body in the
sagittal plane?
Solution:
If the body were supine or prone, the MRI scanner would move from
top to bottom to produce frontal sections, which would divide the body
into anterior and posterior portions, as in “cutting” a deck of cards.
Again, if the body were supine or prone, to produce sagittal sections,
the scanner would move from left to right or from right to left to divide
the body lengthwise into left and right portions.
Exercise:
Problem:
If a bullet were to penetrate a lung, which three anterior thoracic body
cavities would it enter, and which layer of the serous membrane would
it encounter first?
Solution:
The bullet would enter the ventral, thoracic, and pleural cavities, and it
would encounter the parietal layer of serous membrane first.
Glossary
abdominopelvic cavity
division of the anterior (ventral) cavity that houses the abdominal and
pelvic viscera
anatomical position
standard reference position used for describing locations and directions
on the human body
anterior
describes the front or direction toward the front of the body; also
referred to as ventral
anterior cavity
larger body cavity located anterior to the posterior (dorsal) body
cavity; includes the serous membrane-lined pleural cavities for the
lungs, pericardial cavity for the heart, and peritoneal cavity for the
abdominal and pelvic organs; also referred to as ventral cavity
caudal
describes a position below or lower than another part of the body
proper; near or toward the tail (in humans, the coccyx, or lowest part
of the spinal column); also referred to as inferior
cranial
describes a position above or higher than another part of the body
proper; also referred to as superior
cranial cavity
division of the posterior (dorsal) cavity that houses the brain
deep
describes a position farther from the surface of the body
distal
describes a position farther from the point of attachment or the trunk of
the body
dorsal
describes the back or direction toward the back of the body; also
referred to as posterior
dorsal cavity
posterior body cavity that houses the brain and spinal cord; also
referred to the posterior body cavity
frontal plane
two-dimensional, vertical plane that divides the body or organ into
anterior and posterior portions
inferior
describes a position below or lower than another part of the body
proper; near or toward the tail (in humans, the coccyx, or lowest part
of the spinal column); also referred to as caudal
lateral
describes the side or direction toward the side of the body
medial
describes the middle or direction toward the middle of the body
pericardium
sac that encloses the heart
peritoneum
serous membrane that lines the abdominopelvic cavity and covers the
organs found there
plane
imaginary two-dimensional surface that passes through the body
pleura
serous membrane that lines the pleural cavity and covers the lungs
posterior
describes the back or direction toward the back of the body; also
referred to as dorsal
posterior cavity
posterior body cavity that houses the brain and spinal cord; also
referred to as dorsal cavity
prone
face down
proximal
describes a position nearer to the point of attachment or the trunk of
the body
sagittal plane
two-dimensional, vertical plane that divides the body or organ into
right and left sides
section
in anatomy, a single flat surface of a three-dimensional structure that
has been cut through
serous Membrane
membrane that covers organs and reduces friction; also referred to as
serosa
serosa
membrane that covers organs and reduces friction; also referred to as
serous Membrane
spinal cavity
division of the dorsal cavity that houses the spinal cord; also referred
to as vertebral cavity
superficial
describes a position nearer to the surface of the body
superior
describes a position above or higher than another part of the body
proper; also referred to as cranial
supine
face up
thoracic cavity
division of the anterior (ventral) cavity that houses the heart, lungs,
esophagus, and trachea
transverse plane
two-dimensional, horizontal plane that divides the body or organ into
superior and inferior portions
ventral
describes the front or direction toward the front of the body; also
referred to as anterior
ventral cavity
larger body cavity located anterior to the posterior (dorsal) body
cavity; includes the serous membrane-lined pleural cavities for the
lungs, pericardial cavity for the heart, and peritoneal cavity for the
abdominal and pelvic organs; also referred to as anterior body cavity
Medical Imaging
By the end of this section, you will be able to:
e Discuss the uses and drawbacks of X-ray imaging
e Identify four modern medical imaging techniques and how they are
used
For thousands of years, fear of the dead and legal sanctions limited the
ability of anatomists and physicians to study the internal structures of the
human body. An inability to control bleeding, infection, and pain made
surgeries infrequent, and those that were performed—such as wound
suturing, amputations, tooth and tumor removals, skull drilling, and
cesarean births—did not greatly advance knowledge about internal
anatomy. Theories about the function of the body and about disease were
therefore largely based on external observations and imagination. During
the fourteenth and fifteenth centuries, however, the detailed anatomical
drawings of Italian artist and anatomist Leonardo da Vinci and Flemish
anatomist Andreas Vesalius were published, and interest in human anatomy
began to increase. Medical schools began to teach anatomy using human
dissection; although some resorted to grave robbing to obtain corpses. Laws
were eventually passed that enabled students to dissect the corpses of
criminals and those who donated their bodies for research. Still, it was not
until the late nineteenth century that medical researchers discovered non-
surgical methods to look inside the living body.
X-Rays
German physicist Wilhelm R6éntgen (1845-1923) was experimenting with
electrical current when he discovered that a mysterious and invisible “ray”
would pass through his flesh but leave an outline of his bones on a screen
coated with a metal compound. In 1895, R6ntgen made the first durable
record of the internal parts of a living human: an “X-ray” image (as it came
to be called) of his wife’s hand. Scientists around the world quickly began
their own experiments with X-rays, and by 1900, X-rays were widely used
to detect a variety of injuries and diseases. In 1901, R6ntgen was awarded
the first Nobel Prize for physics for his work in this field.
The X-ray is a form of high energy electromagnetic radiation with a short
wavelength capable of penetrating solids and ionizing gases. As they are
used in medicine, X-rays are emitted from an X-ray machine and directed
toward a specially treated metallic plate placed behind the patient’s body.
The beam of radiation results in darkening of the X-ray plate. X-rays are
slightly impeded by soft tissues, which show up as gray on the X-ray plate,
whereas hard tissues, such as bone, largely block the rays, producing a
light-toned “shadow.” Thus, X-rays are best used to visualize hard body
structures such as teeth and bones ({link]). Like many forms of high energy
radiation, however, X-rays are capable of damaging cells and initiating
changes that can lead to cancer. This danger of excessive exposure to X-
rays was not fully appreciated for many years after their widespread use.
X-Ray of a Hand
High energy
electromagnetic radiation
allows the internal
structures of the body, such
as bones, to be seen in X-
rays like these. (credit:
Trace Meek/flickr)
Refinements and enhancements of X-ray techniques have continued
throughout the twentieth and twenty-first centuries. Although often
supplanted by more sophisticated imaging techniques, the X-ray remains a
“workhorse” in medical imaging, especially for viewing fractures and for
dentistry. The disadvantage of irradiation to the patient and the operator is
now attenuated by proper shielding and by limiting exposure.
Modern Medical Imaging
X-rays can depict a two-dimensional image of a body region, and only from
a single angle. In contrast, more recent medical imaging technologies
produce data that is integrated and analyzed by computers to produce three-
dimensional images or images that reveal aspects of body functioning.
Computed Tomography
Tomography refers to imaging by sections. Computed tomography (CT)
is a noninvasive imaging technique that uses computers to analyze several
cross-sectional X-rays in order to reveal minute details about structures in
the body ({link]a). The technique was invented in the 1970s and is based on
the principle that, as X-rays pass through the body, they are absorbed or
reflected at different levels. In the technique, a patient lies on a motorized
platform while a computerized axial tomography (CAT) scanner rotates 360
degrees around the patient, taking X-ray images. A computer combines
these images into a two-dimensional view of the scanned area, or “slice.”
Medical Imaging Techniques
(a) The results of a CT scan of the head are shown as
successive transverse sections. (b) An MRI machine
generates a magnetic field around a patient. (c) PET
scans use radiopharmaceuticals to create images of
active blood flow and physiologic activity of the organ
or organs being targeted. (d) Ultrasound technology is
used to monitor pregnancies because it is the least
invasive of imaging techniques and uses no
electromagnetic radiation. (credit a: Akira
Ohgaki/flickr; credit b: “Digital Cate”/flickr; credit c:
“Raziel”/Wikimedia Commons; credit d:
“Tsis”/Wikimedia Commons)
Since 1970, the development of more powerful computers and more
sophisticated software has made CT scanning routine for many types of
diagnostic evaluations. It is especially useful for soft tissue scanning, such
as of the brain and the thoracic and abdominal viscera. Its level of detail is
so precise that it can allow physicians to measure the size of a mass down to
a millimeter. The main disadvantage of CT scanning is that it exposes
patients to a dose of radiation many times higher than that of X-rays. In
fact, children who undergo CT scans are at increased risk of developing
cancer, as are adults who have multiple CT scans.
Note:
wae
— :
mess OPenstax COLLEGE
A CT or CAT scan relies on a circling scanner that revolves around the
patient’s body. Watch this video to learn more about CT and CAT scans.
What type of radiation does a CT scanner use?
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a noninvasive medical imaging
technique based on a phenomenon of nuclear physics discovered in the
1930s, in which matter exposed to magnetic fields and radio waves was
found to emit radio signals. In 1970, a physician and researcher named
Raymond Damadian noticed that malignant (cancerous) tissue gave off
different signals than normal body tissue. He applied for a patent for the
first MRI scanning device, which was in use clinically by the early 1980s.
The early MRI scanners were crude, but advances in digital computing and
electronics led to their advancement over any other technique for precise
imaging, especially to discover tumors. MRI also has the major advantage
of not exposing patients to radiation.
Drawbacks of MRI scans include their much higher cost, and patient
discomfort with the procedure. The MRI scanner subjects the patient to
such powerful electromagnets that the scan room must be shielded. The
patient must be enclosed in a metal tube-like device for the duration of the
scan (see [link]b), sometimes as long as thirty minutes, which can be
uncomfortable and impractical for ill patients. The device is also so noisy
that, even with earplugs, patients can become anxious or even fearful.
These problems have been overcome somewhat with the development of
“open” MRI scanning, which does not require the patient to be entirely
enclosed in the metal tube. Patients with iron-containing metallic implants
(internal sutures, some prosthetic devices, and so on) cannot undergo MRI
scanning because it can dislodge these implants.
Functional MRIs ({MRIs), which detect the concentration of blood flow in
certain parts of the body, are increasingly being used to study the activity in
parts of the brain during various body activities. This has helped scientists
learn more about the locations of different brain functions and more about
brain abnormalities and diseases.
Note:
zis
A patient undergoing an MRI is surrounded by a tube-shaped scanner.
Watch this video to learn more about MRIs. What is the function of
magnets in an MRI?
Positron Emission Tomography
Positron emission tomography (PET) is a medical imaging technique
involving the use of so-called radiopharmaceuticals, substances that emit
radiation that is short-lived and therefore relatively safe to administer to the
body. Although the first PET scanner was introduced in 1961, it took 15
more years before radiopharmaceuticals were combined with the technique
and revolutionized its potential. The main advantage is that PET (see
[link]c) can illustrate physiologic activity—including nutrient metabolism
and blood flow—of the organ or organs being targeted, whereas CT and
MRI scans can only show static images. PET is widely used to diagnose a
multitude of conditions, such as heart disease, the spread of cancer, certain
forms of infection, brain abnormalities, bone disease, and thyroid disease.
PET relies on radioactive substances administered several minutes before
the scan. Watch this video to learn more about PET. How is PET used in
chemotherapy?
Ultrasonography
Ultrasonography is an imaging technique that uses the transmission of
high-frequency sound waves into the body to generate an echo signal that is
converted by a computer into a real-time image of anatomy and physiology
(see [link]d). Ultrasonography is the least invasive of all imaging
techniques, and it is therefore used more freely in sensitive situations such
as pregnancy. The technology was first developed in the 1940s and 1950s.
Ultrasonography is used to study heart function, blood flow in the neck or
extremities, certain conditions such as gallbladder disease, and fetal growth
and development. The main disadvantages of ultrasonography are that the
image quality is heavily operator-dependent and that it is unable to
penetrate bone and gas.
Chapter Review
Detailed anatomical drawings of the human body first became available in
the fifteenth and sixteenth centuries; however, it was not until the end of the
nineteenth century, and the discovery of X-rays, that anatomists and
physicians discovered non-surgical methods to look inside a living body.
Since then, many other techniques, including CT scans, MRI scans, PET
scans, and ultrasonography, have been developed, providing more accurate
and detailed views of the form and function of the human body.
Interactive Link Questions
Exercise:
Problem:
A CT or CAT scan relies on a circling scanner that revolves around the
patient’s body. Watch this video to learn more about CT and CAT
scans. What type of radiation does a CT scanner use?
Solution:
X-rays.
Exercise:
Problem:
A patient undergoing an MRI is surrounded by a tube-shaped scanner.
Watch this video to learn more about MRIs. What is the function of
magnets in an MRI?
Solution:
The magnets induce tissue to emit radio signals that can show
differences between different types of tissue.
Exercise:
Problem:
PET relies on radioactive substances administered several minutes
before the scan. Watch this video to learn more about PET. How is
PET used in chemotherapy?
Solution:
PET scans can indicate how patients are responding to chemotherapy.
Review Questions
Exercise:
Problem:
In 1901, Wilhelm Réntgen was the first person to win the Nobel Prize
for physics. For what discovery did he win?
a. nuclear physics
b. radiopharmaceuticals
c. the link between radiation and cancer
d. X-rays
Solution:
D
Exercise:
Problem:
Which of the following imaging techniques would be best to use to
study the uptake of nutrients by rapidly multiplying cancer cells?
a. CT
b. MRI
c. PET
d. ultrasonography
Solution:
C
Exercise:
Problem:
Which of the following imaging studies can be used most safely during
pregnancy?
a. CT scans
b. PET scans
c. ultrasounds
d. X-rays
Solution:
C
Exercise:
Problem: What are two major disadvantages of MRI scans?
a. release of radiation and poor quality images
b. high cost and the need for shielding from the magnetic signals
c. can only view metabolically active tissues and inadequate
availability of equipment
d. release of radiation and the need for a patient to be confined to
metal tube for up to 30 minutes
Solution:
Critical Thinking Questions
Exercise:
Problem:
Which medical imaging technique is most dangerous to use repeatedly,
and why?
Solution:
CT scanning subjects patients to much higher levels of radiation than
X-rays, and should not be performed repeatedly.
Exercise:
Problem:
Explain why ultrasound imaging is the technique of choice for
studying fetal growth and development.
Solution:
Ultrasonography does not expose a mother or fetus to radiation, to
radiopharmaceuticals, or to magnetic fields. At this time, there are no
known medical risks of ultrasonography.
Glossary
computed tomography (CT)
medical imaging technique in which a computer-enhanced cross-
sectional X-ray image is obtained
magnetic resonance imaging (MRI)
medical imaging technique in which a device generates a magnetic
field to obtain detailed sectional images of the internal structures of the
body
positron emission tomography (PET)
medical imaging technique in which radiopharmaceuticals are traced
to reveal metabolic and physiological functions in tissues
ultrasonography
application of ultrasonic waves to visualize subcutaneous body
structures such as tendons and organs
X-ray
form of high energy electromagnetic radiation with a short wavelength
capable of penetrating solids and ionizing gases; used in medicine as a
diagnostic aid to visualize body structures such as bones
Types of Tissues
By the end of this section, you will be able to:
e Identify the four main tissue types
e Discuss the functions of each tissue type
e Relate the structure of each tissue type to their function
e Discuss the embryonic origin of tissue
e Identify the three major germ layers
e Identify the main types of tissue membranes
The term tissue is used to describe a group of cells found together in the
body. The cells within a tissue share a common embryonic origin.
Microscopic observation reveals that the cells in a tissue share
morphological features and are arranged in an orderly pattern that achieves
the tissue’s functions. From the evolutionary perspective, tissues appear in
more complex organisms. For example, multicellular protists, ancient
eukaryotes, do not have cells organized into tissues.
Although there are many types of cells in the human body, they are
organized into four broad categories of tissues: epithelial, connective,
muscle, and nervous. Each of these categories is characterized by specific
functions that contribute to the overall health and maintenance of the body.
A disruption of the structure is a sign of injury or disease. Such changes can
be detected through histology, the microscopic study of tissue appearance,
organization, and function.
The Four Types of Tissues
Epithelial tissue, also referred to as epithelium, refers to the sheets of cells
that cover exterior surfaces of the body, lines internal cavities and
passageways, and forms certain glands. Connective tissue, as its name
implies, binds the cells and organs of the body together and functions in the
protection, support, and integration of all parts of the body. Muscle tissue is
excitable, responding to stimulation and contracting to provide movement,
and occurs as three major types: skeletal (voluntary) muscle, smooth
muscle, and cardiac muscle in the heart. Nervous tissue is also excitable,
allowing the propagation of electrochemical signals in the form of nerve
impulses that communicate between different regions of the body ([link]).
The next level of organization is the organ, where several types of tissues
come together to form a working unit. Just as knowing the structure and
function of cells helps you in your study of tissues, knowledge of tissues
will help you understand how organs function. The epithelial and
connective tissues are discussed in detail in this chapter. Muscle and
nervous tissues will be discussed only briefly in this chapter.
Four Types of Tissue: Body
Nervous tissue
Brain
Spinal cord
Nerves
Muscle tissue
Cardiac muscle
Smooth muscle
Skeletal muscle
Epithelial tissue
Lining of Gl tract organs
and other hollow organs
Skin surface (epidermis)
Connective tissue
Fat and other soft
padding tissue
Bone
Tendon
The four types of tissues are exemplified in nervous
tissue, stratified squamous epithelial tissue, cardiac
muscle tissue, and connective tissue in small intestine.
Clockwise from nervous tissue, LM x 872, LM x 282,
LM x 460, LM x 800. (Micrographs provided by the
Regents of University of Michigan Medical School ©
2012)
Embryonic Origin of Tissues
The zygote, or fertilized egg, is a single cell formed by the fusion of an egg
and sperm. After fertilization the zygote gives rise to rapid mitotic cycles,
generating many cells to form the embryo. The first embryonic cells
generated have the ability to differentiate into any type of cell in the body
and, as such, are called totipotent, meaning each has the capacity to divide,
differentiate, and develop into a new organism. As cell proliferation
progresses, three major cell lineages are established within the embryo. As
explained in a later chapter, each of these lineages of embryonic cells forms
the distinct germ layers from which all the tissues and organs of the human
body eventually form. Each germ layer is identified by its relative position:
ectoderm (ecto- = “outer”), mesoderm (meso- = “middle”), and endoderm
(endo- = “inner’’). [link] shows the types of tissues and organs associated
with the each of the three germ layers. Note that epithelial tissue originates
in all three layers, whereas nervous tissue derives primarily from the
ectoderm and muscle tissue from mesoderm.
Embryonic Origin of Tissues and Major Organs
Ectoderm Epidermis, glands on skin, some cranial bones, pituitary and adrenal medulla, the nervous
system, the mouth between cheek and gums, the anus
= ue, ANZ 1Q( | a
= is S P= y =. ] : y/ | . = Nos /
Skin cells Neurons Pigment cell
Mesoderm Connective tissues proper, bone, cartilage, blood, endothelium of blood vessels, muscle,
synovial membranes, serous membranes lining body cavities, kidneys, lining of gonads
_———
—— = — =
—
be _<<ff = =
Cardiac Skeletal Tubule cell Red blood Smooth
muscle muscle of kidney cells muscle
Endoderm Lining of airways and digestive system except the mouth and distal part of digestive system
(rectum and anal canal); glands (digestive glands, endocrine glands, adrenal cortex)
Lung cell Thyroid cell Pancreatic cell
= = :
mmm §~OPENStax COLLEGE
View this slideshow to learn more about stem cells. How do somatic stem
cells differ from embryonic stem cells?
Tissue Membranes
A tissue membrane is a thin layer or sheet of cells that covers the outside
of the body (for example, skin), the organs (for example, pericardium),
internal passageways that lead to the exterior of the body (for example,
abdominal mesenteries), and the lining of the moveable joint cavities. There
are two basic types of tissue membranes: connective tissue and epithelial
membranes ((link]).
Tissue Membranes
Mucous membranes line the
digestive, respiratory, urinary,
and reproductive tracts. They
are coated with the secretions
of mucous glands.
Serous membranes line body
cavities closed to the exterior
of the body: the peritoneal,
pleural, and pericardial
cavities.
Cutaneous membrane, or the
skin, covers the body surface.
Synovial membranes line joint
cavities and produce the fluid
within the joint.
The two broad categories of tissue membranes
in the body are (1) connective tissue
membranes, which include synovial
membranes, and (2) epithelial membranes,
which include mucous membranes, serous
membranes, and the cutaneous membrane, in
other words, the skin.
Connective Tissue Membranes
The connective tissue membrane is formed solely from connective tissue.
These membranes encapsulate organs, such as the kidneys, and line our
movable joints. A synovial membrane is a type of connective tissue
membrane that lines the cavity of a freely movable joint. For example,
synovial membranes surround the joints of the shoulder, elbow, and knee.
Fibroblasts in the inner layer of the synovial membrane release hyaluronan
into the joint cavity. The hyaluronan effectively traps available water to
form the synovial fluid, a natural lubricant that enables the bones of a joint
to move freely against one another without much friction. This synovial
fluid readily exchanges water and nutrients with blood, as do all body
fluids.
Epithelial Membranes
The epithelial membrane is composed of epithelium attached to a layer of
connective tissue, for example, your skin. The mucous membrane is also a
composite of connective and epithelial tissues. Sometimes called mucosae,
these epithelial membranes line the body cavities and hollow passageways
that open to the external environment, and include the digestive, respiratory,
excretory, and reproductive tracts. Mucous, produced by the epithelial
exocrine glands, covers the epithelial layer. The underlying connective
tissue, called the lamina propria (literally “own layer”), help support the
fragile epithelial layer.
A serous membrane is an epithelial membrane composed of mesodermally
derived epithelium called the mesothelium that is supported by connective
tissue. These membranes line the coelomic cavities of the body, that is,
those cavities that do not open to the outside, and they cover the organs
located within those cavities. They are essentially membranous bags, with
mesothelium lining the inside and connective tissue on the outside. Serous
fluid secreted by the cells of the thin squamous mesothelium lubricates the
membrane and reduces abrasion and friction between organs. Serous
membranes are identified according locations. Three serous membranes line
the thoracic cavity; the two pleura that cover the lungs and the pericardium
that covers the heart. A fourth, the peritoneum, is the serous membrane in
the abdominal cavity that covers abdominal organs and forms double sheets
of mesenteries that suspend many of the digestive organs.
The skin is an epithelial membrane also called the cutaneous membrane. It
is a stratified squamous epithelial membrane resting on top of connective
tissue. The apical surface of this membrane is exposed to the external
environment and is covered with dead, keratinized cells that help protect the
body from desiccation and pathogens.
Chapter Review
The human body contains more than 200 types of cells that can all be
classified into four types of tissues: epithelial, connective, muscle, and
nervous. Epithelial tissues act as coverings controlling the movement of
materials across the surface. Connective tissue integrates the various parts
of the body and provides support and protection to organs. Muscle tissue
allows the body to move. Nervous tissues propagate information.
The study of the shape and arrangement of cells in tissue is called histology.
All cells and tissues in the body derive from three germ layers in the
embryo: the ectoderm, mesoderm, and endoderm.
Different types of tissues form membranes that enclose organs, provide a
friction-free interaction between organs, and keep organs together. Synovial
membranes are connective tissue membranes that protect and line the joints.
Epithelial membranes are formed from epithelial tissue attached to a layer
of connective tissue. There are three types of epithelial membranes:
mucous, which contain glands; serous, which secrete fluid; and cutaneous
which makes up the skin.
Interactive Link Questions
Exercise:
Problem:
View this slideshow to learn more about stem cells. How do somatic
stem cells differ from embryonic stem cells?
Solution:
Most somatic stem cells give rise to only a few cell types.
Review Questions
Exercise:
Problem: Which of the following is not a type of tissue?
a. muscle
b. nervous
c. embryonic
d. epithelial
Solution:
c
Exercise:
Problem:
The process by which a less specialized cell matures into a more
specialized cell is called
a. differentiation
b. maturation
c. modification
d. specialization
Solution:
A
Exercise:
Problem:
Differentiated cells in a developing embryo derive from
a. endothelium, mesothelium, and epithelium
b. ectoderm, mesoderm, and endoderm
c. connective tissue, epithelial tissue, and muscle tissue
d. epidermis, mesoderm, and endothelium
Solution:
B
Exercise:
Problem:
Which of the following lines the body cavities exposed to the external
environment?
a. mesothelium
b. lamina propria
c. mesenteries
d. mucosa
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Identify the four types of tissue in the body, and describe the major
functions of each tissue.
Solution:
The four types of tissue in the body are epithelial, connective, muscle,
and nervous. Epithelial tissue is made of layers of cells that cover the
surfaces of the body that come into contact with the exterior world,
line internal cavities, and form glands. Connective tissue binds the
cells and organs of the body together and performs many functions,
especially in the protection, support, and integration of the body.
Muscle tissue, which responds to stimulation and contracts to provide
movement, is divided into three major types: skeletal (voluntary)
muscles, smooth muscles, and the cardiac muscle in the heart. Nervous
tissue allows the body to receive signals and transmit information as
electric impulses from one region of the body to another.
Exercise:
Problem:
The zygote is described as totipotent because it ultimately gives rise to
all the cells in your body including the highly specialized cells of your
nervous system. Describe this transition, discussing the steps and
processes that lead to these specialized cells.
Solution:
The zygote divides into many cells. As these cells become specialized,
they lose their ability to differentiate into all tissues. At first they form
the three primary germ layers. Following the cells of the ectodermal
germ layer, they too become more restricted in what they can form.
Ultimately, some of these ectodermal cells become further restricted
and differentiate in to nerve cells.
Exercise:
Problem: What is the function of synovial membranes?
Solution:
Synovial membranes are a type of connective tissue membrane that
supports mobility in joints. The membrane lines the joint cavity and
contains fibroblasts that produce hyaluronan, which leads to the
production of synovial fluid, a natural lubricant that enables the bones
of a joint to move freely against one another.
Glossary
connective tissue
type of tissue that serves to hold in place, connect, and integrate the
body’s organs and systems
connective tissue membrane
connective tissue that encapsulates organs and lines movable joints
cutaneous membrane
skin; epithelial tissue made up of a stratified squamous epithelial cells
that cover the outside of the body
ectoderm
outermost embryonic germ layer from which the epidermis and the
nervous tissue derive
endoderm
innermost embryonic germ layer from which most of the digestive
system and lower respiratory system derive
epithelial membrane
epithelium attached to a layer of connective tissue
epithelial tissue
type of tissue that serves primarily as a covering or lining of body
parts, protecting the body; it also functions in absorption, transport,
and secretion
histology
microscopic study of tissue architecture, organization, and function
lamina propria
areolar connective tissue underlying a mucous membrane
mesoderm
middle embryonic germ layer from which connective tissue, muscle
tissue, and some epithelial tissue derive
mucous membrane
tissue membrane that is covered by protective mucous and lines tissue
exposed to the outside environment
muscle tissue
type of tissue that is capable of contracting and generating tension in
response to stimulation; produces movement.
nervous tissue
type of tissue that is capable of sending and receiving impulses
through electrochemical signals.
serous membrane
type of tissue membrane that lines body cavities and lubricates them
with serous fluid
synovial membrane
connective tissue membrane that lines the cavities of freely movable
joints, producing synovial fluid for lubrication
tissue
group of cells that are similar in form and perform related functions
tissue membrane
thin layer or sheet of cells that covers the outside of the body, organs,
and internal cavities
totipotent
embryonic cells that have the ability to differentiate into any type of
cell and organ in the body
Epithelial Tissue
By the end of this section, you will be able to:
e Explain the structure and function of epithelial tissue
e Distinguish between tight junctions, anchoring junctions, and gap
junctions
e Distinguish between simple epithelia and stratified epithelia, as well as
between squamous, cuboidal, and columnar epithelia
e Describe the structure and function of endocrine and exocrine glands
and their respective secretions
Most epithelial tissues are essentially large sheets of cells covering all the
surfaces of the body exposed to the outside world and lining the outside of
organs. Epithelium also forms much of the glandular tissue of the body.
Skin is not the only area of the body exposed to the outside. Other areas
include the airways, the digestive tract, as well as the urinary and
reproductive systems, all of which are lined by an epithelium. Hollow
organs and body cavities that do not connect to the exterior of the body,
which includes, blood vessels and serous membranes, are lined by
endothelium (plural = endothelia), which is a type of epithelium.
Epithelial cells derive from all three major embryonic layers. The epithelia
lining the skin, parts of the mouth and nose, and the anus develop from the
ectoderm. Cells lining the airways and most of the digestive system
originate in the endoderm. The epithelium that lines vessels in the
lymphatic and cardiovascular system derives from the mesoderm and is
called an endothelium.
All epithelia share some important structural and functional features. This
tissue is highly cellular, with little or no extracellular material present
between cells. Adjoining cells form a specialized intercellular connection
between their cell membranes called a cell junction. The epithelial cells
exhibit polarity with differences in structure and function between the
exposed or apical facing surface of the cell and the basal surface close to
the underlying body structures. The basal lamina, a mixture of
glycoproteins and collagen, provides an attachment site for the epithelium,
separating it from underlying connective tissue. The basal lamina attaches
to a reticular lamina, which is secreted by the underlying connective
tissue, forming a basement membrane that helps hold it all together.
Epithelial tissues are nearly completely avascular. For instance, no blood
vessels cross the basement membrane to enter the tissue, and nutrients must
come by diffusion or absorption from underlying tissues or the surface.
Many epithelial tissues are capable of rapidly replacing damaged and dead
cells. Sloughing off of damaged or dead cells is a characteristic of surface
epithelium and allows our airways and digestive tracts to rapidly replace
damaged cells with new cells.
Generalized Functions of Epithelial Tissue
Epithelial tissues provide the body’s first line of protection from physical,
chemical, and biological wear and tear. The cells of an epithelium act as
gatekeepers of the body controlling permeability and allowing selective
transfer of materials across a physical barrier. All substances that enter the
body must cross an epithelium. Some epithelia often include structural
features that allow the selective transport of molecules and ions across their
cell membranes.
Many epithelial cells are capable of secretion and release mucous and
specific chemical compounds onto their apical surfaces. The epithelium of
the small intestine releases digestive enzymes, for example. Cells lining the
respiratory tract secrete mucous that traps incoming microorganisms and
particles. A glandular epithelium contains many secretory cells.
The Epithelial Cell
Epithelial cells are typically characterized by the polarized distribution of
organelles and membrane-bound proteins between their basal and apical
surfaces. Particular structures found in some epithelial cells are an
adaptation to specific functions. Certain organelles are segregated to the
basal sides, whereas other organelles and extensions, such as cilia, when
present, are on the apical surface.
Cilia are microscopic extensions of the apical cell membrane that are
supported by microtubules. They beat in unison and move fluids as well as
trapped particles. Ciliated epithelium lines the ventricles of the brain where
it helps circulate the cerebrospinal fluid. The ciliated epithelium of your
airway forms a mucociliary escalator that sweeps particles of dust and
pathogens trapped in the secreted mucous toward the throat. It is called an
escalator because it continuously pushes mucous with trapped particles
upward. In contrast, nasal cilia sweep the mucous blanket down towards
your throat. In both cases, the transported materials are usually swallowed,
and end up in the acidic environment of your stomach.
Cell to Cell Junctions
Cells of epithelia are closely connected and are not separated by
intracellular material. Three basic types of connections allow varying
degrees of interaction between the cells: tight junctions, anchoring
junctions, and gap junctions ([link]).
Types of Cell Junctions
Tight junction
plasma
membranes
Strands of <@
transmembrane
proteins
Intercellullar
Anchoring junctions
Gap junction
Adjacent
plasma
membranes
Gap between
cells
Connexons
(composed of
connexins)
Adjacent
plasma
membranes
Plaque
Transmembrane
Integrins
Basal
lamina ) (
Hemidesmosome
glycoprotein glycoprotein
(cadherin) (cadherin)
Intermediate Actin filament
filament
(keratin)
Intercellullar ——_-i Intercellullar
space space _
it ee
il
Desmosome ( Adherens ) C)
——
Adjacent
plasma
membranes
Plaque
Transmembrane
The three basic types of cell-to-cell junctions are tight
junctions, gap junctions, and anchoring junctions.
At one end of the spectrum is the tight junction, which separates the cells
into apical and basal compartments. When two adjacent epithelial cells
form a tight junction, there is no extracellular space between them and the
movement of substances through the extracellular space between the cells is
blocked. This enables the epithelia to act as selective barriers. An
anchoring junction includes several types of cell junctions that help
stabilize epithelial tissues. Anchoring junctions are common on the lateral
and basal surfaces of cells where they provide strong and flexible
connections. There are three types of anchoring junctions: desmosomes,
hemidesmosomes, and adherens. Desmosomes occur in patches on the
membranes of cells. The patches are structural proteins on the inner surface
of the cell’s membrane. The adhesion molecule, cadherin, is embedded in
these patches and projects through the cell membrane to link with the
cadherin molecules of adjacent cells. These connections are especially
important in holding cells together. Hemidesmosomes, which look like half
a desmosome, link cells to the extracellular matrix, for example, the basal
lamina. While similar in appearance to desmosomes, they include the
adhesion proteins called integrins rather than cadherins. Adherens junctions
use either cadherins or integrins depending on whether they are linking to
other cells or matrix. The junctions are characterized by the presence of the
contractile protein actin located on the cytoplasmic surface of the cell
membrane. The actin can connect isolated patches or form a belt-like
structure inside the cell. These junctions influence the shape and folding of
the epithelial tissue.
In contrast with the tight and anchoring junctions, a gap junction forms an
intercellular passageway between the membranes of adjacent cells to
facilitate the movement of small molecules and ions between the cytoplasm
of adjacent cells. These junctions allow electrical and metabolic coupling of
adjacent cells, which coordinates function in large groups of cells.
Classification of Epithelial Tissues
Epithelial tissues are classified according to the shape of the cells and
number of the cell layers formed ((link]). Cell shapes can be squamous
(flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar
(rectangular, taller than it is wide). Similarly, the number of cell layers in
the tissue can be one—where every cell rests on the basal lamina—which is
a simple epithelium, or more than one, which is a stratified epithelium and
only the basal layer of cells rests on the basal lamina. Pseudostratified
(pseudo- = “false”) describes tissue with a single layer of irregularly shaped
cells that give the appearance of more than one layer. Transitional describes
a form of specialized stratified epithelium in which the shape of the cells
can vary.
Cells of Epithelial Tissue
[Simp
Stratified
Squamous
—S—S==—=
Simple squamous epithelium
Cuboidal elelelelele
Simple cuboidal epithelium Stratified cuboidal epithelium Pseudostratified
Simple columnar epithelium Stratified columnar epithelium Pseudostratified columnar epithelium
Simple epithelial tissue is organized as a single layer of
cells and stratified epithelial tissue is formed by several
layers of cells.
Simple Epithelium
The shape of the cells in the single cell layer of simple epithelium reflects
the functioning of those cells. The cells in simple squamous epithelium
have the appearance of thin scales. Squamous cell nuclei tend to be flat,
horizontal, and elliptical, mirroring the form of the cell. The endothelium
is the epithelial tissue that lines vessels of the lymphatic and cardiovascular
system, and it is made up of a single layer of squamous cells. Simple
Squamous epithelium, because of the thinness of the cell, is present where
rapid passage of chemical compounds is observed. The alveoli of lungs
where gases diffuse, segments of kidney tubules, and the lining of
capillaries are also made of simple squamous epithelial tissue. The
mesothelium is a simple squamous epithelium that forms the surface layer
of the serous membrane that lines body cavities and internal organs. Its
primary function is to provide a smooth and protective surface. Mesothelial
cells are squamous epithelial cells that secrete a fluid that lubricates the
mesothelium.
In simple cuboidal epithelium, the nucleus of the box-like cells appears
round and is generally located near the center of the cell. These epithelia are
active in the secretion and absorptions of molecules. Simple cuboidal
epithelia are observed in the lining of the kidney tubules and in the ducts of
glands.
In simple columnar epithelium, the nucleus of the tall column-like cells
tends to be elongated and located in the basal end of the cells. Like the
cuboidal epithelia, this epithelium is active in the absorption and secretion
of molecules. Simple columnar epithelium forms the lining of some
sections of the digestive system and parts of the female reproductive tract.
Ciliated columnar epithelium is composed of simple columnar epithelial
cells with cilia on their apical surfaces. These epithelial cells are found in
the lining of the fallopian tubes and parts of the respiratory system, where
the beating of the cilia helps remove particulate matter.
Pseudostratified columnar epithelium is a type of epithelium that appears
to be stratified but instead consists of a single layer of irregularly shaped
and differently sized columnar cells. In pseudostratified epithelium, nuclei
of neighboring cells appear at different levels rather than clustered in the
basal end. The arrangement gives the appearance of stratification; but in
fact all the cells are in contact with the basal lamina, although some do not
reach the apical surface. Pseudostratified columnar epithelium is found in
the respiratory tract, where some of these cells have cilia.
Both simple and pseudostratified columnar epithelia are heterogeneous
epithelia because they include additional types of cells interspersed among
the epithelial cells. For example, a goblet cell is a mucous-secreting
unicellular “gland” interspersed between the columnar epithelial cells of
mucous membranes ([link]).
Goblet Cell
Microvilli
Secretory vesicles
containing mucin
Nucleus
(a) In the lining of the small intestine, columnar
epithelium cells are interspersed with goblet cells. (b)
The arrows in this micrograph point to the mucous-
secreting goblet cells. LM x 1600. (Micrograph
provided by the Regents of University of Michigan
Medical School © 2012)
Note:
CEs.)
a
—
mss Openstax COLLEGE
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
Stratified Epithelium
A stratified epithelium consists of several stacked layers of cells. This
epithelium protects against physical and chemical wear and tear. The
stratified epithelium is named by the shape of the most apical layer of cells,
closest to the free space. Stratified squamous epithelium is the most
common type of stratified epithelium in the human body. The apical cells
are squamous, whereas the basal layer contains either columnar or cuboidal
cells. The top layer may be covered with dead cells filled with keratin.
Mammalian skin is an example of this dry, keratinized, stratified squamous
epithelium. The lining of the mouth cavity is an example of an
unkeratinized, stratified squamous epithelium. Stratified cuboidal
epithelium and stratified columnar epithelium can also be found in
certain glands and ducts, but are uncommon in the human body.
Another kind of stratified epithelium is transitional epithelium, so-called
because of the gradual changes in the shapes of the apical cells as the
bladder fills with urine. It is found only in the urinary system, specifically
the ureters and urinary bladder. When the bladder is empty, this epithelium
is convoluted and has cuboidal apical cells with convex, umbrella shaped,
apical surfaces. As the bladder fills with urine, this epithelium loses its
convolutions and the apical cells transition from cuboidal to squamous. It
appears thicker and more multi-layered when the bladder is empty, and
more stretched out and less stratified when the bladder is full and distended.
[link] summarizes the different categories of epithelial cell tissue cells.
Summary of Epithelial Tissue Cells
Simple cuboidal epithelium
Stratified cuboidal epithelium
Air sacs of lungs and the lining
of the heart, blood vessels,
and lymphatic vessels
In ducts and secretory portions
of small glands and in kidney
tubules
Ciliated tissues are in bronchi,
uterine tubes, and uterus;
smooth (nonciliated tissues)
are in the digestive tract,
bladder
Ciliated tissue lines the trachea
and much of the upper
respiratory tract
Lines the esophagus, mouth,
and vagina
Sweat glands, salivary glands,
and the mammary glands
The male urethra and the
ducts of some glands
Lines the bladder, uretha, and
the ureters
SS
Simple squamous epithelium
Allows materials to pass
through by diffusion and
filtration, and secretes
lubricating substance
Secretes and absorbs
Absorbs; it also secretes
mucous and enzymes
Secretes mucus; ciliated tissue
moves mucus
Protects against abrasion
Protective tissue
Secretes and protects
Allows the urinary organs to
expand and stretch
Watch this video to find out more about the anatomy of epithelial tissues.
Where in the body would one find non-keratinizing stratified squamous
epithelium?
Glandular Epithelium
A gland is a structure made up of one or more cells modified to synthesize
and secrete chemical substances. Most glands consist of groups of epithelial
cells. A gland can be classified as an endocrine gland, a ductless gland that
releases secretions directly into surrounding tissues and fluids (endo- =
“inside”), or an exocrine gland whose secretions leave through a duct that
opens directly, or indirectly, to the external environment (exo- = “outside’”’).
Endocrine Glands
The secretions of endocrine glands are called hormones. Hormones are
released into the interstitial fluid, diffused into the bloodstream, and
delivered to targets, in other words, cells that have receptors to bind the
hormones. The endocrine system is part of a major regulatory system
coordinating the regulation and integration of body responses. A few
examples of endocrine glands include the anterior pituitary, thymus, adrenal
cortex, and gonads.
Exocrine Glands
Exocrine glands release their contents through a duct that leads to the
epithelial surface. Mucous, sweat, saliva, and breast milk are all examples
of secretions from exocrine glands. They are all discharged through tubular
ducts. Secretions into the lumen of the gastrointestinal tract, technically
outside of the body, are of the exocrine category.
Glandular Structure
Exocrine glands are classified as either unicellular or multicellular. The
unicellular glands are scattered single cells, such as goblet cells, found in
the mucous membranes of the small and large intestine.
The multicellular exocrine glands known as serous glands develop from
simple epithelium to form a secretory surface that secretes directly into an
inner cavity. These glands line the internal cavities of the abdomen and
chest and release their secretions directly into the cavities. Other
multicellular exocrine glands release their contents through a tubular duct.
The duct is single in a simple gland but in compound glands is divided into
one or more branches ([link]). In tubular glands, the ducts can be straight or
coiled, whereas tubes that form pockets are alveolar (acinar), such as the
exocrine portion of the pancreas. Combinations of tubes and pockets are
known as tubuloalveolar (tubuloacinar) compound glands. In a branched
gland, a duct is connected to more than one secretory group of cells.
Types of Exocrine Glands
Simple ducts
a
Alveolar
(acinar)
Not found in
adult; a stage in
development of
simple
branched
glands
Simp!
Sebaceous (oil)
glands
Compound alveolar (acinar)
RRRROESSAS
Compound tubuloalveolar
>
o
ix y
ing
a
-
Salivary glands; glands of
respiratory passages;
and pancreas
Mammary glands
Simple tubular —
Simple branched tubular
aTIr- - AP
Intestinal glands
Merocrine sweat
glands
Gastric glands;
and mucous glands
of esophagus, tongue,
duodenum
Compound tubular
DOOUDWODLE ot ee oes
Mucous glands (in mouth);
bulbourethral glands (in male
reproductive system); and testes
(seminiferous tubules) )
Compound ducts 4
Exocrine glands are classified by their structure.
Methods and Types of Secretion
Exocrine glands can be classified by their mode of secretion and the nature
Tubular
of the substances released, as well as by the structure of the glands and
shape of ducts ([link]). Merocrine secretion is the most common type of
exocrine secretion. The secretions are enclosed in vesicles that move to the
apical surface of the cell where the contents are released by exocytosis. For
example, watery mucous containing the glycoprotein mucin, a lubricant that
offers some pathogen protection is a merocrine secretion. The eccrine
glands that produce and secrete sweat are another example.
Modes of Glandular Secretion
Secretion
Secretory vesicle
(a) Merocrine Golgi complex
secretion
Nucleus
Pinched off portion
of cell is the secretion
(b) Apocrine
secretion
Mature cell dies
and becomes
secretory product
(c) Holocrine
secretion
(a) In merocrine secretion, the cell remains
intact. (b) In apocrine secretion, the apical
portion of the cell is released, as well. (c) In
holocrine secretion, the cell is destroyed as it
releases its product and the cell itself
becomes part of the secretion.
Apocrine secretion accumulates near the apical portion of the cell. That
portion of the cell and its secretory contents pinch off from the cell and are
released. Apocrine sweat glands in the axillary and genital areas release
fatty secretions that local bacteria break down; this causes body odor. Both
merocrine and apocrine glands continue to produce and secrete their
contents with little damage caused to the cell because the nucleus and golgi
regions remain intact after secretion.
In contrast, the process of holocrine secretion involves the rupture and
destruction of the entire gland cell. The cell accumulates its secretory
products and releases them only when it bursts. New gland cells
differentiate from cells in the surrounding tissue to replace those lost by
secretion. The sebaceous glands that produce the oils on the skin and hair
are holocrine glands/cells ({link]).
Sebaceous Glands
Hair
Sebaceous
gland
These glands secrete oils that lubricate and protect the skin.
They are holocrine glands and they are destroyed after
releasing their contents. New glandular cells form to
replace the cells that are lost. LM x 400. (Micrograph
provided by the Regents of University of Michigan Medical
School © 2012)
Glands are also named after the products they produce. The serous gland
produces watery, blood-plasma-like secretions rich in enzymes such as
alpha amylase, whereas the mucous gland releases watery to viscous
products rich in the glycoprotein mucin. Both serous and mucous glands are
common in the salivary glands of the mouth. Mixed exocrine glands contain
both serous and mucous glands and release both types of secretions.
Chapter Review
In epithelial tissue, cells are closely packed with little or no extracellular
matrix except for the basal lamina that separates the epithelium from
underlying tissue. The main functions of epithelia are protection from the
environment, coverage, secretion and excretion, absorption, and filtration.
Cells are bound together by tight junctions that form an impermeable
barrier. They can also be connected by gap junctions, which allow free
exchange of soluble molecules between cells, and anchoring junctions,
which attach cell to cell or cell to matrix. The different types of epithelial
tissues are characterized by their cellular shapes and arrangements:
squamous, cuboidal, or columnar epithelia. Single cell layers form simple
epithelia, whereas stacked cells form stratified epithelia. Very few
capillaries penetrate these tissues.
Glands are secretory tissues and organs that are derived from epithelial
tissues. Exocrine glands release their products through ducts. Endocrine
glands secrete hormones directly into the interstitial fluid and blood stream.
Glands are classified both according to the type of secretion and by their
structure. Merocrine glands secrete products as they are synthesized.
Apocrine glands release secretions by pinching off the apical portion of the
cell, whereas holocrine gland cells store their secretions until they rupture
and release their contents. In this case, the cell becomes part of the
secretion.
Interactive Link Questions
Exercise:
Problem:
Watch this video to find out more about the anatomy of epithelial
tissues. Where in the body would one find non-keratinizing stratified
squamous epithelium?
Solution:
The inside of the mouth, esophagus, vaginal canal, and anus.
Review Questions
Exercise:
Problem:
In observing epithelial cells under a microscope, the cells are arranged
in a single layer and look tall and narrow, and the nucleus is located
close to the basal side of the cell. The specimen is what type of
epithelial tissue?
a. columnar
b. stratified
c. squamous
d. transitional
Solution:
A
Exercise:
Problem:
Which of the following is the epithelial tissue that lines the interior of
blood vessels?
a. columnar
b. pseudostratified
c. simple squamous
d. transitional
Solution:
C
Exercise:
Problem:
Which type of epithelial tissue specializes in moving particles across
its surface and is found in airways and lining of the oviduct?
a. transitional
b. stratified columnar
c. pseudostratified ciliated columnar
d. stratified squamous
Solution:
B
Exercise:
Problem:
The exocrine gland stores its secretion until the glandular
cell ruptures, whereas the gland releases its apical region
and reforms.
a. holocrine; apocrine
b. eccrine; endocrine
c. apocrine; holocrine
d. eccrine; apocrine
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
The structure of a tissue usually is optimized for its function. Describe
how the structure of the mucosa and its cells match its function of
nutrient absorption.
Solution:
The mucosa of the intestine is highly folded, increasing the surface
area for nutrient absorption. A greater surface area for absorption
allows more nutrients to be absorbed per unit time. In addition, the
nutrient-absorbing cells of the mucosa have finger-like projections
called microvilli that further increase the surface area for nutrient
absorption.
Glossary
anchoring junction
mechanically attaches adjacent cells to each other or to the basement
membrane
apical
that part of a cell or tissue which, in general, faces an open space
apocrine secretion
release of a substance along with the apical portion of the cell
basal lamina
thin extracellular layer that lies underneath epithelial cells and
separates them from other tissues
basement membrane
in epithelial tissue, a thin layer of fibrous material that anchors the
epithelial tissue to the underlying connective tissue; made up of the
basal lamina and reticular lamina
cell junction
point of cell-to-cell contact that connects one cell to another in a tissue
endocrine gland
groups of cells that release chemical signals into the intercellular fluid
to be picked up and transported to their target organs by blood
endothelium
tissue that lines vessels of the lymphatic and cardiovascular system,
made up of a simple squamous epithelium
exocrine gland
group of epithelial cells that secrete substances through ducts that open
to the skin or to internal body surfaces that lead to the exterior of the
body
gap junction
allows cytoplasmic communications to occur between cells
goblet cell
unicellular gland found in columnar epithelium that secretes mucous
holocrine secretion
release of a substance caused by the rupture of a gland cell, which
becomes part of the secretion
merocrine secretion
release of a substance from a gland via exocytosis
mesothelium
simple squamous epithelial tissue which covers the major body
cavities and is the epithelial portion of serous membranes
mucous gland
group of cells that secrete mucous, a thick, slippery substance that
keeps tissues moist and acts as a lubricant
pseudostratified columnar epithelium
tissue that consists of a single layer of irregularly shaped and sized
cells that give the appearance of multiple layers; found in ducts of
certain glands and the upper respiratory tract
reticular lamina
matrix containing collagen and elastin secreted by connective tissue; a
component of the basement membrane
serous gland
group of cells within the serous membrane that secrete a lubricating
substance onto the surface
simple columnar epithelium
tissue that consists of a single layer of column-like cells; promotes
secretion and absorption in tissues and organs
simple cuboidal epithelium
tissue that consists of a single layer of cube-shaped cells; promotes
secretion and absorption in ducts and tubules
simple squamous epithelium
tissue that consists of a single layer of flat scale-like cells; promotes
diffusion and filtration across surface
stratified columnar epithelium
tissue that consists of two or more layers of column-like cells, contains
glands and is found in some ducts
stratified cuboidal epithelium
tissue that consists of two or more layers of cube-shaped cells, found
in some ducts
stratified squamous epithelium
tissue that consists of multiple layers of cells with the most apical
being flat scale-like cells; protects surfaces from abrasion
tight junction
forms an impermeable barrier between cells
transitional epithelium
form of stratified epithelium found in the urinary tract, characterized
by an apical layer of cells that change shape in response to the
presence of urine
Connective Tissue Supports and Protects
By the end of this section, you will be able to:
e Identify and distinguish between the types of connective tissue: proper,
supportive, and fluid
e Explain the functions of connective tissues
As may be obvious from its name, one of the major functions of connective
tissue is to connect tissues and organs. Unlike epithelial tissue, which is
composed of cells closely packed with little or no extracellular space in
between, connective tissue cells are dispersed in a matrix. The matrix
usually includes a large amount of extracellular material produced by the
connective tissue cells that are embedded within it. The matrix plays a
major role in the functioning of this tissue. The major component of the
matrix is a ground substance often crisscrossed by protein fibers. This
ground substance is usually a fluid, but it can also be mineralized and solid,
as in bones. Connective tissues come in a vast variety of forms, yet they
typically have in common three characteristic components: cells, large
amounts of amorphous ground substance, and protein fibers. The amount
and structure of each component correlates with the function of the tissue,
from the rigid ground substance in bones supporting the body to the
inclusion of specialized cells; for example, a phagocytic cell that engulfs
pathogens and also rids tissue of cellular debris.
Functions of Connective Tissues
Connective tissues perform many functions in the body, but most
importantly, they support and connect other tissues; from the connective
tissue sheath that surrounds muscle cells, to the tendons that attach muscles
to bones, and to the skeleton that supports the positions of the body.
Protection is another major function of connective tissue, in the form of
fibrous capsules and bones that protect delicate organs and, of course, the
skeletal system. Specialized cells in connective tissue defend the body from
microorganisms that enter the body. Transport of fluid, nutrients, waste, and
chemical messengers is ensured by specialized fluid connective tissues,
such as blood and lymph. Adipose cells store surplus energy in the form of
fat and contribute to the thermal insulation of the body.
Embryonic Connective Tissue
All connective tissues derive from the mesodermal layer of the embryo (see
[link]). The first connective tissue to develop in the embryo is
mesenchyme, the stem cell line from which all connective tissues are later
derived. Clusters of mesenchymal cells are scattered throughout adult tissue
and supply the cells needed for replacement and repair after a connective
tissue injury. A second type of embryonic connective tissue forms in the
umbilical cord, called mucous connective tissue or Wharton’s jelly. This
tissue is no longer present after birth, leaving only scattered mesenchymal
cells throughout the body.
Classification of Connective Tissues
The three broad categories of connective tissue are classified according to
the characteristics of their ground substance and the types of fibers found
within the matrix ({link]). Connective tissue proper includes loose
connective tissue and dense connective tissue. Both tissues have a variety
of cell types and protein fibers suspended in a viscous ground substance.
Dense connective tissue is reinforced by bundles of fibers that provide
tensile strength, elasticity, and protection. In loose connective tissue, the
fibers are loosely organized, leaving large spaces in between. Supportive
connective tissue—bone and cartilage—provide structure and strength to
the body and protect soft tissues. A few distinct cell types and densely
packed fibers in a matrix characterize these tissues. In bone, the matrix is
rigid and described as calcified because of the deposited calcium salts. In
fluid connective tissue, in other words, lymph and blood, various
specialized cells circulate in a watery fluid containing salts, nutrients, and
dissolved proteins.
Connective Tissue Examples
Connective tisssiee Exaifipjasortive Fluid connective
proper connective tissue tissue
Connective tissue Supportive Fluid connective
proper connective tissue tissue
Loose connective :
an Cartilage
e Hyaline
e Areolar y Blood
a aadinose e Fibrocartilage
e Reticular Pane
Dense connective
tissue
Bones
Regular
; ere e Compact bone Lymph
eicsalat e Cancellous bone
elastic
Connective Tissue Proper
Fibroblasts are present in all connective tissue proper ([link]). Fibrocytes,
adipocytes, and mesenchymal cells are fixed cells, which means they
remain within the connective tissue. Other cells move in and out of the
connective tissue in response to chemical signals. Macrophages, mast cells,
lymphocytes, plasma cells, and phagocytic cells are found in connective
tissue proper but are actually part of the immune system protecting the
body.
Connective Tissue Proper
Reticular fibers
Adipocytes
Mesenchymal cell
Elastic fibers
Collagen fibers
Fibroblast
Macrophage
Fibroblasts produce this fibrous tissue. Connective tissue proper
includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells.
LM x 400. (Micrograph provided by the Regents of University of
Michigan Medical School © 2012)
Cell Types
The most abundant cell in connective tissue proper is the fibroblast.
Polysaccharides and proteins secreted by fibroblasts combine with extra-
cellular fluids to produce a viscous ground substance that, with embedded
fibrous proteins, forms the extra-cellular matrix. As you might expect, a
fibrocyte, a less active form of fibroblast, is the second most common cell
type in connective tissue proper.
Adipocytes are cells that store lipids as droplets that fill most of the
cytoplasm. There are two basic types of adipocytes: white and brown. The
brown adipocytes store lipids as many droplets, and have high metabolic
activity. In contrast, white fat adipocytes store lipids as a single large drop
and are metabolically less active. Their effectiveness at storing large
amounts of fat is witnessed in obese individuals. The number and type of
adipocytes depends on the tissue and location, and vary among individuals
in the population.
The mesenchymal cell is a multipotent adult stem cell. These cells can
differentiate into any type of connective tissue cells needed for repair and
healing of damaged tissue.
The macrophage cell is a large cell derived from a monocyte, a type of
blood cell, which enters the connective tissue matrix from the blood vessels.
The macrophage cells are an essential component of the immune system,
which is the body’s defense against potential pathogens and degraded host
cells. When stimulated, macrophages release cytokines, small proteins that
act as chemical messengers. Cytokines recruit other cells of the immune
system to infected sites and stimulate their activities. Roaming, or free,
macrophages move rapidly by amoeboid movement, engulfing infectious
agents and cellular debris. In contrast, fixed macrophages are permanent
residents of their tissues.
The mast cell, found in connective tissue proper, has many cytoplasmic
granules. These granules contain the chemical signals histamine and
heparin. When irritated or damaged, mast cells release histamine, an
inflammatory mediator, which causes vasodilation and increased blood flow
at a site of injury or infection, along with itching, swelling, and redness you
recognize as an allergic response. Like blood cells, mast cells are derived
from hematopoietic stem cells and are part of the immune system.
Connective Tissue Fibers and Ground Substance
Three main types of fibers are secreted by fibroblasts: collagen fibers,
elastic fibers, and reticular fibers. Collagen fiber is made from fibrous
protein subunits linked together to form a long and straight fiber. Collagen
fibers, while flexible, have great tensile strength, resist stretching, and give
ligaments and tendons their characteristic resilience and strength. These
fibers hold connective tissues together, even during the movement of the
body.
Elastic fiber contains the protein elastin along with lesser amounts of other
proteins and glycoproteins. The main property of elastin is that after being
stretched or compressed, it will return to its original shape. Elastic fibers are
prominent in elastic tissues found in skin and the elastic ligaments of the
vertebral column.
Reticular fiber is also formed from the same protein subunits as collagen
fibers; however, these fibers remain narrow and are arrayed in a branching
network. They are found throughout the body, but are most abundant in the
reticular tissue of soft organs, such as liver and spleen, where they anchor
and provide structural support to the parenchyma (the functional cells,
blood vessels, and nerves of the organ).
All of these fiber types are embedded in ground substance. Secreted by
fibroblasts, ground substance is made of polysaccharides, specifically
hyaluronic acid, and proteins. These combine to form a proteoglycan with a
protein core and polysaccharide branches. The proteoglycan attracts and
traps available moisture forming the clear, viscous, colorless matrix you
now know as ground substance.
Loose Connective Tissue
Loose connective tissue is found between many organs where it acts both to
absorb shock and bind tissues together. It allows water, salts, and various
nutrients to diffuse through to adjacent or imbedded cells and tissues.
Adipose tissue consists mostly of fat storage cells, with little extracellular
matrix ({link]). A large number of capillaries allow rapid storage and
mobilization of lipid molecules. White adipose tissue is most abundant. It
can appear yellow and owes its color to carotene and related pigments from
plant food. White fat contributes mostly to lipid storage and can serve as
insulation from cold temperatures and mechanical injuries. White adipose
tissue can be found protecting the kidneys and cushioning the back of the
eye. Brown adipose tissue is more common in infants, hence the term “baby
fat.” In adults, there is a reduced amount of brown fat and it is found mainly
in the neck and clavicular regions of the body. The many mitochondria in
the cytoplasm of brown adipose tissue help explain its efficiency at
metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that
as it breaks down fats, it releases metabolic heat, rather than producing
adenosine triphosphate (ATP), a key molecule used in metabolism.
Adipose Tissue
| es —————
This is a loose connective tissue that consists of fat cells
with little extracellular matrix. It stores fat for energy
and provides insulation. LM x 800. (Micrograph
provided by the Regents of University of Michigan
Medical School © 2012)
Areolar tissue shows little specialization. It contains all the cell types and
fibers previously described and is distributed in a random, web-like fashion.
It fills the spaces between muscle fibers, surrounds blood and lymph
vessels, and supports organs in the abdominal cavity. Areolar tissue
underlies most epithelia and represents the connective tissue component of
epithelial membranes, which are described further in a later section.
Reticular tissue is a mesh-like, supportive framework for soft organs such
as lymphatic tissue, the spleen, and the liver ([link]). Reticular cells produce
the reticular fibers that form the network onto which other cells attach. It
derives its name from the Latin reticulus, which means “little net.”
Reticular Tissue
This is a loose connective tissue made up of a network of
reticular fibers that provides a supportive framework for
soft organs. LM x 1600. (Micrograph provided by the
Regents of University of Michigan Medical School ©
2012)
Dense Connective Tissue
Dense connective tissue contains more collagen fibers than does loose
connective tissue. As a consequence, it displays greater resistance to
stretching. There are two major categories of dense connective tissue:
regular and irregular. Dense regular connective tissue fibers are parallel to
each other, enhancing tensile strength and resistance to stretching in the
direction of the fiber orientations. Ligaments and tendons are made of dense
regular connective tissue, but in ligaments not all fibers are parallel. Dense
regular elastic tissue contains elastin fibers in addition to collagen fibers,
which allows the ligament to return to its original length after stretching.
The ligaments in the vocal folds and between the vertebrae in the vertebral
column are elastic.
In dense irregular connective tissue, the direction of fibers is random. This
arrangement gives the tissue greater strength in all directions and less
strength in one particular direction. In some tissues, fibers crisscross and
form a mesh. In other tissues, stretching in several directions is achieved by
alternating layers where fibers run in the same orientation in each layer, and
it is the layers themselves that are stacked at an angle. The dermis of the
skin is an example of dense irregular connective tissue rich in collagen
fibers. Dense irregular elastic tissues give arterial walls the strength and the
ability to regain original shape after stretching ([link]).
Dense Connective Tissue
Fibroblast
nuclei
Fibroblast
nuclei
Collagen
fiber
bundles
(b) Irregular dense
(a) Dense regular connective tissue consists of
collagenous fibers packed into parallel bundles. (b)
Dense irregular connective tissue consists of collagenous
fibers interwoven into a mesh-like network. From top,
LM x 1000, LM x 200. (Micrographs provided by the
Regents of University of Michigan Medical School ©
2012)
Note:
Disorders of the...
Connective Tissue: Tendinitis
Your opponent stands ready as you prepare to hit the serve, but you are
confident that you will smash the ball past your opponent. As you toss the
ball high in the air, a bumming pain shoots across your wrist and you drop
the tennis racket. That dull ache in the wrist that you ignored through the
summer is now an unbearable pain. The game is over for now.
After examining your swollen wrist, the doctor in the emergency room
announces that you have developed wrist tendinitis. She recommends icing
the tender area, taking non-steroidal anti-inflammatory medication to ease
the pain and to reduce swelling, and complete rest for a few weeks. She
interrupts your protests that you cannot stop playing. She issues a stern
warning about the risk of aggravating the condition and the possibility of
surgery. She consoles you by mentioning that well known tennis players
such as Venus and Serena Williams and Rafael Nadal have also suffered
from tendinitis related injuries.
What is tendinitis and how did it happen? Tendinitis is the inflammation of
a tendon, the thick band of fibrous connective tissue that attaches a muscle
to a bone. The condition causes pain and tenderness in the area around a
joint. On rare occasions, a sudden serious injury will cause tendinitis. Most
often, the condition results from repetitive motions over time that strain the
tendons needed to perform the tasks.
Persons whose jobs and hobbies involve performing the same movements
over and over again are often at the greatest risk of tendinitis. You hear of
tennis and golfer’s elbow, jumper's knee, and swimmer’s shoulder. In all
cases, overuse of the joint causes a microtrauma that initiates the
inflammatory response. Tendinitis is routinely diagnosed through a clinical
examination. In case of severe pain, X-rays can be examined to rule out the
possibility of a bone injury. Severe cases of tendinitis can even tear loose a
tendon. Surgical repair of a tendon is painful. Connective tissue in the
tendon does not have abundant blood supply and heals slowly.
While older adults are at risk for tendinitis because the elasticity of tendon
tissue decreases with age, active people of all ages can develop tendinitis.
Young athletes, dancers, and computer operators; anyone who performs the
same movements constantly is at risk for tendinitis. Although repetitive
motions are unavoidable in many activities and may lead to tendinitis,
precautions can be taken that can lessen the probability of developing
tendinitis. For active individuals, stretches before exercising and cross
training or changing exercises are recommended. For the passionate
athlete, it may be time to take some lessons to improve technique. All of
the preventive measures aim to increase the strength of the tendon and
decrease the stress put on it. With proper rest and managed care, you will
be back on the court to hit that slice-spin serve over the net.
Note:
Pelee Le
Watch this animation to learn more about tendonitis, a painful condition
caused by swollen or injured tendons.
Supportive Connective Tissues
Two major forms of supportive connective tissue, cartilage and bone, allow
the body to maintain its posture and protect internal organs.
Cartilage
The distinctive appearance of cartilage is due to polysaccharides called
chondroitin sulfates, which bind with ground substance proteins to form
proteoglycans. Embedded within the cartilage matrix are chondrocytes, or
cartilage cells, and the space they occupy are called lacunae (singular =
lacuna). A layer of dense irregular connective tissue, the perichondrium,
encapsulates the cartilage. Cartilaginous tissue is avascular, thus all
nutrients need to diffuse through the matrix to reach the chondrocytes. This
is a factor contributing to the very slow healing of cartilaginous tissues.
The three main types of cartilage tissue are hyaline cartilage, fibrocartilage,
and elastic cartilage ([{link]). Hyaline cartilage, the most common type of
cartilage in the body, consists of short and dispersed collagen fibers and
contains large amounts of proteoglycans. Under the microscope, tissue
samples appear clear. The surface of hyaline cartilage is smooth. Both
strong and flexible, it is found in the rib cage and nose and covers bones
where they meet to form moveable joints. It makes up a template of the
embryonic skeleton before bone formation. A plate of hyaline cartilage at
the ends of bone allows continued growth until adulthood. Fibrocartilage
is tough because it has thick bundles of collagen fibers dispersed through its
matrix. Menisci in the knee joint and the intervertebral discs are examples
of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen
and proteoglycans. This tissue gives rigid support as well as elasticity. Tug
gently at your ear lobes, and notice that the lobes return to their initial
shape. The external ear contains elastic cartilage.
Types of Cartilage
(a) Hyaline cartilage
Chondrocytes
in lacunae
Matrix
(b) Fibrocartilage
Chondrocyte
in lacuna
Collagen fiber
in matrix
(c) Elastic cartilage
Chondrocyte
in lacuna
Elastic fibers
in matrix
Cartilage is a connective tissue consisting of
collagenous fibers embedded in a firm matrix of
chondroitin sulfates. (a) Hyaline cartilage provides
support with some flexibility. The example is from
dog tissue. (b) Fibrocartilage provides some
compressibility and can absorb pressure. (c) Elastic
cartilage provides firm but elastic support. From top,
LM x 300, LM x 1200, LM x 1016. (Micrographs
provided by the Regents of University of Michigan
Medical School © 2012)
Bone
Bone is the hardest connective tissue. It provides protection to internal
organs and supports the body. Bone’s rigid extracellular matrix contains
mostly collagen fibers embedded in a mineralized ground substance
containing hydroxyapatite, a form of calcium phosphate. Both components
of the matrix, organic and inorganic, contribute to the unusual properties of
bone. Without collagen, bones would be brittle and shatter easily. Without
mineral crystals, bones would flex and provide little support. Osteocytes,
bone cells like chondrocytes, are located within lacunae. The histology of
transverse tissue from long bone shows a typical arrangement of osteocytes
in concentric circles around a central canal. Bone is a highly vascularized
tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively
short time.
Cancellous bone looks like a sponge under the microscope and contains
empty spaces between trabeculae, or arches of bone proper. It is lighter than
compact bone and found in the interior of some bones and at the end of long
bones. Compact bone is solid and has greater structural strength.
Fluid Connective Tissue
Blood and lymph are fluid connective tissues. Cells circulate in a liquid
extracellular matrix. The formed elements circulating in blood are all
derived from hematopoietic stem cells located in bone marrow ([link]).
Erythrocytes, red blood cells, transport oxygen and some carbon dioxide.
Leukocytes, white blood cells, are responsible for defending against
potentially harmful microorganisms or molecules. Platelets are cell
fragments involved in blood clotting. Some white blood cells have the
ability to cross the endothelial layer that lines blood vessels and enter
adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid
matrix and transported through the body.
Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries
are extremely permeable, allowing larger molecules and excess fluid from
interstitial spaces to enter the lymphatic vessels. Lymph drains into blood
vessels, delivering molecules to the blood that could not otherwise directly
enter the bloodstream. In this way, specialized lymphatic capillaries
transport absorbed fats away from the intestine and deliver these molecules
to the blood.
Blood: A Fluid Connective Tissue
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Blood is a fluid connective tissue containing
erythrocytes and various types of leukocytes
that circulate in a liquid extracellular matrix.
LM x 1600. (Micrograph provided by the
Regents of University of Michigan Medical
School © 2012)
Note:
ac|
openstax COLLEGE”
Wii
ae
View the University of Michigan Webscope to explore the tissue sample in
greater detail.
Note:
="
meee OFENStAX COLLEGE”
Visit this link to test your connective tissue knowledge with this 10-
question quiz. Can you name the 10 tissue types shown in the histology
slides?
Chapter Review
Connective tissue is a heterogeneous tissue with many cell shapes and
tissue architecture. Structurally, all connective tissues contain cells that are
embedded in an extracellular matrix stabilized by proteins. The chemical
nature and physical layout of the extracellular matrix and proteins vary
enormously among tissues, reflecting the variety of functions that
connective tissue fulfills in the body. Connective tissues separate and
cushion organs, protecting them from shifting or traumatic injury. Connect
tissues provide support and assist movement, store and transport energy
molecules, protect against infections, and contribute to temperature
homeostasis.
Many different cells contribute to the formation of connective tissues. They
originate in the mesodermal germ layer and differentiate from mesenchyme
and hematopoietic tissue in the bone marrow. Fibroblasts are the most
abundant and secrete many protein fibers, adipocytes specialize in fat
storage, hematopoietic cells from the bone marrow give rise to all the blood
cells, chondrocytes form cartilage, and osteocytes form bone. The
extracellular matrix contains fluid, proteins, polysaccharide derivatives,
and, in the case of bone, mineral crystals. Protein fibers fall into three major
groups: collagen fibers that are thick, strong, flexible, and resist stretch;
reticular fibers that are thin and form a supportive mesh; and elastin fibers
that are thin and elastic.
The major types of connective tissue are connective tissue proper,
supportive tissue, and fluid tissue. Loose connective tissue proper includes
adipose tissue, areolar tissue, and reticular tissue. These serve to hold
organs and other tissues in place and, in the case of adipose tissue, isolate
and store energy reserves. The matrix is the most abundant feature for loose
tissue although adipose tissue does not have much extracellular matrix.
Dense connective tissue proper is richer in fibers and may be regular, with
fibers oriented in parallel as in ligaments and tendons, or irregular, with
fibers oriented in several directions. Organ capsules (collagenous type) and
walls of arteries (elastic type) contain dense irregular connective tissue.
Cartilage and bone are supportive tissue. Cartilage contains chondrocytes
and is somewhat flexible. Hyaline cartilage is smooth and clear, covers
joints, and is found in the growing portion of bones. Fibrocartilage is tough
because of extra collagen fibers and forms, among other things, the
intervertebral discs. Elastic cartilage can stretch and recoil to its original
shape because of its high content of elastic fibers. The matrix contains very
few blood vessels. Bones are made of a rigid, mineralized matrix containing
calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is
highly vascularized. Cancellous bone is spongy and less solid than compact
bone. Fluid tissue, for example blood and lymph, is characterized by a
liquid matrix and no supporting fibers.
Interactive Link Questions
Exercise:
Problem:
Visit this link to test your connective tissue knowledge with this 10-
question quiz. Can you name the 10 tissue types shown in the
histology slides?
Solution:
Click at the bottom of the quiz for the answers.
Review Questions
Exercise:
Problem:
Connective tissue is made of which three essential components?
a. cells, ground substance, and carbohydrate fibers
b. cells, ground substance, and protein fibers
c. collagen, ground substance, and protein fibers
d. matrix, ground substance, and fluid
Solution:
B
Exercise:
Problem:
Under the microscope, a tissue specimen shows cells located in spaces
scattered in a transparent background. This is probably
a. loose connective tissue
b. a tendon
c. bone
d. hyaline cartilage
Solution:
D
Exercise:
Problem: Which connective tissue specializes in storage of fat?
a. tendon
b. adipose tissue
c. reticular tissue
d. dense connective tissue
Solution:
B
Exercise:
Problem:
Ligaments connect bones together and withstand a lot of stress. What
type of connective tissue should you expect ligaments to contain?
a. areolar tissue
b. adipose tissue
c. dense regular connective tissue
d. dense irregular connective tissue
Solution:
C
Exercise:
Problem:
In adults, new connective tissue cells originate from the
a. mesoderm
b. mesenchyme
c. ectoderm
d. endoderm
Solution:
B
Exercise:
Problem:In bone, the main cells are
a. fibroblasts
b. chondrocytes
c. lymphocytes
d. osteocytes
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
One of the main functions of connective tissue is to integrate organs
and organ systems in the body. Discuss how blood fulfills this role.
Solution:
Blood is a fluid connective tissue, a variety of specialized cells that
circulate in a watery fluid containing salts, nutrients, and dissolved
proteins in a liquid extracellular matrix. Blood contains formed
elements derived from bone marrow. Erythrocytes, or red blood cells,
transport the gases oxygen and carbon dioxide. Leukocytes, or white
blood cells, are responsible for the defense of the organism against
potentially harmful microorganisms or molecules. Platelets are cell
fragments involved in blood clotting. Some cells have the ability to
cross the endothelial layer that lines vessels and enter adjacent tissues.
Nutrients, salts, and waste are dissolved in the liquid matrix and
transported through the body.
Exercise:
Problem:
Why does an injury to cartilage, especially hyaline cartilage, heal
much more slowly than a bone fracture?
Solution:
A layer of dense irregular connective tissue covers cartilage. No blood
vessels supply cartilage tissue. Injuries to cartilage heal very slowly
because cells and nutrients needed for repair diffuse slowly to the
injury site.
Glossary
adipocytes
lipid storage cells
adipose tissue
specialized areolar tissue rich in stored fat
areolar tissue
(also, loose connective tissue) a type of connective tissue proper that
shows little specialization with cells dispersed in the matrix
chondrocytes
cells of the cartilage
collagen fiber
flexible fibrous proteins that give connective tissue tensile strength
connective tissue proper
connective tissue containing a viscous matrix, fibers, and cells.
dense connective tissue
connective tissue proper that contains many fibers that provide both
elasticity and protection
elastic cartilage
type of cartilage, with elastin as the major protein, characterized by
rigid support as well as elasticity
elastic fiber
fibrous protein within connective tissue that contains a high percentage
of the protein elastin that allows the fibers to stretch and return to
original size
fibroblast
most abundant cell type in connective tissue, secretes protein fibers
and matrix into the extracellular space
fibrocartilage
tough form of cartilage, made of thick bundles of collagen fibers
embedded in chondroitin sulfate ground substance
fibrocyte
less active form of fibroblast
fluid connective tissue
specialized cells that circulate in a watery fluid containing salts,
nutrients, and dissolved proteins
ground substance
fluid or semi-fluid portion of the matrix
hyaline cartilage
most common type of cartilage, smooth and made of short collagen
fibers embedded in a chondroitin sulfate ground substance
lacunae
(singular = lacuna) small spaces in bone or cartilage tissue that cells
occupy
loose connective tissue
(also, areolar tissue) type of connective tissue proper that shows little
specialization with cells dispersed in the matrix
matrix
extracellular material which is produced by the cells embedded in it,
containing ground substance and fibers
mesenchymal cell
adult stem cell from which most connective tissue cells are derived
mesenchyme
embryonic tissue from which connective tissue cells derive
mucous connective tissue
specialized loose connective tissue present in the umbilical cord
parenchyma
functional cells of a gland or organ, in contrast with the supportive or
connective tissue of a gland or organ
reticular fiber
fine fibrous protein, made of collagen subunits, which cross-link to
form supporting “nets” within connective tissue
reticular tissue
type of loose connective tissue that provides a supportive framework
to soft organs, such as lymphatic tissue, spleen, and the liver
supportive connective tissue
type of connective tissue that provides strength to the body and
protects soft tissue
Muscle Tissue and Motion
By the end of this section, you will be able to:
e Identify the three types of muscle tissue
¢ Compare and contrast the functions of each muscle tissue type
e Explain how muscle tissue can enable motion
Muscle tissue is characterized by properties that allow movement. Muscle
cells are excitable; they respond to a stimulus. They are contractile,
meaning they can shorten and generate a pulling force. When attached
between two movable objects, in other words, bones, contractions of the
muscles cause the bones to move. Some muscle movement is voluntary,
which means it is under conscious control. For example, a person decides to
open a book and read a chapter on anatomy. Other movements are
involuntary, meaning they are not under conscious control, such as the
contraction of your pupil in bright light. Muscle tissue is classified into
three types according to structure and function: skeletal, cardiac, and
smooth ({link]).
Comparison of Structure and Properties of Muscle Tissue Types
Tissue
Skeletal
Histology
Long
cylindrical
fiber,
Striated,
many
peripherally
located
nuclei
Function
Voluntary
movement, produces
heat, protects organs
Location
Attached to
bones and
around
entrance
points to
body (e.g.,
mouth,
anus)
Comparison of Structure and Properties of Muscle Tissue Types
Tissue Histology Function Location
Short,
branched,
: rl ntr m
aie st ated, Contracts to pump ean
single blood
central
nucleus
Involuntar
Short, vey
: movement, moves
ae food, involuntar
shaped, no ‘ y Walls of
: control of :
evident Pea major
Smooth ihe respiration, moves
Striation, organs and
single secretions, regulates passageways
F flow of blood in
SSA eae arteries b
each fiber y
contraction
Skeletal muscle is attached to bones and its contraction makes possible
locomotion, facial expressions, posture, and other voluntary movements of
the body. Forty percent of your body mass is made up of skeletal muscle.
Skeletal muscles generate heat as a byproduct of their contraction and thus
participate in thermal homeostasis. Shivering is an involuntary contraction
of skeletal muscles in response to perceived lower than normal body
temperature. The muscle cell, or myocyte, develops from myoblasts
derived from the mesoderm. Myocytes and their numbers remain relatively
constant throughout life. Skeletal muscle tissue is arranged in bundles
surrounded by connective tissue. Under the light microscope, muscle cells
appear striated with many nuclei squeezed along the membranes. The
striation is due to the regular alternation of the contractile proteins actin
and myosin, along with the structural proteins that couple the contractile
proteins to connective tissues. The cells are multinucleated as a result of the
fusion of the many myoblasts that fuse to form each long muscle fiber.
Cardiac muscle forms the contractile walls of the heart. The cells of
cardiac muscle, known as cardiomyocytes, also appear striated under the
microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells
typically with a single centrally located nucleus. A principal characteristic
of cardiomyocytes is that they contract on their own intrinsic rhythms
without any external stimulation. Cardiomyocyte attach to one another with
specialized cell junctions called intercalated discs. Intercalated discs have
both anchoring junctions and gap junctions. Attached cells form long,
branching cardiac muscle fibers that are, essentially, a mechanical and
electrochemical syncytium allowing the cells to synchronize their actions.
The cardiac muscle pumps blood through the body and is under involuntary
control. The attachment junctions hold adjacent cells together across the
dynamic pressures changes of the cardiac cycle.
Smooth muscle tissue contraction is responsible for involuntary
movements in the internal organs. It forms the contractile component of the
digestive, urinary, and reproductive systems as well as the airways and
arteries. Each cell is spindle shaped with a single nucleus and no visible
striations ((link]).
Muscle Tissue
(a) Skeletal muscle cells have
prominent striation and
nuclei on their periphery. (b)
Smooth muscle cells have a
single nucleus and no visible
striations. (c) Cardiac muscle
cells appear striated and have
a single nucleus. From top,
LM x 1600, LM x 1600, LM
x 1600. (Micrographs
provided by the Regents of
University of Michigan
Medical School © 2012)
Note:
| acl
ele, "a
—:
mess Openstax COLLEGE
—— .
.
Watch this video to learn more about muscle tissue. In looking through a
microscope how could you distinguish skeletal muscle tissue from smooth
muscle?
Chapter Review
The three types of muscle cells are skeletal, cardiac, and smooth. Their
morphologies match their specific functions in the body. Skeletal muscle is
voluntary and responds to conscious stimuli. The cells are striated and
multinucleated appearing as long, unbranched cylinders. Cardiac muscle is
involuntary and found only in the heart. Each cell is striated with a single
nucleus and they attach to one another to form long fibers. Cells are
attached to one another at intercalated disks. The cells are interconnected
physically and electrochemically to act as a syncytium. Cardiac muscle
cells contract autonomously and involuntarily. Smooth muscle is
involuntary. Each cell is a spindle-shaped fiber and contains a single
nucleus. No striations are evident because the actin and myosin filaments
do not align in the cytoplasm.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn more about muscle tissue. In looking through
a microscope how could you distinguish skeletal muscle tissue from
smooth muscle?
Solution:
Skeletal muscle cells are striated.
Review Questions
Exercise:
Problem:
Striations, cylindrical cells, and multiple nuclei are observed in
a. Skeletal muscle only
b. cardiac muscle only
c. smooth muscle only
d. skeletal and cardiac muscles
Solution:
A
Exercise:
Problem:The cells of muscles, myocytes, develop from
a. myoblasts
b. endoderm
c. fibrocytes
d. chondrocytes
Solution:
A
Exercise:
Problem:
Skeletal muscle is composed of very hard working cells. Which
organelles do you expect to find in abundance in skeletal muscle cell?
a. nuclei
b. striations
c. golgi bodies
d. mitochondria
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
You are watching cells in a dish spontaneously contract. They are all
contracting at different rates; some fast, some slow. After a while,
several cells link up and they begin contracting in synchrony. Discuss
what is going on and what type of cells you are looking at.
Solution:
The cells in the dish are cardiomyocytes, cardiac muscle cells. They
have an intrinsic ability to contract. When they link up, they form
intercalating discs that allow the cells to communicate with each other
and begin contracting in synchrony.
Exercise:
Problem: Why does skeletal muscle look striated?
Solution:
Under the light microscope, cells appear striated due to the
arrangement of the contractile proteins actin and myosin.
Glossary
cardiac muscle
heart muscle, under involuntary control, composed of striated cells that
attach to form fibers, each cell contains a single nucleus, contracts
autonomously
myocyte
muscle cells
skeletal muscle
usually attached to bone, under voluntary control, each cell is a fiber
that is multinucleated and striated
smooth muscle
under involuntary control, moves internal organs, cells contain a single
nucleus, are spindle-shaped, and do not appear striated; each cell is a
fiber
striation
alignment of parallel actin and myosin filaments which form a banded
pattern
Nervous Tissue Mediates Perception and Response
By the end of this section, you will be able to:
e Identify the classes of cells that make up nervous tissue
e Discuss how nervous tissue mediates perception and response
Nervous tissue is characterized as being excitable and capable of sending
and receiving electrochemical signals that provide the body with
information. Two main classes of cells make up nervous tissue: the neuron
and neuroglia ([link]). Neurons propagate information via electrochemical
impulses, called action potentials, which are biochemically linked to the
release of chemical signals. Neuroglia play an essential role in supporting
neurons and modulating their information propagation.
The Neuron
Contact with
other cells
a Nucleus
Microfibrils and
microtubules
Axon
Dendrites
The cell body of a neuron, also called the soma,
contains the nucleus and mitochondria. The dendrites
transfer the nerve impulse to the soma. The axon
carries the action potential away to another excitable
cell. LM x 1600. (Micrograph provided by the
Regents of University of Michigan Medical School ©
2012)
Note:
=
mss Openstax COLLEGE
Follow this link to learn more about nervous tissue. What are the main
parts of a nerve cell?
Neurons display distinctive morphology, well suited to their role as
conducting cells, with three main parts. The cell body includes most of the
cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell
body and appear as thin extensions. A long “tail,” the axon, extends from
the neuron body and can be wrapped in an insulating layer known as
myelin, which is formed by accessory cells. The synapse is the gap between
nerve cells, or between a nerve cell and its target, for example, a muscle or
a gland, across which the impulse is transmitted by chemical compounds
known as neurotransmitters. Neurons categorized as multipolar neurons
have several dendrites and a single prominent axon. Bipolar neurons
possess a single dendrite and axon with the cell body, while unipolar
neurons have only a single process extending out from the cell body, which
divides into a functional dendrite and into a functional axon. When a neuron
is sufficiently stimulated, it generates an action potential that propagates
down the axon towards the synapse. If enough neurotransmitters are
released at the synapse to stimulate the next neuron or target, a response is
generated.
The second class of neural cells comprises the neuroglia or glial cells,
which have been characterized as having a simple support role. The word
“glia” comes from the Greek word for glue. Recent research is shedding
light on the more complex role of neuroglia in the function of the brain and
nervous system. Astrocyte cells, named for their distinctive star shape, are
abundant in the central nervous system. The astrocytes have many
functions, including regulation of ion concentration in the intercellular
space, uptake and/or breakdown of some neurotransmitters, and formation
of the blood-brain barrier, the membrane that separates the circulatory
system from the brain. Microglia protect the nervous system against
infection but are not nervous tissue because they are related to
macrophages. Oligodendrocyte cells produce myelin in the central nervous
system (brain and spinal cord) while the Schwann cell produces myelin in
the peripheral nervous system ([link]).
Nervous Tissue
Neurons
Microglial cell Sa Astrocytes y
' “s
aK. OZ
Oligodendrocyte
Nervous tissue is made up of neurons and neuroglia. The cells
of nervous tissue are specialized to transmit and receive
impulses. LM x 872. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)
Chapter Review
The most prominent cell of the nervous tissue, the neuron, is characterized
mainly by its ability to receive stimuli and respond by generating an
electrical signal, known as an action potential, which can travel rapidly over
great distances in the body. A typical neuron displays a distinctive
morphology: a large cell body branches out into short extensions called
dendrites, which receive chemical signals from other neurons, and a long
tail called an axon, which relays signals away from the cell to other
neurons, muscles, or glands. Many axons are wrapped by a myelin sheath, a
lipid derivative that acts as an insulator and speeds up the transmission of
the action potential. Other cells in the nervous tissue, the neuroglia, include
the astrocytes, microglia, oligodendrocytes, and Schwann cells.
Interactive Link Questions
Exercise:
Problem:
Follow this link to learn more about nervous tissue. What are the main
parts of a nerve cell?
Solution:
Dendrites, cell body, and the axon.
Review Questions
Exercise:
Problem:
The cells responsible for the transmission of the nerve impulse are
a. Neurons
b. oligodendrocytes
c. astrocytes
d. microglia
Solution:
A
Exercise:
Problem:
The nerve impulse travels down a(n) , away from the cell
body.
a. dendrite
b. axon
c. microglia
d. collagen fiber
Solution:
B
Exercise:
Problem:
Which of the following central nervous system cells regulate ions,
regulate the uptake and/or breakdown of some neurotransmitters, and
contribute to the formation of the blood-brain barrier?
a. microglia
b. neuroglia
c. oligodendrocytes
d. astrocytes
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Which morphological adaptations of neurons make them suitable for
the transmission of nerve impulse?
Solution:
Neurons are well suited for the transmission of nerve impulses because
short extensions, dendrites, receive impulses from other neurons, while
a long tail extension, an axon, carries electrical impulses away from
the cell to other neurons.
Exercise:
Problem: What are the functions of astrocytes?
Solution:
Astrocytes regulate ions and uptake and/or breakdown of some
neurotransmitters and contribute to the formation of the blood-brain-
barrier.
References
Stern, P. Focus issue: getting excited about glia. Science [Internet]. 2010
[cited 2012 Dec 4]; 3(147):330-773. Available from:
http://stke.sciencemag,org/cgi/content/abstract/sigtrans;3/147/eg11
Ming GL, Song H. Adult neurogenesis in the mammalian central nervous
system. Annu. Rev. Neurosci. 2005 [cited 2012 Dec 4]; 28:223—250.
Glossary
astrocyte
star-shaped cell in the central nervous system that regulates ions and
uptake and/or breakdown of some neurotransmitters and contributes to
the formation of the blood-brain barrier
myelin
layer of lipid inside some neuroglial cells that wraps around the axons
of some neurons
neuroglia
supportive neural cells
neuron
excitable neural cell that transfer nerve impulses
oligodendrocyte
neuroglial cell that produces myelin in the brain
Schwann cell
neuroglial cell that produces myelin in the peripheral nervous system
Tissue Injury and Aging
By the end of this section, you will be able to:
e Identify the cardinal signs of inflammation
List the body’s response to tissue injury
e Explain the process of tissue repair
Discuss the progressive impact of aging on tissue
e Describe cancerous mutations’ effect on tissue
Tissues of all types are vulnerable to injury and, inevitably, aging. In the
former case, understanding how tissues respond to damage can guide
strategies to aid repair. In the latter case, understanding the impact of aging
can help in the search for ways to diminish its effects.
Tissue Injury and Repair
Inflammation is the standard, initial response of the body to injury.
Whether biological, chemical, physical, or radiation burns, all injuries lead
to the same sequence of physiological events. Inflammation limits the
extent of injury, partially or fully eliminates the cause of injury, and initiates
repair and regeneration of damaged tissue. Necrosis, or accidental cell
death, causes inflammation. Apoptosis is programmed cell death, a normal
step-by-step process that destroys cells no longer needed by the body. By
mechanisms still under investigation, apoptosis does not initiate the
inflammatory response. Acute inflammation resolves over time by the
healing of tissue. If inflammation persists, it becomes chronic and leads to
diseased conditions. Arthritis and tuberculosis are examples of chronic
inflammation. The suffix “-itis” denotes inflammation of a specific organ or
type, for example, peritonitis is the inflammation of the peritoneum, and
meningitis refers to the inflammation of the meninges, the tough
membranes that surround the central nervous system
The four cardinal signs of inflammation—redness, swelling, pain, and local
heat—were first recorded in antiquity. Cornelius Celsus is credited with
documenting these signs during the days of the Roman Empire, as early as
the first century AD. A fifth sign, loss of function, may also accompany
inflammation.
Upon tissue injury, damaged cells release inflammatory chemical signals
that evoke local vasodilation, the widening of the blood vessels. Increased
blood flow results in apparent redness and heat. In response to injury, mast
cells present in tissue degranulate, releasing the potent vasodilator
histamine. Increased blood flow and inflammatory mediators recruit white
blood cells to the site of inflammation. The endothelium lining the local
blood vessel becomes “leaky” under the influence of histamine and other
inflammatory mediators allowing neutrophils, macrophages, and fluid to
move from the blood into the interstitial tissue spaces. The excess liquid in
tissue causes swelling, more properly called edema. The swollen tissues
squeezing pain receptors cause the sensation of pain. Prostaglandins
released from injured cells also activate pain neurons. Non-steroidal anti-
inflammatory drugs (NSAIDs) reduce pain because they inhibit the
synthesis of prostaglandins. High levels of NSAIDs reduce inflammation.
Antihistamines decrease allergies by blocking histamine receptors and as a
result the histamine response.
After containment of an injury, the tissue repair phase starts with removal of
toxins and waste products. Clotting (coagulation) reduces blood loss from
damaged blood vessels and forms a network of fibrin proteins that trap
blood cells and bind the edges of the wound together. A scab forms when
the clot dries, reducing the risk of infection. Sometimes a mixture of dead
leukocytes and fluid called pus accumulates in the wound. As healing
progresses, fibroblasts from the surrounding connective tissues replace the
collagen and extracellular material lost by the injury. Angiogenesis, the
growth of new blood vessels, results in vascularization of the new tissue
known as granulation tissue. The clot retracts pulling the edges of the
wound together, and it slowly dissolves as the tissue is repaired. When a
large amount of granulation tissue forms and capillaries disappear, a pale
scar is often visible in the healed area. A primary union describes the
healing of a wound where the edges are close together. When there is a
gaping wound, it takes longer to refill the area with cells and collagen. The
process called secondary union occurs as the edges of the wound are
pulled together by what is called wound contraction. When a wound is
more than one quarter of an inch deep, sutures (stitches) are recommended
to promote a primary union and avoid the formation of a disfiguring scar.
Regeneration is the addition of new cells of the same type as the ones that
were injured ((link]).
Tissue Healing
Clotting occurs, caused by clotting proteins Epithelial cells multiply and fill Restored epthelium thickens; the
and plasma proteins, and a scab is formed in over the granulation tissue area matures and contracts
Inflammatory chemicals White blood cells seep Granulation tissue restores Underlying area
are released from injury _ into the injured area the vascular supply of scar tissue
During wound repair, collagen fibers are laid down
randomly by fibroblasts that move into repair the area.
Note:
Oa ec
openstax COLLEGE”
. 75
.
O)
a r
Watch this video to see a hand heal. Over what period of time do you think
these images were taken?
Tissue and Aging
According to poet Ralph Waldo Emerson, “The surest poison is time.” In
fact, biology confirms that many functions of the body decline with age. All
the cells, tissues, and organs are affected by senescence, with noticeable
variability between individuals owing to different genetic makeup and
lifestyles. The outward signs of aging are easily recognizable. The skin and
other tissues become thinner and drier, reducing their elasticity, contributing
to wrinkles and high blood pressure. Hair turns gray because follicles
produce less melanin, the brown pigment of hair and the iris of the eye. The
face looks flabby because elastic and collagen fibers decrease in connective
tissue and muscle tone is lost. Glasses and hearing aids may become parts
of life as the senses slowly deteriorate, all due to reduced elasticity. Overall
height decreases as the bones lose calcium and other minerals. With age,
fluid decreases in the fibrous cartilage disks intercalated between the
vertebrae in the spine. Joints lose cartilage and stiffen. Many tissues,
including those in muscles, lose mass through a process called atrophy.
Lumps and rigidity become more widespread. As a consequence, the
passageways, blood vessels, and airways become more rigid. The brain and
spinal cord lose mass. Nerves do not transmit impulses with the same speed
and frequency as in the past. Some loss of thought clarity and memory can
accompany aging. More severe problems are not necessarily associated
with the aging process and may be symptoms of underlying illness.
As exterior signs of aging increase, so do the interior signs, which are not as
noticeable. The incidence of heart diseases, respiratory syndromes, and type
2 diabetes increases with age, though these are not necessarily age-
dependent effects. Wound healing is slower in the elderly, accompanied by
a higher frequency of infection as the capacity of the immune system to
fend off pathogen declines.
Aging is also apparent at the cellular level because all cells experience
changes with aging. Telomeres, regions of the chromosomes necessary for
cell division, shorten each time cells divide. As they do, cells are less able
to divide and regenerate. Because of alterations in cell membranes,
transport of oxygen and nutrients into the cell and removal of carbon
dioxide and waste products from the cell are not as efficient in the elderly.
Cells may begin to function abnormally, which may lead to diseases
associated with aging, including arthritis, memory issues, and some cancers.
The progressive impact of aging on the body varies considerably among
individuals, but Studies indicate, however, that exercise and healthy
lifestyle choices can slow down the deterioration of the body that comes
with old age.
Note:
Homeostatic Imbalances
Tissues and Cancer
Cancer is a generic term for many diseases in which cells escape regulatory
signals. Uncontrolled growth, invasion into adjacent tissues, and
colonization of other organs, if not treated early enough, are its hallmarks.
Health suffers when tumors “rob” blood supply from the “normal” organs.
A mutation is defined as a permanent change in the DNA of a cell.
Epigenetic modifications, changes that do not affect the code of the DNA
but alter how the DNA is decoded, are also known to generate abnormal
cells. Alterations in the genetic material may be caused by environmental
agents, infectious agents, or errors in the replication of DNA that
accumulate with age. Many mutations do not cause any noticeable change
in the functions of a cell. However, if the modification affects key proteins
that have an impact on the cell’s ability to proliferate in an orderly fashion,
the cell starts to divide abnormally. As changes in cells accumulate, they
lose their ability to form regular tissues. A tumor, a mass of cells
displaying abnormal architecture, forms in the tissue. Many tumors are
benign, meaning they do not metastasize nor cause disease. A tumor
becomes malignant, or cancerous, when it breaches the confines of its
tissue, promotes angiogenesis, attracts the growth of capillaries, and
metastasizes to other organs ((link]). The specific names of cancers reflect
the tissue of origin. Cancers derived from epithelial cells are referred to as
carcinomas. Cancer in myeloid tissue or blood cells form myelomas.
Leukemias are cancers of white blood cells, whereas sarcomas derive from
connective tissue. Cells in tumors differ both in structure and function.
Some cells, called cancer stem cells, appear to be a subtype of cell
responsible for uncontrolled growth. Recent research shows that contrary
to what was previously assumed, tumors are not disorganized masses of
cells, but have their own structures.
Development of Cancer
Cell division
takes place
to replace
lost tissue
Cell division
accelerates
.—\—
WSS
SS
SW:
nN
Carcinoma
breaks into
underlying
tissue
Underlying
tissue
Note the change in cell size,
nucleus size, and organization
in the tissue.
Note:
Coker aa)
ae
—
wm, OPENSTAX COLLEGE
—
.
-
tO) hs
Watch this video to learn more about tumors. What is a tumor?
Cancer treatments vary depending on the disease’s type and stage.
Traditional approaches, including surgery, radiation, chemotherapy, and
hormonal therapy, aim to remove or kill rapidly dividing cancer cells, but
these strategies have their limitations. Depending on a tumor’s location, for
example, cancer surgeons may be unable to remove it. Radiation and
chemotherapy are difficult, and it is often impossible to target only the
cancer cells. The treatments inevitably destroy healthy tissue as well. To
address this, researchers are working on pharmaceuticals that can target
specific proteins implicated in cancer-associated molecular pathways.
Chapter Review
Inflammation is the classic response of the body to injury and follows a
common sequence of events. The area is red, feels warm to the touch,
swells, and is painful. Injured cells, mast cells, and resident macrophages
release chemical signals that cause vasodilation and fluid leakage in the
surrounding tissue. The repair phase includes blood clotting, followed by
regeneration of tissue as fibroblasts deposit collagen. Some tissues
regenerate more readily than others. Epithelial and connective tissues
replace damaged or dead cells from a supply of adult stem cells. Muscle
and nervous tissues undergo either slow regeneration or do not repair at all.
Age affects all the tissues and organs of the body. Damaged cells do not
regenerate as rapidly as in younger people. Perception of sensation and
effectiveness of response are lost in the nervous system. Muscles atrophy,
and bones lose mass and become brittle. Collagen decreases in some
connective tissue, and joints stiffen.
Interactive Link Questions
Exercise:
Problem:
Watch this video to see a hand heal. Over what period of time do you
think these images were taken?
Solution:
Approximately one month.
Exercise:
Problem:
Watch this video to learn more about tumors. What is a tumor?
Solution:
A mass of cancer cells that continue to grow and divide.
Review Questions
Exercise:
Problem:
Which of the following processes is not a cardinal sign of
inflammation?
a. redness
b. heat
c. fever
d. swelling
Solution:
C
Exercise:
Problem:
When a mast cell reacts to an irritation, which of the following
chemicals does it release?
a. collagen
b. histamine
c. hyaluronic acid
d. meylin
Solution:
B
Exercise:
Problem: Atrophy refers to
a. loss of elasticity
b. loss of mass
c. loss of rigidity
d. loss of permeability
Solution:
B
Exercise:
Problem:
Individuals can slow the rate of aging by modifying all of these
lifestyle aspects except for
a. diet
b. exercise
c. genetic factors
d. stress
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Why is it important to watch for increased redness, swelling and pain
after a cut or abrasion has been cleaned and bandaged?
Solution:
These symptoms would indicate that infection is present.
Exercise:
Problem:
Aspirin is a non-steroidal anti-inflammatory drug (NSAID) that
inhibits the formation of blood clots and is taken regularly by
individuals with a heart condition. Steroids such as cortisol are used to
control some autoimmune diseases and severe arthritis by down-
regulating the inflammatory response. After reading the role of
inflammation in the body’s response to infection, can you predict an
undesirable consequence of taking anti-inflammatory drugs on a
regular basis?
Solution:
Since NSAIDs or other anti-inflammatory drugs inhibit the formation
of blood clots, regular and prolonged use of these drugs may promote
internal bleeding, such as bleeding in the stomach. Excessive levels of
cortisol would suppress inflammation, which could slow the wound
healing process.
Exercise:
Problem:
As an individual ages, a constellation of symptoms begins the decline
to the point where an individual’s functioning is compromised.
Identify and discuss two factors that have a role in factors leading to
the compromised situation.
Solution:
The genetic makeup and the lifestyle of each individual are factors
which determine the degree of decline in cells, tissues, and organs as
an individual ages.
Exercise:
Problem: Discuss changes that occur in cells as a person ages.
Solution:
All cells experience changes with aging. They become larger, and
many cannot divide and regenerate. Because of alterations in cell
membranes, transport of oxygen and nutrients into the cell and
removal of carbon dioxide and waste products are not as efficient in
the elderly. Cells lose their ability to function, or they begin to function
abnormally, leading to disease and cancer.
References
Emerson, RW. Old age. Atlantic. 1862 [cited 2012 Dec 4]; 9(51):134—140.
Glossary
apoptosis
programmed cell death
atrophy
loss of mass and function
clotting
also called coagulation; complex process by which blood components
form a plug to stop bleeding
histamine
chemical compound released by mast cells in response to injury that
causes vasodilation and endothelium permeability
inflammation
response of tissue to injury
necrosis
accidental death of cells and tissues
primary union
condition of a wound where the wound edges are close enough to be
brought together and fastened if necessary, allowing quicker and more
thorough healing
secondary union
wound healing facilitated by wound contraction
vasodilation
widening of blood vessels
wound contraction
process whereby the borders of a wound are physically drawn together
Layers of the Skin
By the end of this section, you will be able to:
e Identify the components of the integumentary system
Describe the layers of the skin and the functions of each layer
e Identify and describe the hypodermis and deep fascia
Describe the role of keratinocytes and their life cycle
e Describe the role of melanocytes in skin pigmentation
Although you may not typically think of the skin as an organ, it is in fact
made of tissues that work together as a single structure to perform unique
and critical functions. The skin and its accessory structures make up the
integumentary system, which provides the body with overall protection.
The skin is made of multiple layers of cells and tissues, which are held to
underlying structures by connective tissue ({link]). The deeper layer of skin
is well vascularized (has numerous blood vessels). It also has numerous
sensory, and autonomic and sympathetic nerve fibers ensuring
communication to and from the brain.
Layers of Skin
Hair shaft
Pore of sweat gland duct
Epidermis
|
Arrector pili
muscle
Hair follicle
Sebaceous (oil)
gland
Hypodermis
Hair root
Hair follicle
receptor Eccrine sweat gland
Pacinian corpuscle
Cutaneous vascular
plexus
Adipose tissue
Sensory nerve fiber
The skin is composed of two main layers: the epidermis,
made of closely packed epithelial cells, and the dermis,
made of dense, irregular connective tissue that houses
blood vessels, hair follicles, sweat glands, and other
structures. Beneath the dermis lies the hypodermis,
which is composed mainly of loose connective and fatty
tissues.
ee
mess’ OPENStAX COLLEGE”
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io ae T
The skin consists of two main layers and a closely associated layer. View
this animation to learn more about layers of the skin. What are the basic
functions of each of these layers?
The Epidermis
The epidermis is composed of keratinized, stratified squamous epithelium.
It is made of four or five layers of epithelial cells, depending on its location
in the body. It does not have any blood vessels within it (i.e., it is
avascular). Skin that has four layers of cells is referred to as “thin skin.”
From deep to superficial, these layers are the stratum basale, stratum
spinosum, stratum granulosum, and stratum corneum. Most of the skin can
be classified as thin skin. “Thick skin” is found only on the palms of the
hands and the soles of the feet. It has a fifth layer, called the stratum
lucidum, located between the stratum corneum and the stratum granulosum
(Llink]).
Thin Skin versus Thick Skin
These slides show cross-
sections of the epidermis and
dermis of (a) thin and (b)
thick skin. Note the
significant difference in the
thickness of the epithelial
layer of the thick skin. From
top, LM x 40, LM ~x 40.
(Micrographs provided by the
Regents of University of
Michigan Medical School ©
2012)
The cells in all of the layers except the stratum basale are called
keratinocytes. A keratinocyte is a cell that manufactures and stores the
protein keratin. Keratin is an intracellular fibrous protein that gives hair,
nails, and skin their hardness and water-resistant properties. The
keratinocytes in the stratum corneum are dead and regularly slough away,
being replaced by cells from the deeper layers ({link]).
Epidermis
The epidermis is epithelium
composed of multiple layers
of cells. The basal layer
consists of cuboidal cells,
whereas the outer layers are
squamous, keratinized cells,
so the whole epithelium is
often described as being
keratinized stratified
squamous epithelium. LM x
40. (Micrograph provided by
the Regents of University of
Michigan Medical School ©
2012)
Note:
. .
openstax COLLEGE
View the University of Michigan WebScope to explore the tissue sample in
greater detail. If you zoom on the cells at the outermost layer of this
section of skin, what do you notice about the cells?
Stratum Basale
The stratum basale (also called the stratum germinativum) is the deepest
epidermal layer and attaches the epidermis to the basal lamina, below which
lie the layers of the dermis. The cells in the stratum basale bond to the
dermis via intertwining collagen fibers, referred to as the basement
membrane. A finger-like projection, or fold, known as the dermal papilla
(plural = dermal papillae) is found in the superficial portion of the dermis.
Dermal papillae increase the strength of the connection between the
epidermis and dermis; the greater the folding, the stronger the connections
made ([link]).
Layers of the Epidermis
Dead cells filled
with keratin
5 =
Stratum corneum =
eS =o SSSqNqBPHOO
Stratum lucidum ——_{_
Stratum granulosum
: ——_,_
Stratum spinosum =
Stratum basale — j
Melanocyte
Lamellar granules
Keratinocyte
Merkel cell
: Sensory neuron
Dermis
The epidermis of thick skin has five layers: stratum
basale, stratum spinosum, stratum granulosum, stratum
lucidum, and stratum corneum.
The stratum basale is a single layer of cells primarily made of basal cells. A
basal cell is a cuboidal-shaped stem cell that is a precursor of the
keratinocytes of the epidermis. All of the keratinocytes are produced from
this single layer of cells, which are constantly going through mitosis to
produce new cells. As new cells are formed, the existing cells are pushed
superficially away from the stratum basale. Two other cell types are found
dispersed among the basal cells in the stratum basale. The first is a Merkel
cell, which functions as a receptor and is responsible for stimulating
sensory nerves that the brain perceives as touch. These cells are especially
abundant on the surfaces of the hands and feet. The second is a melanocyte,
a cell that produces the pigment melanin. Melanin gives hair and skin its
color, and also helps protect the living cells of the epidermis from
ultraviolet (UV) radiation damage.
In a growing fetus, fingerprints form where the cells of the stratum basale
meet the papillae of the underlying dermal layer (papillary layer), resulting
in the formation of the ridges on your fingers that you recognize as
fingerprints. Fingerprints are unique to each individual and are used for
forensic analyses because the patterns do not change with the growth and
aging processes.
Stratum Spinosum
As the name suggests, the stratum spinosum is spiny in appearance due to
the protruding cell processes that join the cells via a structure called a
desmosome. The desmosomes interlock with each other and strengthen the
bond between the cells. It is interesting to note that the “spiny” nature of
this layer is an artifact of the staining process. Unstained epidermis samples
do not exhibit this characteristic appearance. The stratum spinosum is
composed of eight to 10 layers of keratinocytes, formed as a result of cell
division in the stratum basale ([link]). Interspersed among the keratinocytes
of this layer is a type of dendritic cell called the Langerhans cell, which
functions as a macrophage by engulfing bacteria, foreign particles, and
damaged cells that occur in this layer.
Cells of the Epidermis
065 - Epidermis_001.svs
WebScope 1
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Epiaermis
Stratum ,,
corneum i
Stratum ~
granulosum
Remnants of
cross-sectioned
Stratum shed hair and
spinosum its follicle
Stratum basal
or germinativum
Capillai
Dermi poy
EJM Mag. 2,700 X
The cells in the different layers of the epidermis originate
from basal cells located in the stratum basale, yet the
cells of each layer are distinctively different. EM x 2700.
(Micrograph provided by the Regents of University of
Michigan Medical School © 2012)
Note:
poe
openstax COLLEGE”
View the University of Michigan WebScope to explore the tissue sample in
greater detail. If you zoom on the cells at the outermost layer of this
section of skin, what do you notice about the cells?
The keratinocytes in the stratum spinosum begin the synthesis of keratin
and release a water-repelling glycolipid that helps prevent water loss from
the body, making the skin relatively waterproof. As new keratinocytes are
produced atop the stratum basale, the keratinocytes of the stratum spinosum
are pushed into the stratum granulosum.
Stratum Granulosum
The stratum granulosum has a grainy appearance due to further changes
to the keratinocytes as they are pushed from the stratum spinosum. The
cells (three to five layers deep) become flatter, their cell membranes
thicken, and they generate large amounts of the proteins keratin, which is
fibrous, and keratohyalin, which accumulates as lamellar granules within
the cells (see [link]). These two proteins make up the bulk of the
keratinocyte mass in the stratum granulosum and give the layer its grainy
appearance. The nuclei and other cell organelles disintegrate as the cells
die, leaving behind the keratin, keratohyalin, and cell membranes that will
form the stratum lucidum, the stratum comeum, and the accessory
structures of hair and nails.
Stratum Lucidum
The stratum lucidum is a smooth, seemingly translucent layer of the
epidermis located just above the stratum granulosum and below the stratum
corneum. This thin layer of cells is found only in the thick skin of the
palms, soles, and digits. The keratinocytes that compose the stratum
lucidum are dead and flattened (see [link]). These cells are densely packed
with eleiden, a clear protein rich in lipids, derived from keratohyalin, which
gives these cells their transparent (i.e., lucid) appearance and provides a
barrier to water.
Stratum Corneum
The stratum corneum is the most superficial layer of the epidermis and is
the layer exposed to the outside environment (see [link]). The increased
keratinization (also called cornification) of the cells in this layer gives it its
name. There are usually 15 to 30 layers of cells in the stratum corneum.
This dry, dead layer helps prevent the penetration of microbes and the
dehydration of underlying tissues, and provides a mechanical protection
against abrasion for the more delicate, underlying layers. Cells in this layer
are shed periodically and are replaced by cells pushed up from the stratum
granulosum (or stratum lucidum in the case of the palms and soles of feet).
The entire layer is replaced during a period of about 4 weeks. Cosmetic
procedures, such as microdermabrasion, help remove some of the dry, upper
layer and aim to keep the skin looking “fresh” and healthy.
Dermis
The dermis might be considered the “core” of the integumentary system
(derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over”
and hypodermis (hypo- = “below”). It contains blood and lymph vessels,
nerves, and other structures, such as hair follicles and sweat glands. The
dermis is made of two layers of connective tissue that compose an
interconnected mesh of elastin and collagenous fibers, produced by
fibroblasts ({link]).
Layers of the Dermis
This stained slide shows the two
components of the dermis—the
papillary layer and the reticular
layer. Both are made of connective
tissue with fibers of collagen
extending from one to the other,
making the border between the
two somewhat indistinct. The
dermal papillae extending into the
epidermis belong to the papillary
layer, whereas the dense collagen
fiber bundles below belong to the
reticular layer. LM x 10. (credit:
modification of work by
“kilbad”/Wikimedia Commons)
Papillary Layer
The papillary layer is made of loose, areolar connective tissue, which
means the collagen and elastin fibers of this layer form a loose mesh. This
superficial layer of the dermis projects into the stratum basale of the
epidermis to form finger-like dermal papillae (see [link]). Within the
papillary layer are fibroblasts, a small number of fat cells (adipocytes), and
an abundance of small blood vessels. In addition, the papillary layer
contains phagocytes, defensive cells that help fight bacteria or other
infections that have breached the skin. This layer also contains lymphatic
capillaries, nerve fibers, and touch receptors called the Meissner corpuscles.
Reticular Layer
Underlying the papillary layer is the much thicker reticular layer,
composed of dense, irregular connective tissue. This layer is well
vascularized and has a rich sensory and sympathetic nerve supply. The
reticular layer appears reticulated (net-like) due to a tight meshwork of
fibers. Elastin fibers provide some elasticity to the skin, enabling
movement. Collagen fibers provide structure and tensile strength, with
strands of collagen extending into both the papillary layer and the
hypodermis. In addition, collagen binds water to keep the skin hydrated.
Collagen injections and Retin-A creams help restore skin turgor by either
introducing collagen externally or stimulating blood flow and repair of the
dermis, respectively.
Hypodermis
The hypodermis (also called the subcutaneous layer or superficial fascia) is
a layer directly below the dermis and serves to connect the skin to the
underlying fascia (fibrous tissue) of the bones and muscles. It is not strictly
a part of the skin, although the border between the hypodermis and dermis
can be difficult to distinguish. The hypodermis consists of well-
vascularized, loose, areolar connective tissue and adipose tissue, which
functions as a mode of fat storage and provides insulation and cushioning
for the integument.
Note:
Everyday Connection
Lipid Storage
The hypodermis is home to most of the fat that concerns people when they
are trying to keep their weight under control. Adipose tissue present in the
hypodermis consists of fat-storing cells called adipocytes. This stored fat
can serve as an energy reserve, insulate the body to prevent heat loss, and
act as a cushion to protect underlying structures from trauma.
Where the fat is deposited and accumulates within the hypodermis depends
on hormones (testosterone, estrogen, insulin, glucagon, leptin, and others),
as well as genetic factors. Fat distribution changes as our bodies mature
and age. Men tend to accumulate fat in different areas (neck, arms, lower
back, and abdomen) than do women (breasts, hips, thighs, and buttocks).
The body mass index (BMI) is often used as a measure of fat, although this
measure is, in fact, derived from a mathematical formula that compares
body weight (mass) to height. Therefore, its accuracy as a health indicator
can be called into question in individuals who are extremely physically fit.
In many animals, there is a pattern of storing excess calories as fat to be
used in times when food is not readily available. In much of the developed
world, insufficient exercise coupled with the ready availability and
consumption of high-calorie foods have resulted in unwanted
accumulations of adipose tissue in many people. Although periodic
accumulation of excess fat may have provided an evolutionary advantage
to our ancestors, who experienced unpredictable bouts of famine, it is now
becoming chronic and considered a major health threat. Recent studies
indicate that a distressing percentage of our population is overweight
and/or clinically obese. Not only is this a problem for the individuals
affected, but it also has a severe impact on our healthcare system. Changes
in lifestyle, specifically in diet and exercise, are the best ways to control
body fat accumulation, especially when it reaches levels that increase the
risk of heart disease and diabetes.
Pigmentation
The color of skin is influenced by a number of pigments, including melanin,
carotene, and hemoglobin. Recall that melanin is produced by cells called
melanocytes, which are found scattered throughout the stratum basale of the
epidermis. The melanin is transferred into the keratinocytes via a cellular
vesicle called a melanosome ((link]).
Skin Pigmentation
Surface
Upper
keratinocytes
Melanosomes
Basal
keratinocytes
Melanocytes
The relative coloration of the skin depends of the amount of
melanin produced by melanocytes in the stratum basale and
taken up by keratinocytes.
Melanin occurs in two primary forms. Eumelanin exists as black and
brown, whereas pheomelanin provides a red color. Dark-skinned
individuals produce more melanin than those with pale skin. Exposure to
the UV rays of the sun or a tanning salon causes melanin to be
manufactured and built up in keratinocytes, as sun exposure stimulates
keratinocytes to secrete chemicals that stimulate melanocytes. The
accumulation of melanin in keratinocytes results in the darkening of the
skin, or a tan. This increased melanin accumulation protects the DNA of
epidermal cells from UV ray damage and the breakdown of folic acid, a
nutrient necessary for our health and well-being. In contrast, too much
melanin can interfere with the production of vitamin D, an important
nutrient involved in calcium absorption. Thus, the amount of melanin
present in our skin is dependent on a balance between available sunlight
and folic acid destruction, and protection from UV radiation and vitamin D
production.
It requires about 10 days after initial sun exposure for melanin synthesis to
peak, which is why pale-skinned individuals tend to suffer sunburns of the
epidermis initially. Dark-skinned individuals can also get sunburns, but are
more protected than are pale-skinned individuals. Melanosomes are
temporary structures that are eventually destroyed by fusion with
lysosomes; this fact, along with melanin-filled keratinocytes in the stratum
corneum sloughing off, makes tanning impermanent.
Too much sun exposure can eventually lead to wrinkling due to the
destruction of the cellular structure of the skin, and in severe cases, can
cause sufficient DNA damage to result in skin cancer. When there is an
irregular accumulation of melanocytes in the skin, freckles appear. Moles
are larger masses of melanocytes, and although most are benign, they
should be monitored for changes that might indicate the presence of cancer
({link]).
Moles
Moles range from benign accumulations
of melanocytes to melanomas. These
structures populate the landscape of our
skin. (credit: the National Cancer
Institute)
Note:
Disorders of the...
Integumentary System
The first thing a clinician sees is the skin, and so the examination of the
skin should be part of any thorough physical examination. Most skin
disorders are relatively benign, but a few, including melanomas, can be
fatal if untreated. A couple of the more noticeable disorders, albinism and
vitiligo, affect the appearance of the skin and its accessory organs.
Although neither is fatal, it would be hard to claim that they are benign, at
least to the individuals so afflicted.
Albinism is a genetic disorder that affects (completely or partially) the
coloring of skin, hair, and eyes. The defect is primarily due to the inability
of melanocytes to produce melanin. Individuals with albinism tend to
appear white or very pale due to the lack of melanin in their skin and hair.
Recall that melanin helps protect the skin from the harmful effects of UV
radiation. Individuals with albinism tend to need more protection from UV
radiation, as they are more prone to sunburns and skin cancer. They also
tend to be more sensitive to light and have vision problems due to the lack
of pigmentation on the retinal wall. Treatment of this disorder usually
involves addressing the symptoms, such as limiting UV light exposure to
the skin and eyes. In vitiligo, the melanocytes in certain areas lose their
ability to produce melanin, possibly due to an autoimmune reaction. This
leads to a loss of color in patches ({link]). Neither albinism nor vitiligo
directly affects the lifespan of an individual.
Vitiligo
Individuals with
vitiligo experience
depigmentation that
results in lighter
colored patches of skin.
The condition is
especially noticeable
on darker skin. (credit:
Klaus D. Peter)
Other changes in the appearance of skin coloration can be indicative of
diseases associated with other body systems. Liver disease or liver cancer
can cause the accumulation of bile and the yellow pigment bilirubin,
leading to the skin appearing yellow or jaundiced (jaune is the French
word for “yellow”). Tumors of the pituitary gland can result in the
secretion of large amounts of melanocyte-stimulating hormone (MSH),
which results in a darkening of the skin. Similarly, Addison’s disease can
stimulate the release of excess amounts of adrenocorticotropic hormone
(ACTH), which can give the skin a deep bronze color. A sudden drop in
oxygenation can affect skin color, causing the skin to initially turn ashen
(white). With a prolonged reduction in oxygen levels, dark red
deoxyhemoglobin becomes dominant in the blood, making the skin appear
blue, a condition referred to as cyanosis (kyanos is the Greek word for
“blue”). This happens when the oxygen supply is restricted, as when
someone is experiencing difficulty in breathing because of asthma or a
heart attack. However, in these cases the effect on skin color has nothing
do with the skin’s pigmentation.
mess’ OPENStAX COLLEGE”
seh : T
This ABC video follows the story of a pair of fraternal African-American
twins, one of whom is albino. Watch this video to learn about the
challenges these children and their family face. Which ethnicities do you
think are exempt from the possibility of albinism?
Chapter Review
The skin is composed of two major layers: a superficial epidermis and a
deeper dermis. The epidermis consists of several layers beginning with the
innermost (deepest) stratum basale (germinatum), followed by the stratum
spinosum, stratum granulosum, stratum lucidum (when present), and ending
with the outermost layer, the stratum corneum. The topmost layer, the
stratum corneum, consists of dead cells that shed periodically and is
progressively replaced by cells formed from the basal layer. The stratum
basale also contains melanocytes, cells that produce melanin, the pigment
primarily responsible for giving skin its color. Melanin is transferred to
keratinocytes in the stratum spinosum to protect cells from UV rays.
The dermis connects the epidermis to the hypodermis, and provides strength
and elasticity due to the presence of collagen and elastin fibers. It has only
two layers: the papillary layer with papillae that extend into the epidermis
and the lower, reticular layer composed of loose connective tissue. The
hypodermis, deep to the dermis of skin, is the connective tissue that
connects the dermis to underlying structures; it also harbors adipose tissue
for fat storage and protection.
Interactive Link Questions
Exercise:
Problem:
The skin consists of two layers and a closely associated layer. View
this animation to learn more about layers of the skin. What are the
basic functions of each of these layers?
Solution:
The epidermis provides protection, the dermis provides support and
flexibility, and the hypodermis (fat layer) provides insulation and
padding.
Exercise:
Problem:
[link] If you zoom on the cells at the outermost layer of this section of
skin, what do you notice about the cells?
Solution:
[link] These cells do not have nuclei, so you can deduce that they are
dead. They appear to be sloughing off.
Exercise:
Problem:
[link] If you zoom on the cells of the stratum spinosum, what is
distinctive about them?
Solution:
[link] These cells have desmosomes, which give the cells their spiny
appearance.
Exercise:
Problem:
This ABC video follows the story of a pair of fraternal African-
American twins, one of whom is albino. Watch this video to learn
about the challenges these children and their family face. Which
ethnicities do you think are exempt from the possibility of albinism?
Solution:
There are none.
Review Questions
Exercise:
Problem:
The papillary layer of the dermis is most closely associated with which
layer of the epidermis?
a. stratum spinosum
b. stratum corneum
c. stratum granulosum
d. stratum basale
Solution:
D
Exercise:
Problem:Langerhans cells are commonly found in the
a. stratum spinosum
b. stratum corneum
c. stratum granulosum
d. stratum basale
Solution:
A
Exercise:
Problem:
The papillary and reticular layers of the dermis are composed mainly
of
a. melanocytes
b. keratinocytes
c. connective tissue
d. adipose tissue
Solution:
C
Exercise:
Problem: Collagen lends to the skin.
a. elasticity
b. structure
c. color
d. UV protection
Solution:
B
Exercise:
Problem: Which of the following is not a function of the hypodermis?
a. protects underlying organs
b. helps maintain body temperature
c. source of blood vessels in the epidermis
d. a site to long-term energy storage
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
What determines the color of skin, and what is the process that darkens
skin when it is exposed to UV light?
Solution:
The pigment melanin, produced by melanocytes, is primarily
responsible for skin color. Melanin comes in different shades of brown
and black. Individuals with darker skin have darker, more abundant
melanin, whereas fair-skinned individuals have a lighter shade of skin
and less melanin. Exposure to UV irradiation stimulates the
melanocytes to produce and secrete more melanin.
Exercise:
Problem:
Cells of the epidermis derive from stem cells of the stratum basale.
Describe how the cells change as they become integrated into the
different layers of the epidermis.
Solution:
As the cells move into the stratum spinosum, they begin the synthesis
of keratin and extend cell processes, desmosomes, which link the cells.
As the stratum basale continues to produce new cells, the keratinocytes
of the stratum spinosum are pushed into the stratum granulosum. The
cells become flatter, their cell membranes thicken, and they generate
large amounts of the proteins keratin and keratohyalin. The nuclei and
other cell organelles disintegrate as the cells die, leaving behind the
keratin, keratohyalin, and cell membranes that form the stratum
lucidum and the stratum corneum. The keratinocytes in these layers are
mostly dead and flattened. Cells in the stratum corneum are
periodically shed.
Glossary
albinism
genetic disorder that affects the skin, in which there is no melanin
production
basal cell
type of stem cell found in the stratum basale and in the hair matrix that
continually undergoes cell division, producing the keratinocytes of the
epidermis
dermal papilla
(plural = dermal papillae) extension of the papillary layer of the dermis
that increases surface contact between the epidermis and dermis
dermis
layer of skin between the epidermis and hypodermis, composed mainly
of connective tissue and containing blood vessels, hair follicles, sweat
glands, and other structures
desmosome
structure that forms an impermeable junction between cells
elastin fibers
fibers made of the protein elastin that increase the elasticity of the
dermis
eleiden
clear protein-bound lipid found in the stratum lucidum that is derived
from keratohyalin and helps to prevent water loss
epidermis
outermost tissue layer of the skin
hypodermis
connective tissue connecting the integument to the underlying bone
and muscle
integumentary system
skin and its accessory structures
keratin
type of structural protein that gives skin, hair, and nails its hard, water-
resistant properties
keratinocyte
cell that produces keratin and is the most predominant type of cell
found in the epidermis
keratohyalin
granulated protein found in the stratum granulosum
Langerhans cell
specialized dendritic cell found in the stratum spinosum that functions
as a macrophage
melanin
pigment that determines the color of hair and skin
melanocyte
cell found in the stratum basale of the epidermis that produces the
pigment melanin
melanosome
intercellular vesicle that transfers melanin from melanocytes into
keratinocytes of the epidermis
Merkel cell
receptor cell in the stratum basale of the epidermis that responds to the
sense of touch
papillary layer
superficial layer of the dermis, made of loose, areolar connective tissue
reticular layer
deeper layer of the dermis; it has a reticulated appearance due to the
presence of abundant collagen and elastin fibers
stratum basale
deepest layer of the epidermis, made of epidermal stem cells
stratum corneum
most superficial layer of the epidermis
stratum granulosum
layer of the epidermis superficial to the stratum spinosum
stratum lucidum
layer of the epidermis between the stratum granulosum and stratum
corneum, found only in thick skin covering the palms, soles of the feet,
and digits
stratum spinosum
layer of the epidermis superficial to the stratum basale, characterized
by the presence of desmosomes
vitiligo
skin condition in which melanocytes in certain areas lose the ability to
produce melanin, possibly due an autoimmune reaction that leads to
loss of color in patches
Accessory Structures of the Skin
By the end of this section, you will be able to:
e Identify the accessory structures of the skin
e Describe the structure and function of hair and nails
e Describe the structure and function of sweat glands and sebaceous
glands
Accessory structures of the skin include hair, nails, sweat glands, and
sebaceous glands. These structures embryologically originate from the
epidermis and can extend down through the dermis into the hypodermis.
Hair
Hair is a keratinous filament growing out of the epidermis. It is primarily
made of dead, keratinized cells. Strands of hair originate in an epidermal
penetration of the dermis called the hair follicle. The hair shaft is the part
of the hair not anchored to the follicle, and much of this is exposed at the
skin’s surface. The rest of the hair, which is anchored in the follicle, lies
below the surface of the skin and is referred to as the hair root. The hair
root ends deep in the dermis at the hair bulb, and includes a layer of
mitotically active basal cells called the hair matrix. The hair bulb
surrounds the hair papilla, which is made of connective tissue and contains
blood capillaries and nerve endings from the dermis ([link]).
Hair
Medulla
Cortex
Cuticle
Sebaceous
gland
Inner root
sheath
Outer root
sheath
Hair matrix
Hair
papilla
Hair follicles originate in the
epidermis and have many
different parts.
Just as the basal layer of the epidermis forms the layers of epidermis that
get pushed to the surface as the dead skin on the surface sheds, the basal
cells of the hair bulb divide and push cells outward in the hair root and shaft
as the hair grows. The medulla forms the central core of the hair, which is
surrounded by the cortex, a layer of compressed, keratinized cells that is
covered by an outer layer of very hard, keratinized cells known as the
cuticle. These layers are depicted in a longitudinal cross-section of the hair
follicle ([link]), although not all hair has a medullary layer. Hair texture
(straight, curly) is determined by the shape and structure of the cortex, and
to the extent that it is present, the medulla. The shape and structure of these
layers are, in turn, determined by the shape of the hair follicle. Hair growth
begins with the production of keratinocytes by the basal cells of the hair
bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed
through the follicle toward the surface. Keratinization is completed as the
cells are pushed to the skin surface to form the shaft of hair that is
externally visible. The external hair is completely dead and composed
entirely of keratin. For this reason, our hair does not have sensation.
Furthermore, you can cut your hair or shave without damaging the hair
structure because the cut is superficial. Most chemical hair removers also
act superficially; however, electrolysis and yanking both attempt to destroy
the hair bulb so hair cannot grow.
Hair Follicle
The slide shows a cross-section of a hair
follicle. Basal cells of the hair matrix in
the center differentiate into cells of the
inner root sheath. Basal cells at the base
of the hair root form the outer root sheath.
LM x 4. (credit: modification of work by
“kilbad”/Wikimedia Commons)
The wall of the hair follicle is made of three concentric layers of cells. The
cells of the internal root sheath surround the root of the growing hair and
extend just up to the hair shaft. They are derived from the basal cells of the
hair matrix. The external root sheath, which is an extension of the
epidermis, encloses the hair root. It is made of basal cells at the base of the
hair root and tends to be more keratinous in the upper regions. The glassy
membrane is a thick, clear connective tissue sheath covering the hair root,
connecting it to the tissue of the dermis.
Note:
Dasara
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—— .
The hair follicle is made of multiple layers of cells that form from basal
cells in the hair matrix and the hair root. Cells of the hair matrix divide and
differentiate to form the layers of the hair. Watch this video to learn more
about hair follicles.
Hair serves a variety of functions, including protection, sensory input,
thermoregulation, and communication. For example, hair on the head
protects the skull from the sun. The hair in the nose and ears, and around
the eyes (eyelashes) defends the body by trapping and excluding dust
particles that may contain allergens and microbes. Hair of the eyebrows
prevents sweat and other particles from dripping into and bothering the
eyes. Hair also has a sensory function due to sensory innervation by a hair
root plexus surrounding the base of each hair follicle. Hair is extremely
sensitive to air movement or other disturbances in the environment, much
more so than the skin surface. This feature is also useful for the detection of
the presence of insects or other potentially damaging substances on the skin
surface. Each hair root is connected to a smooth muscle called the arrector
pili that contracts in response to nerve signals from the sympathetic nervous
system, making the external hair shaft “stand up.” The primary purpose for
this is to trap a layer of air to add insulation. This is visible in humans as
goose bumps and even more obvious in animals, such as when a frightened
cat raises its fur. Of course, this is much more obvious in organisms with a
heavier coat than most humans, such as dogs and cats.
Hair Growth
Hair grows and is eventually shed and replaced by new hair. This occurs in
three phases. The first is the anagen phase, during which cells divide
rapidly at the root of the hair, pushing the hair shaft up and out. The length
of this phase is measured in years, typically from 2 to 7 years. The catagen
phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle’s
active growth. Finally, during the telogen phase, the hair follicle is at rest
and no new growth occurs. At the end of this phase, which lasts about 2 to 4
months, another anagen phase begins. The basal cells in the hair matrix then
produce a new hair follicle, which pushes the old hair out as the growth
cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day
during the anagen phase. On average, 50 hairs are lost and replaced per day.
Hair loss occurs if there is more hair shed than what is replaced and can
happen due to hormonal or dietary changes. Hair loss can also result from
the aging process, or the influence of hormones.
Hair Color
Similar to the skin, hair gets its color from the pigment melanin, produced
by melanocytes in the hair papilla. Different hair color results from
differences in the type of melanin, which is genetically determined. As a
person ages, the melanin production decreases, and hair tends to lose its
color and becomes gray and/or white.
Nails
The nail bed is a specialized structure of the epidermis that is found at the
tips of our fingers and toes. The nail body is formed on the nail bed, and
protects the tips of our fingers and toes as they are the farthest extremities
and the parts of the body that experience the maximum mechanical stress
({link]). In addition, the nail body forms a back-support for picking up
small objects with the fingers. The nail body is composed of densely packed
dead keratinocytes. The epidermis in this part of the body has evolved a
specialized structure upon which nails can form. The nail body forms at the
nail root, which has a matrix of proliferating cells from the stratum basale
that enables the nail to grow continuously. The lateral nail fold overlaps the
nail on the sides, helping to anchor the nail body. The nail fold that meets
the proximal end of the nail body forms the nail cuticle, also called the
eponychium. The nail bed is rich in blood vessels, making it appear pink,
except at the base, where a thick layer of epithelium over the nail matrix
forms a crescent-shaped region called the lunula (the “little moon”). The
area beneath the free edge of the nail, furthest from the cuticle, is called the
hyponychium. It consists of a thickened layer of stratum corneum.
Nails
Free edge Eponychium
a ee Proximal nail fold
. Lunula
Nail /
Nail body
Lateral nail fold
Lunula
Eponychium
Proximal nail fold
Epidermis Dermis Phalanx Hyponychium
The nail is an accessory structure of the integumentary
system.
Note:
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Nails are accessory structures of the integumentary system. Visit this link
to learn more about the origin and growth of fingernails.
Sweat Glands
When the body becomes warm, sudoriferous glands produce sweat to cool
the body. Sweat glands develop from epidermal projections into the dermis
and are classified as merocrine glands; that is, the secretions are excreted by
exocytosis through a duct without affecting the cells of the gland. There are
two types of sweat glands, each secreting slightly different products.
An eccrine sweat gland is type of gland that produces a hypotonic sweat
for thermoregulation. These glands are found all over the skin’s surface, but
are especially abundant on the palms of the hand, the soles of the feet, and
the forehead ([link]). They are coiled glands lying deep in the dermis, with
the duct rising up to a pore on the skin surface, where the sweat is released.
This type of sweat, released by exocytosis, is hypotonic and composed
mostly of water, with some salt, antibodies, traces of metabolic waste, and
dermicidin, an antimicrobial peptide. Eccrine glands are a primary
component of thermoregulation in humans and thus help to maintain
homeostasis.
Eccrine Gland
Skin surface
Eccrine sweat
gland
Eccrine glands are coiled glands in the
dermis that release sweat that is
mostly water.
An apocrine sweat gland is usually associated with hair follicles in densely
hairy areas, such as armpits and genital regions. Apocrine sweat glands are
larger than eccrine sweat glands and lie deeper in the dermis, sometimes
even reaching the hypodermis, with the duct normally emptying into the
hair follicle. In addition to water and salts, apocrine sweat includes organic
compounds that make the sweat thicker and subject to bacterial
decomposition and subsequent smell. The release of this sweat is under
both nervous and hormonal control, and plays a role in the poorly
understood human pheromone response. Most commercial antiperspirants
use an aluminum-based compound as their primary active ingredient to stop
sweat. When the antiperspirant enters the sweat gland duct, the aluminum-
based compounds precipitate due to a change in pH and form a physical
block in the duct, which prevents sweat from coming out of the pore.
— aa COLLEGE”
Sweating regulates body temperature. The composition of the sweat
determines whether body odor is a byproduct of sweating. Visit this link to
learn more about sweating and body odor.
Sebaceous Glands
A sebaceous gland is a type of oil gland that is found all over the body and
helps to lubricate and waterproof the skin and hair. Most sebaceous glands
are associated with hair follicles. They generate and excrete sebum, a
mixture of lipids, onto the skin surface, thereby naturally lubricating the dry
and dead layer of keratinized cells of the stratum corneum, keeping it
pliable. The fatty acids of sebum also have antibacterial properties, and
prevent water loss from the skin in low-humidity environments. The
secretion of sebum is stimulated by hormones, many of which do not
become active until puberty. Thus, sebaceous glands are relatively inactive
during childhood.
Chapter Review
Accessory structures of the skin include hair, nails, sweat glands, and
sebaceous glands. Hair is made of dead keratinized cells, and gets its color
from melanin pigments. Nails, also made of dead keratinized cells, protect
the extremities of our fingers and toes from mechanical damage. Sweat
glands and sebaceous glands produce sweat and sebum, respectively. Each
of these fluids has a role to play in maintaining homeostasis. Sweat cools
the body surface when it gets overheated and helps excrete small amounts
of metabolic waste. Sebum acts as a natural moisturizer and keeps the dead,
flaky, outer keratin layer healthy.
Review Questions
Exercise:
Problem:
In response to stimuli from the sympathetic nervous system, the
arrector pili
a. are glands on the skin surface
b. can lead to excessive sweating
c. are responsible for goose bumps
d. secrete sebum
Solution:
C
Exercise:
Problem:The hair matrix contains
a. the hair follicle
b. the hair shaft
c. the glassy membrane
d. a layer of basal cells
Solution:
D
Exercise:
Problem:Eccrine sweat glands
a. are present on hair
b. are present in the skin throughout the body and produce watery
Sweat
c. produce sebum
d. act as a moisturizer
Solution:
B
Exercise:
Problem: Sebaceous glands
a. are a type of sweat gland
b. are associated with hair follicles
c. may function in response to touch
d. release a watery solution of salt and metabolic waste
Solution:
B
Exercise:
Problem:
Similar to the hair, nails grow continuously throughout our lives.
Which of the following is furthest from the nail growth center?
a. nail bed
b. hyponychium
c. nail root
d. eponychium
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Explain the differences between eccrine and apocrine sweat glands.
Solution:
Eccrine sweat glands are all over the body, especially the forehead and
palms of the hand. They release a watery sweat, mixed with some
metabolic waste and antibodies. Apocrine glands are associated with
hair follicles. They are larger than eccrine sweat glands and lie deeper
in the dermis, sometimes even reaching the hypodermis. They release
a thicker sweat that is often decomposed by bacteria on the skin,
resulting in an unpleasant odor.
Exercise:
Problem: Describe the structure and composition of nails.
Solution:
Nails are composed of densely packed dead keratinocytes. They
protect the fingers and toes from mechanical stress. The nail body is
formed on the nail bed, which is at the nail root. Nail folds, folds of
skin that overlap the nail on its side, secure the nail to the body. The
crescent-shaped region at the base of the nail is the lunula.
Glossary
anagen
active phase of the hair growth cycle
apocrine sweat gland
type of sweat gland that is associated with hair follicles in the armpits
and genital regions
arrector pili
smooth muscle that is activated in response to external stimuli that pull
on hair follicles and make the hair “stand up”
catagen
transitional phase marking the end of the anagen phase of the hair
growth cycle
cortex
in hair, the second or middle layer of keratinocytes originating from
the hair matrix, as seen in a cross-section of the hair bulb
cuticle
in hair, the outermost layer of keratinocytes originating from the hair
matrix, as seen in a cross-section of the hair bulb
eccrine sweat gland
type of sweat gland that is common throughout the skin surface; it
produces a hypotonic sweat for thermoregulation
eponychium
nail fold that meets the proximal end of the nail body, also called the
cuticle
external root sheath
outer layer of the hair follicle that is an extension of the epidermis,
which encloses the hair root
glassy membrane
layer of connective tissue that surrounds the base of the hair follicle,
connecting it to the dermis
hair
keratinous filament growing out of the epidermis
hair bulb
structure at the base of the hair root that surrounds the dermal papilla
hair follicle
cavity or sac from which hair originates
hair matrix
layer of basal cells from which a strand of hair grows
hair papilla
mass of connective tissue, blood capillaries, and nerve endings at the
base of the hair follicle
hair root
part of hair that is below the epidermis anchored to the follicle
hair shaft
part of hair that is above the epidermis but is not anchored to the
follicle
hyponychium
thickened layer of stratum corneum that lies below the free edge of the
nail
internal root sheath
innermost layer of keratinocytes in the hair follicle that surround the
hair root up to the hair shaft
lunula
basal part of the nail body that consists of a crescent-shaped layer of
thick epithelium
medulla
in hair, the innermost layer of keratinocytes originating from the hair
matrix
nail bed
layer of epidermis upon which the nail body forms
nail body
main keratinous plate that forms the nail
nail cuticle
fold of epithelium that extends over the nail bed, also called the
eponychium
nail fold
fold of epithelium at that extend over the sides of the nail body,
holding it in place
nail root
part of the nail that is lodged deep in the epidermis from which the nail
gTOWS
sebaceous gland
type of oil gland found in the dermis all over the body and helps to
lubricate and waterproof the skin and hair by secreting sebum
sebum
oily substance that is composed of a mixture of lipids that lubricates
the skin and hair
sudoriferous gland
sweat gland
telogen
resting phase of the hair growth cycle initiated with catagen and
terminated by the beginning of a new anagen phase of hair growth
Diseases, Disorders, and Injuries of the Integumentary System
By the end of this section, you will be able to:
e Describe several different diseases and disorders of the skin
e Describe the effect of injury to the skin and the process of healing
The integumentary system is susceptible to a variety of diseases, disorders,
and injuries. These range from annoying but relatively benign bacterial or
fungal infections that are categorized as disorders, to skin cancer and severe
burns, which can be fatal. In this section, you will learn several of the most
common skin conditions.
Diseases
One of the most talked about diseases is skin cancer. Cancer is a broad term
that describes diseases caused by abnormal cells in the body dividing
uncontrollably. Most cancers are identified by the organ or tissue in which
the cancer originates. One common form of cancer is skin cancer. The Skin
Cancer Foundation reports that one in five Americans will experience some
type of skin cancer in their lifetime. The degradation of the ozone layer in
the atmosphere and the resulting increase in exposure to UV radiation has
contributed to its rise. Overexposure to UV radiation damages DNA, which
can lead to the formation of cancerous lesions. Although melanin offers
some protection against DNA damage from the sun, often it is not enough.
The fact that cancers can also occur on areas of the body that are normally
not exposed to UV radiation suggests that there are additional factors that
can lead to cancerous lesions.
In general, cancers result from an accumulation of DNA mutations. These
mutations can result in cell populations that do not die when they should
and uncontrolled cell proliferation that leads to tumors. Although many
tumors are benign (harmless), some produce cells that can mobilize and
establish tumors in other organs of the body; this process is referred to as
metastasis. Cancers are characterized by their ability to metastasize.
Basal Cell Carcinoma
Basal cell carcinoma is a form of cancer that affects the mitotically active
stem cells in the stratum basale of the epidermis. It is the most common of
all cancers that occur in the United States and is frequently found on the
head, neck, arms, and back, which are areas that are most susceptible to
long-term sun exposure. Although UV rays are the main culprit, exposure to
other agents, such as radiation and arsenic, can also lead to this type of
cancer. Wounds on the skin due to open sores, tattoos, burns, etc. may be
predisposing factors as well. Basal cell carcinomas start in the stratum
basale and usually spread along this boundary. At some point, they begin to
grow toward the surface and become an uneven patch, bump, growth, or
scar on the skin surface ([link]). Like most cancers, basal cell carcinomas
respond best to treatment when caught early. Treatment options include
surgery, freezing (cryosurgery), and topical ointments (Mayo Clinic 2012).
Basal Cell Carcinoma
Basal cell carcinoma
can take several
different forms. Similar
to other forms of skin
cancer, it is readily
cured if caught early
and treated. (credit:
John Hendrix, MD)
Squamous Cell Carcinoma
Squamous cell carcinoma is a cancer that affects the keratinocytes of the
stratum spinosum and presents as lesions commonly found on the scalp,
ears, and hands ([link]). It is the second most common skin cancer. The
American Cancer Society reports that two of 10 skin cancers are squamous
cell carcinomas, and it is more aggressive than basal cell carcinoma. If not
removed, these carcinomas can metastasize. Surgery and radiation are used
to cure squamous cell carcinoma.
Squamous Cell Carcinoma
Squamous cell carcinoma
presents here as a lesion on
an individual’s nose. (credit:
the National Cancer Institute)
Melanoma
A melanoma is a cancer characterized by the uncontrolled growth of
melanocytes, the pigment-producing cells in the epidermis. Typically, a
melanoma develops from a mole. It is the most fatal of all skin cancers, as it
is highly metastatic and can be difficult to detect before it has spread to
other organs. Melanomas usually appear as asymmetrical brown and black
patches with uneven borders and a raised surface ([{link]). Treatment
typically involves surgical excision and immunotherapy.
Melanoma
Melanomas typically
present as large brown
or black patches with
uneven borders and a
raised surface. (credit:
the National Cancer
Institute)
Doctors often give their patients the following ABCDE mnemonic to help
with the diagnosis of early-stage melanoma. If you observe a mole on your
body displaying these signs, consult a doctor.
e Asymmetry — the two sides are not symmetrical
¢ Borders — the edges are irregular in shape
e Color — the color is varied shades of brown or black
e Diameter — it is larger than 6 mm (0.24 in)
e Evolving — its shape has changed
Some specialists cite the following additional signs for the most serious
form, nodular melanoma:
e Elevated — it is raised on the skin surface
e Firm — it feels hard to the touch
¢ Growing — it is getting larger
Skin Disorders
Two common skin disorders are eczema and acne. Eczema is an
inflammatory condition and occurs in individuals of all ages. Acne involves
the clogging of pores, which can lead to infection and inflammation, and is
often seen in adolescents. Other disorders, not discussed here, include
seborrheic dermatitis (on the scalp), psoriasis, cold sores, impetigo, scabies,
hives, and warts.
Eczema
Eczema is an allergic reaction that manifests as dry, itchy patches of skin
that resemble rashes ([link]). It may be accompanied by swelling of the
skin, flaking, and in severe cases, bleeding. Many who suffer from eczema
have antibodies against dust mites in their blood, but the link between
eczema and allergy to dust mites has not been proven. Symptoms are
usually managed with moisturizers, corticosteroid creams, and
immunosuppressants.
Eczema
Eczema is a common skin
disorder that presents as a
red, flaky rash. (credit:
“Jambula”/Wikimedia
Commons)
Acne
Acne is a skin disturbance that typically occurs on areas of the skin that are
rich in sebaceous glands (face and back). It is most common along with the
onset of puberty due to associated hormonal changes, but can also occur in
infants and continue into adulthood. Hormones, such as androgens,
stimulate the release of sebum. An overproduction and accumulation of
sebum along with keratin can block hair follicles. This plug is initially
white. The sebum, when oxidized by exposure to air, turns black. Acne
results from infection by acne-causing bacteria (Propionibacterium and
Staphylococcus), which can lead to redness and potential scarring due to the
natural wound healing process ((Link]).
Acne
Epidermis Plugged follicle Mild inflammation Marked inflammation
Sebaceous
Accumulation of shed Bacteria proliferate
keratin and sebum
Acne is a result of over-productive sebaceous glands,
which leads to formation of blackheads and
inflammation of the skin.
Note:
Career Connection
Dermatologist
Have you ever had a skin rash that did not respond to over-the-counter
creams, or a mole that you were concerned about? Dermatologists help
patients with these types of problems and more, on a daily basis.
Dermatologists are medical doctors who specialize in diagnosing and
treating skin disorders. Like all medical doctors, dermatologists earn a
medical degree and then complete several years of residency training. In
addition, dermatologists may then participate in a dermatology fellowship
or complete additional, specialized training in a dermatology practice. If
practicing in the United States, dermatologists must pass the United States
Medical Licensing Exam (USMLE), become licensed in their state of
practice, and be certified by the American Board of Dermatology.
Most dermatologists work in a medical office or private-practice setting.
They diagnose skin conditions and rashes, prescribe oral and topical
medications to treat skin conditions, and may perform simple procedures,
such as mole or wart removal. In addition, they may refer patients to an
oncologist if skin cancer that has metastasized is suspected. Recently,
cosmetic procedures have also become a prominent part of dermatology.
Botox injections, laser treatments, and collagen and dermal filler injections
are popular among patients, hoping to reduce the appearance of skin aging.
Dermatology is a competitive specialty in medicine. Limited openings in
dermatology residency programs mean that many medical students
compete for a few select spots. Dermatology is an appealing specialty to
many prospective doctors, because unlike emergency room physicians or
surgeons, dermatologists generally do not have to work excessive hours or
be “on-call” weekends and holidays. Moreover, the popularity of cosmetic
dermatology has made it a growing field with many lucrative
opportunities. It is not unusual for dermatology clinics to market
themselves exclusively as cosmetic dermatology centers, and for
dermatologists to specialize exclusively in these procedures.
Consider visiting a dermatologist to talk about why he or she entered the
field and what the field of dermatology is like. Visit this site for additional
information.
Injuries
Because the skin is the part of our bodies that meets the world most directly,
it is especially vulnerable to injury. Injuries include burns and wounds, as
well as scars and calluses. They can be caused by sharp objects, heat, or
excessive pressure or friction to the skin.
Skin injuries set off a healing process that occurs in several overlapping
stages. The first step to repairing damaged skin is the formation of a blood
clot that helps stop the flow of blood and scabs over with time. Many
different types of cells are involved in wound repair, especially if the
surface area that needs repair is extensive. Before the basal stem cells of the
stratum basale can recreate the epidermis, fibroblasts mobilize and divide
rapidly to repair the damaged tissue by collagen deposition, forming
granulation tissue. Blood capillaries follow the fibroblasts and help increase
blood circulation and oxygen supply to the area. Immune cells, such as
macrophages, roam the area and engulf any foreign matter to reduce the
chance of infection.
Burns
A burn results when the skin is damaged by intense heat, radiation,
electricity, or chemicals. The damage results in the death of skin cells,
which can lead to a massive loss of fluid. Dehydration, electrolyte
imbalance, and renal and circulatory failure follow, which can be fatal. Burn
patients are treated with intravenous fluids to offset dehydration, as well as
intravenous nutrients that enable the body to repair tissues and replace lost
proteins. Another serious threat to the lives of burn patients is infection.
Burned skin is extremely susceptible to bacteria and other pathogens, due to
the loss of protection by intact layers of skin.
Burns are sometimes measured in terms of the size of the total surface area
affected. This is referred to as the “rule of nines,” which associates specific
anatomical areas with a percentage that is a factor of nine ([link]). Burns are
also classified by the degree of their severity. A first-degree burn is a
superficial burn that affects only the epidermis. Although the skin may be
painful and swollen, these burns typically heal on their own within a few
days. Mild sunburn fits into the category of a first-degree burn. A second-
degree burn goes deeper and affects both the epidermis and a portion of
the dermis. These burns result in swelling and a painful blistering of the
skin. It is important to keep the burn site clean and sterile to prevent
infection. If this is done, the burn will heal within several weeks. A third-
degree burn fully extends into the epidermis and dermis, destroying the
tissue and affecting the nerve endings and sensory function. These are
serious burns that may appear white, red, or black; they require medical
attention and will heal slowly without it. A fourth-degree burn is even
more severe, affecting the underlying muscle and bone. Oddly, third and
fourth-degree burns are usually not as painful because the nerve endings
themselves are damaged. Full-thickness burns cannot be repaired by the
body, because the local tissues used for repair are damaged and require
excision (debridement), or amputation in severe cases, followed by grafting
of the skin from an unaffected part of the body, or from skin grown in tissue
culture for grafting purposes.
Calculating the Size of a Burn
Head and
neck 9%
Upper limbs
9% each
Trunk 36%
Genitalia 1%
Lower limbs
18% each
The size of a burn will guide decisions
made about the need for specialized
treatment. Specific parts of the body
are associated with a percentage of
body area.
me oS
= openstax COLLEGE”
r 4
abe
Skin grafts are required when the damage from trauma or infection cannot
be closed with sutures or staples. Watch this video to learn more about skin
grafting procedures.
Scars and Keloids
Most cuts or wounds, with the exception of ones that only scratch the
surface (the epidermis), lead to scar formation. A scar is collagen-rich skin
formed after the process of wound healing that differs from normal skin.
Scarring occurs in cases in which there is repair of skin damage, but the
skin fails to regenerate the original skin structure. Fibroblasts generate scar
tissue in the form of collagen, and the bulk of repair is due to the basket-
weave pattern generated by collagen fibers and does not result in
regeneration of the typical cellular structure of skin. Instead, the tissue is
fibrous in nature and does not allow for the regeneration of accessory
structures, such as hair follicles, sweat glands, or sebaceous glands.
Sometimes, there is an overproduction of scar tissue, because the process of
collagen formation does not stop when the wound is healed; this results in
the formation of a raised or hypertrophic scar called a keloid. In contrast,
scars that result from acne and chickenpox have a sunken appearance and
are called atrophic scars.
Scarring of skin after wound healing is a natural process and does not need
to be treated further. Application of mineral oil and lotions may reduce the
formation of scar tissue. However, modern cosmetic procedures, such as
dermabrasion, laser treatments, and filler injections have been invented as
remedies for severe scarring. All of these procedures try to reorganize the
structure of the epidermis and underlying collagen tissue to make it look
more natural.
Bedsores and Stretch Marks
Skin and its underlying tissue can be affected by excessive pressure. One
example of this is called a bedsore. Bedsores, also called decubitis ulcers,
are caused by constant, long-term, unrelieved pressure on certain body parts
that are bony, reducing blood flow to the area and leading to necrosis (tissue
death). Bedsores are most common in elderly patients who have debilitating
conditions that cause them to be immobile. Most hospitals and long-term
care facilities have the practice of turning the patients every few hours to
prevent the incidence of bedsores. If left untreated by removal of necrotized
tissue, bedsores can be fatal if they become infected.
The skin can also be affected by pressure associated with rapid growth. A
stretch mark results when the dermis is stretched beyond its limits of
elasticity, as the skin stretches to accommodate the excess pressure. Stretch
marks usually accompany rapid weight gain during puberty and pregnancy.
They initially have a reddish hue, but lighten over time. Other than for
cosmetic reasons, treatment of stretch marks is not required. They occur
most commonly over the hips and abdomen.
Calluses
When you wear shoes that do not fit well and are a constant source of
abrasion on your toes, you tend to form a callus at the point of contact. This
occurs because the basal stem cells in the stratum basale are triggered to
divide more often to increase the thickness of the skin at the point of
abrasion to protect the rest of the body from further damage. This is an
example of a minor or local injury, and the skin manages to react and treat
the problem independent of the rest of the body. Calluses can also form on
your fingers if they are subject to constant mechanical stress, such as long
periods of writing, playing string instruments, or video games. A corn is a
specialized form of callus. Corns form from abrasions on the skin that result
from an elliptical-type motion.
Chapter Review
Skin cancer is a result of damage to the DNA of skin cells, often due to
excessive exposure to UV radiation. Basal cell carcinoma and squamous
cell carcinoma are highly curable, and arise from cells in the stratum basale
and stratum spinosum, respectively. Melanoma is the most dangerous form
of skin cancer, affecting melanocytes, which can spread/metastasize to other
organs. Burns are an injury to the skin that occur as a result of exposure to
extreme heat, radiation, or chemicals. First-degree and second-degree burns
usually heal quickly, but third-degree burns can be fatal because they
penetrate the full thickness of the skin. Scars occur when there is repair of
skin damage. Fibroblasts generate scar tissue in the form of collagen, which
forms a basket-weave pattern that looks different from normal skin.
Bedsores and stretch marks are the result of excessive pressure on the skin
and underlying tissue. Bedsores are characterized by necrosis of tissue due
to immobility, whereas stretch marks result from rapid growth. Eczema is
an allergic reaction that manifests as a rash, and acne results from clogged
sebaceous glands. Eczema and acne are usually long-term skin conditions
that may be treated successfully in mild cases. Calluses and corns are the
result of abrasive pressure on the skin.
Review Questions
Exercise:
Problem:In general, skin cancers
a. are easily treatable and not a major health concern
b. occur due to poor hygiene
c. can be reduced by limiting exposure to the sun
d. affect only the epidermis
Solution:
C
Exercise:
Problem: Bedsores
a. can be treated with topical moisturizers
b. can result from deep massages
c. are preventable by eliminating pressure points
d. are caused by dry skin
Solution:
C
Exercise:
Problem:
An individual has spent too much time sun bathing. Not only is his
skin painful to touch, but small blisters have appeared in the affected
area. This indicates that he has damaged which layers of his skin?
a. epidermis only
b. hypodermis only
c. epidermis and hypodermis
d. epidermis and dermis
Solution:
D
Exercise:
Problem:
After a skin injury, the body initiates a wound-healing response. The
first step of this response is the formation of a blood clot to stop
bleeding. Which of the following would be the next response?
a. increased production of melanin by melanocytes
b. increased production of connective tissue
c. an increase in Pacinian corpuscles around the wound
d. an increased activity in the stratum lucidum
Solution:
B
Exercise:
Problem:
Squamous cell carcinomas are the second most common of the skin
cancers and are capable of metastasizing if not treated. This cancer
affects which cells?
a. basal cells of the stratum basale
b. melanocytes of the stratum basale
c. keratinocytes of the stratum spinosum
d. Langerhans cells of the stratum lucidum
Solution:
C
Critical Thinking Questions
Exercise:
Problem:Why do teenagers often experience acne?
Solution:
Acne results from a blockage of sebaceous glands by sebum. The
blockage causes blackheads to form, which are susceptible to
infection. The infected tissue then becomes red and inflamed.
Teenagers experience this at high rates because the sebaceous glands
become active during puberty. Hormones that are especially active
during puberty stimulate the release of sebum, leading in many cases
to blockages.
Exercise:
Problem:Why do scars look different from surrounding skin?
Solution:
Scars are made of collagen and do not have the cellular structure of
normal skin. The tissue is fibrous and does not allow for the
regeneration of accessory structures, such as hair follicles, and sweat
or sebaceous glands.
References
American Cancer Society (US). Skin cancer: basal and squamous cell
[Internet]. c2013 [cited 2012 Nov 1]. Available from:
http://www.cancer.org/acs/groups/cid/documents/webcontent/003139-
pdt.pdf.
Lucile Packard Children’s Hospital at Stanford (US). Classification and
treatment of burns [Internet]. Palo Alto (CA). c2012 [cited 2012 Nov 1].
Available from:
Mayo Clinic (US). Basal cell carcinoma [Internet]. Scottsdale (AZ); c2012
[cited 2012 Nov 1]. Available from:
http://www.mayoclinic.com/health/basal-cell-
carcinoma/ds00925/dsection=treatments-and-drugs.
Beck, J. FYI: how much can a human body sweat before it runs out?
Popular Science [Internet]. New York (NY); c2012 [cited 2012 Nov 1].
much-can-human-body-sweat-it-runs-out.
Skin Cancer Foundation (US). Skin cancer facts [Internet]. New York (NY);
c2013 [cited 2012 Nov 1]. Available from: http://www.skincancer.org/skin-
cancer-information/skin-cancer-facts#top.
Glossary
acne
skin condition due to infected sebaceous glands
basal cell carcinoma
cancer that originates from basal cells in the epidermis of the skin
bedsore
sore on the skin that develops when regions of the body start
necrotizing due to constant pressure and lack of blood supply; also
called decubitis ulcers
callus
thickened area of skin that arises due to constant abrasion
corm
type of callus that is named for its shape and the elliptical motion of
the abrasive force
eczema
skin condition due to an allergic reaction, which resembles a rash
first-degree burn
superficial burn that injures only the epidermis
fourth-degree burn
burn in which full thickness of the skin and underlying muscle and
bone is damaged
keloid
type of scar that has layers raised above the skin surface
melanoma
type of skin cancer that originates from the melanocytes of the skin
metastasis
spread of cancer cells from a source to other parts of the body
scar
collagen-rich skin formed after the process of wound healing that is
different from normal skin
second-degree burn
partial-thickness burn that injures the epidermis and a portion of the
dermis
squamous cell carcinoma
type of skin cancer that originates from the stratum spinosum of the
epidermis
stretch mark
mark formed on the skin due to a sudden growth spurt and expansion
of the dermis beyond its elastic limits
third-degree burn
burn that penetrates and destroys the full thickness of the skin
(epidermis and dermis)
The Functions of the Skeletal System
By the end of this section, you will be able to:
¢ Define bone, cartilage, and the skeletal system
e List and describe the functions of the skeletal system
Bone, or osseous tissue, is a hard, dense connective tissue that forms most
of the adult skeleton, the support structure of the body. In the areas of the
skeleton where bones move (for example, the ribcage and joints), cartilage,
a semi-rigid form of connective tissue, provides flexibility and smooth
surfaces for movement. The skeletal system is the body system composed
of bones and cartilage and performs the following critical functions for the
human body:
e supports the body
e facilitates movement
¢ protects internal organs
e produces blood cells
e stores and releases minerals and fat
Support, Movement, and Protection
The most apparent functions of the skeletal system are the gross functions
—those visible by observation. Simply by looking at a person, you can see
how the bones support, facilitate movement, and protect the human body.
Just as the steel beams of a building provide a scaffold to support its weight,
the bones and cartilage of your skeletal system compose the scaffold that
supports the rest of your body. Without the skeletal system, you would be a
limp mass of organs, muscle, and skin.
Bones also facilitate movement by serving as points of attachment for your
muscles. While some bones only serve as a support for the muscles, others
also transmit the forces produced when your muscles contract. From a
mechanical point of view, bones act as levers and joints serve as fulcrums
({link]). Unless a muscle spans a joint and contracts, a bone is not going to
move. For information on the interaction of the skeletal and muscular
systems, that is, the musculoskeletal system, seek additional content.
Bones Support Movement
Bones act as levers when
muscles span a joint and
contract. (credit: Benjamin J.
DeLong)
Bones also protect internal organs from injury by covering or surrounding
them. For example, your ribs protect your lungs and heart, the bones of
your vertebral column (spine) protect your spinal cord, and the bones of
your cranium (skull) protect your brain ({link]).
Bones Protect Brain
The cranium completely
surrounds and protects
the brain from non-
traumatic injury.
Note:
Career Connection
Orthopedist
An orthopedist is a doctor who specializes in diagnosing and treating
disorders and injuries related to the musculoskeletal system. Some
orthopedic problems can be treated with medications, exercises, braces,
and other devices, but others may be best treated with surgery ([link]).
Arm Brace
An orthopedist will sometimes prescribe
the use of a brace that reinforces the
underlying bone structure it is being used
to support. (credit: Juhan Sonin)
While the origin of the word “orthopedics” (ortho- = “straight”; paed- =
“child”), literally means “straightening of the child,” orthopedists can have
patients who range from pediatric to geriatric. In recent years, orthopedists
have even performed prenatal surgery to correct spina bifida, a congenital
defect in which the neural canal in the spine of the fetus fails to close
completely during embryologic development.
Orthopedists commonly treat bone and joint injuries but they also treat
other bone conditions including curvature of the spine. Lateral curvatures
(scoliosis) can be severe enough to slip under the shoulder blade (scapula)
forcing it up as a hump. Spinal curvatures can also be excessive
dorsoventrally (kyphosis) causing a hunch back and thoracic compression.
These curvatures often appear in preteens as the result of poor posture,
abnormal growth, or indeterminate causes. Mostly, they are readily treated
by orthopedists. As people age, accumulated spinal column injuries and
diseases like osteoporosis can also lead to curvatures of the spine, hence
the stooping you sometimes see in the elderly.
Some orthopedists sub-specialize in sports medicine, which addresses both
simple injuries, such as a sprained ankle, and complex injuries, such as a
torn rotator cuff in the shoulder. Treatment can range from exercise to
surgery.
Mineral Storage, Energy Storage, and Hematopoiesis
On a metabolic level, bone tissue performs several critical functions. For
one, the bone matrix acts as a reservoir for a number of minerals important
to the functioning of the body, especially calcium, and phosphorus. These
minerals, incorporated into bone tissue, can be released back into the
bloodstream to maintain levels needed to support physiological processes.
Calcium ions, for example, are essential for muscle contractions and
controlling the flow of other ions involved in the transmission of nerve
impulses.
Bone also serves as a site for fat storage and blood cell production. The
softer connective tissue that fills the interior of most bone is referred to as
bone marrow ((link]). There are two types of bone marrow: yellow marrow
and red marrow. Yellow marrow contains adipose tissue; the triglycerides
stored in the adipocytes of the tissue can serve as a source of energy. Red
marrow is where hematopoiesis—the production of blood cells—takes
place. Red blood cells, white blood cells, and platelets are all produced in
the red marrow.
Head of Femur Showing Red and Yellow Marrow
Outer surface of bone
Red marrow
Yellow marrow
The head of the femur contains both
yellow and red marrow. Yellow marrow
stores fat. Red marrow is responsible for
hematopoiesis. (credit: modification of
work by “stevenfruitsmaak”/Wikimedia
Commons)
Chapter Review
The major functions of the bones are body support, facilitation of
movement, protection of internal organs, storage of minerals and fat, and
hematopoiesis. Together, the muscular system and skeletal system are
known as the musculoskeletal system.
Review Questions
Exercise:
Problem:
Which function of the skeletal system would be especially important if
you were in a car accident?
a. storage of minerals
b. protection of internal organs
c. facilitation of movement
d. fat storage
Solution:
B
Exercise:
Problem:Bone tissue can be described as
a. dead calcified tissue
b. cartilage
c. the skeletal system
d. dense, hard connective tissue
Solution:
D
Exercise:
Problem: Without red marrow, bones would not be able to
a. store phosphate
b. store calcium
c. make blood cells
d. move like levers
Solution:
C
Exercise:
Problem: Yellow marrow has been identified as
a. an area of fat storage
b. a point of attachment for muscles
c. the hard portion of bone
d. the cause of kyphosis
Solution:
A
Exercise:
Problem: Which of the following can be found in areas of movement?
a. hematopoiesis
b. cartilage
c. yellow marrow
d. red marrow
Solution:
B
Exercise:
Problem:The skeletal system is made of
a. muscles and tendons
b. bones and cartilage
c. vitreous humor
d. minerals and fat
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
The skeletal system is composed of bone and cartilage and has many
functions. Choose three of these functions and discuss what features of
the skeletal system allow it to accomplish these functions.
Solution:
It supports the body. The rigid, yet flexible skeleton acts as a
framework to support the other organs of the body.
It facilitates movement. The movable joints allow the skeleton to
change shape and positions; that is, move.
It protects internal organs. Parts of the skeleton enclose or partly
enclose various organs of the body including our brain, ears, heart, and
lungs. Any trauma to these organs has to be mediated through the
skeletal system.
It produces blood cells. The central cavity of long bones is filled with
marrow. The red marrow is responsible for forming red and white
blood cells.
It stores and releases minerals and fat. The mineral component of
bone, in addition to providing hardness to bone, provides a mineral
reservoir that can be tapped as needed. Additionally, the yellow
marrow, which is found in the central cavity of long bones along with
red marrow, serves as a Storage site for fat.
Glossary
bone
hard, dense connective tissue that forms the structural elements of the
skeleton
cartilage
semi-rigid connective tissue found on the skeleton in areas where
flexibility and smooth surfaces support movement
hematopoiesis
production of blood cells, which occurs in the red marrow of the bones
orthopedist
doctor who specializes in diagnosing and treating musculoskeletal
disorders and injuries
osseous tissue
bone tissue; a hard, dense connective tissue that forms the structural
elements of the skeleton
red marrow
connective tissue in the interior cavity of a bone where hematopoiesis
takes place
skeletal system
organ system composed of bones and cartilage that provides for
movement, support, and protection
yellow marrow
connective tissue in the interior cavity of a bone where fat is stored
Bone Classification
By the end of this section, you will be able to:
¢ Classify bones according to their shapes
e Describe the function of each category of bones
The 206 bones that compose the adult skeleton are divided into five
categories based on their shapes ((link]). Their shapes and their functions
are related such that each categorical shape of bone has a distinct function.
Classifications of Bones
Irregular bone
Flat bone
Vertebra
Sternum
Long bone
Short bones
Lateral
cuneiform
Sesamoid bone
Intermediate
cuneiform
= - Medial
- y cuneiform
Patella ne -
Bones are classified according to their shape.
Long Bones
A long bone is one that is cylindrical in shape, being longer than it is wide.
Keep in mind, however, that the term describes the shape of a bone, not its
size. Long bones are found in the arms (humerus, ulna, radius) and legs
(femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and
toes (metatarsals, phalanges). Long bones function as levers; they move
when muscles contract.
Short Bones
A short bone is one that is cube-like in shape, being approximately equal in
length, width, and thickness. The only short bones in the human skeleton
are in the carpals of the wrists and the tarsals of the ankles. Short bones
provide stability and support as well as some limited motion.
Flat Bones
The term “flat bone” is somewhat of a misnomer because, although a flat
bone is typically thin, it is also often curved. Examples include the cranial
(skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and
the ribs. Flat bones serve as points of attachment for muscles and often
protect internal organs.
Irregular Bones
An irregular bone is one that does not have any easily characterized shape
and therefore does not fit any other classification. These bones tend to have
more complex shapes, like the vertebrae that support the spinal cord and
protect it from compressive forces. Many facial bones, particularly the ones
containing sinuses, are classified as irregular bones.
Sesamoid Bones
A sesamoid bone is a small, round bone that, as the name suggests, is
shaped like a sesame seed. These bones form in tendons (the sheaths of
tissue that connect bones to muscles) where a great deal of pressure is
generated in a joint. The sesamoid bones protect tendons by helping them
overcome compressive forces. Sesamoid bones vary in number and
placement from person to person but are typically found in tendons
associated with the feet, hands, and knees. The patellae (singular = patella)
are the only sesamoid bones found in common with every person. [link]
reviews bone classifications with their associated features, functions, and
examples.
Bone Classifications
Bone
classification
Long
Short
Features
Cylinder-like
shape, longer
than it is wide
Cube-like
shape,
approximately
equal in
length, width,
and thickness
Function(s)
Leverage
Provide
stability,
support,
while
allowing for
some
motion
Examples
Femur, tibia,
fibula,
metatarsals,
humerus,
ulna, radius,
metacarpals,
phalanges
Carpals,
tarsals
Bone Classifications
Bone
classification Features Function(s) Examples
Points of
attachment Sternum,
Flat Thin and for muscles; ribs,
curved protectors scapulae,
of internal cranial bones
organs
Pr
Complex — Vertebrae,
Irregular internal
shape facial bones
organs
Pr
Small and oe
fount: tendons
i : : from Patell
Sesamoid anbeddedan 0) atellae
compressive
tendons
forces
Chapter Review
Bones can be classified according to their shapes. Long bones, such as the
femur, are longer than they are wide. Short bones, such as the carpals, are
approximately equal in length, width, and thickness. Flat bones are thin, but
are often curved, such as the ribs. Irregular bones such as those of the face
have no characteristic shape. Sesamoid bones, such as the patellae, are
small and round, and are located in tendons.
Review Questions
Exercise:
Problem:
Most of the bones of the arms and hands are long bones; however, the
bones in the wrist are categorized as
a. flat bones
b. short bones
c. sesamoid bones
d. irregular bones
Solution:
B
Exercise:
Problem:Sesamoid bones are found embedded in
a. joints
b. muscles
c. ligaments
d. tendons
Solution:
D
Exercise:
Problem:
Bones that surround the spinal cord are classified as bones.
a. irregular
b. sesamoid
c. flat
d. short
Solution:
A
Exercise:
Problem:
Which category of bone is among the most numerous in the skeleton?
a. long bone
b. sesamoid bone
c. short bone
d. flat bone
Solution:
A
Exercise:
Problem:Long bones enable body movement by acting as a
a. counterweight
b. resistive force
c. lever
d. fulcrum
Solution:
GC
Critical Thinking Questions
Exercise:
Problem:
What are the structural and functional differences between a tarsal and
a metatarsal?
Solution:
Structurally, a tarsal is a short bone, meaning its length, width, and
thickness are about equal, while a metatarsal is a long bone whose
length is greater than its width. Functionally, the tarsal provides
limited motion, while the metatarsal acts as a lever.
Exercise:
Problem:
What are the structural and functional differences between the femur
and the patella?
Solution:
Structurally, the femur is a long bone, meaning its length is greater
than its width, while the patella, a sesamoid bone, is small and round.
Functionally, the femur acts as a lever, while the patella protects the
patellar tendon from compressive forces.
Glossary
flat bone
thin and curved bone; serves as a point of attachment for muscles and
protects internal organs
irregular bone
bone of complex shape; protects internal organs from compressive
forces
long bone
cylinder-shaped bone that is longer than it is wide; functions as a lever
sesamoid bone
small, round bone embedded in a tendon; protects the tendon from
compressive forces
short bone
cube-shaped bone that is approximately equal in length, width, and
thickness; provides limited motion
Bone Structure
By the end of this section, you will be able to:
e Identify the anatomical features of a bone
¢ Define and list examples of bone markings
¢ Describe the histology of bone tissue
e Compare and contrast compact and spongy bone
e Identify the structures that compose compact and spongy bone
e Describe how bones are nourished and innervated
Bone tissue (osseous tissue) differs greatly from other tissues in the body.
Bone is hard and many of its functions depend on that characteristic
hardness. Later discussions in this chapter will show that bone is also
dynamic in that its shape adjusts to accommodate stresses. This section will
examine the gross anatomy of bone first and then move on to its histology.
Gross Anatomy of Bone
The structure of a long bone allows for the best visualization of all of the
parts of a bone ([link]). A long bone has two parts: the diaphysis and the
epiphysis. The diaphysis is the tubular shaft that runs between the proximal
and distal ends of the bone. The hollow region in the diaphysis is called the
medullary cavity, which is filled with yellow marrow. The walls of the
diaphysis are composed of dense and hard compact bone.
Anatomy of a Long Bone
Articular cartilage
Proximal
epiphysis
Metaphysis Spongy bone
Epiphyseal line
Red bone marrow
Endosteum
Compact bone
Medullary cavity
Diaphysis Yellow bone marrow
Periosteum
Nutrient artery
Distal
epiphysis
Articular cartilage
A typical long bone shows the
gross anatomical characteristics of
bone.
The wider section at each end of the bone is called the epiphysis (plural =
epiphyses), which is filled with spongy bone. Red marrow fills the spaces in
the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the
narrow area that contains the epiphyseal plate (growth plate), a layer of
hyaline (transparent) cartilage in a growing bone. When the bone stops
growing in early adulthood (approximately 18-21 years), the cartilage is
replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal
line.
The medullary cavity has a delicate membranous lining called the
endosteum (end- = “inside”; oste- = “bone”), where bone growth, repair,
and remodeling occur. The outer surface of the bone is covered with a
fibrous membrane called the periosteum (peri- = “around” or
“surrounding”). The periosteum contains blood vessels, nerves, and
lymphatic vessels that nourish compact bone. Tendons and ligaments also
attach to bones at the periosteum. The periosteum covers the entire outer
surface except where the epiphyses meet other bones to form joints ([link]).
In this region, the epiphyses are covered with articular cartilage, a thin
layer of cartilage that reduces friction and acts as a shock absorber.
Periosteum and Endosteum
Periosteum /
Endosteum
Periosteum Osteoclast
(fibrous layer) Osteocyte
in lacuna Bone matrix
Osteocyte
Periosteum Osteogenic cell
(cellular layer) Osteoblast
The periosteum forms the outer surface of bone, and
the endosteum lines the medullary cavity.
Flat bones, like those of the cranium, consist of a layer of diploé (spongy
bone), lined on either side by a layer of compact bone ([link]). The two
layers of compact bone and the interior spongy bone work together to
protect the internal organs. If the outer layer of a cranial bone fractures, the
brain is still protected by the intact inner layer.
Anatomy of a Flat Bone
| \i
4
‘i ten as
Periosteum RN Ce I ee
~ ° Dee | c=!
OSC” bone (diploé)
This cross-section of a flat bone shows the
spongy bone (diploé) lined on either side by a
layer of compact bone.
Bone Markings
The surface features of bones vary considerably, depending on the function
and location in the body. [link] describes the bone markings, which are
illustrated in ([link]). There are three general classes of bone markings: (1)
articulations, (2) projections, and (3) holes. As the name implies, an
articulation is where two bone surfaces come together (articulus = “joint”).
These surfaces tend to conform to one another, such as one being rounded
and the other cupped, to facilitate the function of the articulation. A
projection is an area of a bone that projects above the surface of the bone.
These are the attachment points for tendons and ligaments. In general, their
size and shape is an indication of the forces exerted through the attachment
to the bone. A hole is an opening or groove in the bone that allows blood
vessels and nerves to enter the bone. As with the other markings, their size
and shape reflect the size of the vessels and nerves that penetrate the bone
at these points.
Bone Markings
Marking
Articulations
Head
Facet
Condyle
Projections
Protuberance
Process
Spine
Tubercle
Tuberosity
Description
Where two
bones meet
Prominent
rounded
surface
Flat surface
Rounded
surface
Raised
markings
Protruding
Prominence
feature
Sharp
process
Small,
rounded
process
Rough
surface
Example
Knee joint
Head of femur
Vertebrae
Occipital condyles
Spinous process of the vertebrae
Chin
Transverse process of vertebra
Ischial spine
Tubercle of humerus
Deltoid tuberosity
Bone Markings
Marking
Line
Crest
Holes
Fossa
Fovea
Sulcus
Canal
Fissure
Foramen
Meatus
Description
Slight,
elongated
ridge
Ridge
Holes and
depressions
Elongated
basin
Small pit
Groove
Passage in
bone
Slit through
bone
Hole
through
bone
Opening
into canal
Example
Temporal lines of the parietal
bones
Iliac crest
Foramen (holes through which
blood vessels can pass through)
Mandibular fossa
Fovea capitis on the head of the
femur
Sigmoid sulcus of the temporal
bones
Auditory canal
Auricular fissure
Foramen magnum in the
occipital bone
External auditory meatus
Bone Markings
Marking Description Example
Air-filled
Sinus space in Nasal sinus
bone
Bone Features
Examples of processes formed where Examples of an elevation or depression
tendons or ligaments attach
Fovea capitis
17 Sulcus
Head
Tubercle Pay
\
Tuberosity
Pelvis
Fossa
Humerus Examples of openings
Tubercle Condyle
Facet
Foramen
Condyles
Examples of processes formed to Canal , \ \ Fissure
articulate with adjacent bones \ \
Protuberance
Skull
The surface features of bones depend on their function,
location, attachment of ligaments and tendons, or the
penetration of blood vessels and nerves.
Bone Cells and Tissue
Bone contains a relatively small number of cells entrenched in a matrix of
collagen fibers that provide a surface for inorganic salt crystals to adhere.
These salt crystals form when calcium phosphate and calcium carbonate
combine to create hydroxyapatite, which incorporates other inorganic salts
like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or
calcifies, on the collagen fibers. The hydroxyapatite crystals give bones
their hardness and strength, while the collagen fibers give them flexibility
so that they are not brittle.
Although bone cells compose a small amount of the bone volume, they are
crucial to the function of bones. Four types of cells are found within bone
tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts ([link]).
Bone Cells
wet &.
Osteocyte Osteoblast Osteogenic cell Osteoclast
(maintains (forms bone matrix) (stem cell) (resorbs bone)
bone tissue)
Four types of cells are found within bone
tissue. Osteogenic cells are undifferentiated
and develop into osteoblasts. When
osteoblasts get trapped within the calcified
matrix, their structure and function changes,
and they become osteocytes. Osteoclasts
develop from monocytes and macrophages
and differ in appearance from other bone
cells.
The osteoblast is the bone cell responsible for forming new bone and is
found in the growing portions of bone, including the periosteum and
endosteum. Osteoblasts, which do not divide, synthesize and secrete the
collagen matrix and calcium salts. As the secreted matrix surrounding the
osteoblast calcifies, the osteoblast become trapped within it; as a result, it
changes in structure and becomes an osteocyte, the primary cell of mature
bone and the most common type of bone cell. Each osteocyte is located in a
space called a lacuna and is surrounded by bone tissue. Osteocytes
maintain the mineral concentration of the matrix via the secretion of
enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can
communicate with each other and receive nutrients via long cytoplasmic
processes that extend through canaliculi (singular = canaliculus), channels
within the bone matrix.
If osteoblasts and osteocytes are incapable of mitosis, then how are they
replenished when old ones die? The answer lies in the properties of a third
category of bone cells—the osteogenic cell. These osteogenic cells are
undifferentiated with high mitotic activity and they are the only bone cells
that divide. Immature osteogenic cells are found in the deep layers of the
periosteum and the marrow. They differentiate and develop into osteoblasts.
The dynamic nature of bone means that new tissue is constantly formed,
and old, injured, or unnecessary bone is dissolved for repair or for calcium
release. The cell responsible for bone resorption, or breakdown, is the
osteoclast. They are found on bone surfaces, are multinucleated, and
originate from monocytes and macrophages, two types of white blood cells,
not from osteogenic cells. Osteoclasts are continually breaking down old
bone while osteoblasts are continually forming new bone. The ongoing
balance between osteoblasts and osteoclasts is responsible for the constant
but subtle reshaping of bone. [link] reviews the bone cells, their functions,
and locations.
Bone Cells
Cell type Function
Osteogenic Develop into
cells osteoblasts
Osteoblasts Bone formation
Maintain mineral
Osteocytes concentration of
matrix
Osteoclasts Bone resorption
Compact and Spongy Bone
Location
Deep layers of the
periosteum and the marrow
Growing portions of bone,
including periosteum and
endosteum
Entrapped in matrix
Bone surfaces and at sites of
old, injured, or unneeded
bone
The differences between compact and spongy bone are best explored via
their histology. Most bones contain compact and spongy osseous tissue, but
their distribution and concentration vary based on the bone’s overall
function. Compact bone is dense so that it can withstand compressive
forces, while spongy (cancellous) bone has open spaces and supports shifts
in weight distribution.
Compact Bone
Compact bone is the denser, stronger of the two types of bone tissue
({link]). It can be found under the periosteum and in the diaphyses of long
bones, where it provides support and protection.
Diagram of Compact Bone
(a) This cross-sectional view of compact bone shows the basic
structural unit, the osteon. (b) In this micrograph of the osteon, you can
clearly see the concentric lamellae and central canals. LM x 40.
(Micrograph provided by the Regents of University of Michigan
Medical School © 2012)
Compact bone Spongy bone
Medullary cavity
Periosteum
Concentric lamellae
Osteon
Lymphatic vessel
Circumferential
lamellae Nerve
Periosteal artery Blood vessels
Periosteal vein rabeéulae
Periosteum:
Outer fibrous layer
Inner osteogenic layer
Interstitial lamellae Medullar cavity
Perforating canal
Spongy bone
Central canal
Blood vessels
Lymphatic vessel
Nerve
Compact bone
The microscopic structural unit of compact bone is called an osteon, or
Haversian system. Each osteon is composed of concentric rings of calcified
matrix called lamellae (singular = lamella). Running down the center of
each osteon is the central canal, or Haversian canal, which contains blood
vessels, nerves, and lymphatic vessels. These vessels and nerves branch off
at right angles through a perforating canal, also known as Volkmann’s
canals, to extend to the periosteum and endosteum.
The osteocytes are located inside spaces called lacunae (singular = lacuna),
found at the borders of adjacent lamellae. As described earlier, canaliculi
connect with the canaliculi of other lacunae and eventually with the central
canal. This system allows nutrients to be transported to the osteocytes and
wastes to be removed from them.
Spongy (Cancellous) Bone
Like compact bone, spongy bone, also known as cancellous bone, contains
osteocytes housed in lacunae, but they are not arranged in concentric
circles. Instead, the lacunae and osteocytes are found in a lattice-like
network of matrix spikes called trabeculae (singular = trabecula) ([link]).
The trabeculae may appear to be a random network, but each trabecula
forms along lines of stress to provide strength to the bone. The spaces of the
trabeculated network provide balance to the dense and heavy compact bone
by making bones lighter so that muscles can move them more easily. In
addition, the spaces in some spongy bones contain red marrow, protected by
the trabeculae, where hematopoiesis occurs.
Osteoclast
Osteoblasts aligned
along trabeculae of
new bone
Canaliculi Endosteum _Lamellae
openings : Ge Canaliculi
on surface aay
Lamellae
Spongy bone is composed of trabeculae that contain
the osteocytes. Red marrow fills the spaces in some
bones.
Note:
Aging and the...
Skeletal System: Paget’s Disease
Paget’s disease usually occurs in adults over age 40. It is a disorder of the
bone remodeling process that begins with overactive osteoclasts. This
means more bone is resorbed than is laid down. The osteoblasts try to
compensate but the new bone they lay down is weak and brittle and
therefore prone to fracture.
While some people with Paget’s disease have no symptoms, others
experience pain, bone fractures, and bone deformities ([link]). Bones of the
pelvis, skull, spine, and legs are the most commonly affected. When
occurring in the skull, Paget’s disease can cause headaches and hearing
loss.
Paget's Disease
Normal Paget’s disease
Normal leg bones are relatively
Straight, but those affected by
Paget’s disease are porous and
curved.
What causes the osteoclasts to become overactive? The answer is still
unknown, but hereditary factors seem to play a role. Some scientists
believe Paget’s disease is due to an as-yet-unidentified virus.
Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may
show bone deformities or areas of bone resorption. Bone scans are also
useful. In these studies, a dye containing a radioactive ion is injected into
the body. Areas of bone resorption have an affinity for the ion, so they will
light up on the scan if the ions are absorbed. In addition, blood levels of an
enzyme called alkaline phosphatase are typically elevated in people with
Paget’s disease.
Bisphosphonates, drugs that decrease the activity of osteoclasts, are often
used in the treatment of Paget’s disease. However, in a small percentage of
cases, bisphosphonates themselves have been linked to an increased risk of
fractures because the old bone that is left after bisphosphonates are
administered becomes worn out and brittle. Still, most doctors feel that the
benefits of bisphosphonates more than outweigh the risk; the medical
professional has to weigh the benefits and risks on a case-by-case basis.
Bisphosphonate treatment can reduce the overall risk of deformities or
fractures, which in turn reduces the risk of surgical repair and its associated
risks and complications.
Blood and Nerve Supply
The spongy bone and medullary cavity receive nourishment from arteries
that pass through the compact bone. The arteries enter through the nutrient
foramen (plural = foramina), small openings in the diaphysis ((link]). The
osteocytes in spongy bone are nourished by blood vessels of the periosteum
that penetrate spongy bone and blood that circulates in the marrow cavities.
As the blood passes through the marrow cavities, it is collected by veins,
which then pass out of the bone through the foramina.
In addition to the blood vessels, nerves follow the same paths into the bone
where they tend to concentrate in the more metabolically active regions of
the bone. The nerves sense pain, and it appears the nerves also play roles in
regulating blood supplies and in bone growth, hence their concentrations in
metabolically active sites of the bone.
Diagram of Blood and Nerve Supply to Bone
ere Articular
—_ ae zm) cartilage
Epiphyseal
artery
and vein
= Metaphyseal
artery
and vein
Periosteum
Compact
bone
Nutrient
artery
and vein
Nutrient
foramen Medullary cavity
Metaphyseal
artery and vein
Metaphysis —||
Epiphyseal
line
Blood vessels and nerves
enter the bone through the
nutrient foramen.
Note:
Watch this video to see the microscopic features of a bone.
Chapter Review
A hollow medullary cavity filled with yellow marrow runs the length of the
diaphysis of a long bone. The walls of the diaphysis are compact bone. The
epiphyses, which are wider sections at each end of a long bone, are filled
with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline
cartilage, is replaced by osseous tissue as the organ grows in length. The
medullary cavity has a delicate membranous lining called the endosteum.
The outer surface of bone, except in regions covered with articular
cartilage, is covered with a fibrous membrane called the periosteum. Flat
bones consist of two layers of compact bone surrounding a layer of spongy
bone. Bone markings depend on the function and location of bones.
Articulations are places where two bones meet. Projections stick out from
the surface of the bone and provide attachment points for tendons and
ligaments. Holes are openings or depressions in the bones.
Bone matrix consists of collagen fibers and organic ground substance,
primarily hydroxyapatite formed from calcium salts. Osteogenic cells
develop into osteoblasts. Osteoblasts are cells that make new bone. They
become osteocytes, the cells of mature bone, when they get trapped in the
matrix. Osteoclasts engage in bone resorption. Compact bone is dense and
composed of osteons, while spongy bone is less dense and made up of
trabeculae. Blood vessels and nerves enter the bone through the nutrient
foramina to nourish and innervate bones.
Review Questions
Exercise:
Problem:
Which of the following occurs in the spongy bone of the epiphysis?
a. bone growth
b. bone remodeling
c. hematopoiesis
d. shock absorption
Solution:
C
Exercise:
Problem:The diaphysis contains
a. the metaphysis
b. fat stores
c. spongy bone
d. compact bone
Solution:
B
Exercise:
Problem:
The fibrous membrane covering the outer surface of the bone is the
a. periosteum
b. epiphysis
c. endosteum
d. diaphysis
Solution:
A
Exercise:
Problem: Which of the following are incapable of undergoing mitosis?
a. osteoblasts and osteoclasts
b. osteocytes and osteoclasts
c. osteoblasts and osteocytes
d. osteogenic cells and osteoclasts
Solution:
Ԥ
Exercise:
Problem: Which cells do not originate from osteogenic cells?
a. osteoblasts
b. osteoclasts
c. osteocytes
d. osteoprogenitor cells
Solution:
D
Exercise:
Problem:
Which of the following are found in compact bone and cancellous
bone?
a. Haversian systems
b. Haversian canals
c. lamellae
d. lacunae
Solution:
C
Exercise:
Problem: Which of the following are only found in cancellous bone?
a. canaliculi
b. Volkmann’s canals
c. trabeculae
d. calcium salts
Solution:
C
Exercise:
Problem:
The area of a bone where the nutrient foramen passes forms what kind
of bone marking?
a. a hole
b. a facet
c. a canal
d. a fissure
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
If the articular cartilage at the end of one of your long bones were to
degenerate, what symptoms do you think you would experience?
Why?
Solution:
If the articular cartilage at the end of one of your long bones were to
deteriorate, which is actually what happens in osteoarthritis, you would
experience joint pain at the end of that bone and limitation of motion at
that joint because there would be no cartilage to reduce friction
between adjacent bones and there would be no cartilage to act as a
shock absorber.
Exercise:
Problem:
In what ways is the structural makeup of compact and spongy bone
well suited to their respective functions?
Solution:
The densely packed concentric rings of matrix in compact bone are
ideal for resisting compressive forces, which is the function of
compact bone. The open spaces of the trabeculated network of spongy
bone allow spongy bone to support shifts in weight distribution, which
is the function of spongy bone.
Glossary
articular cartilage
thin layer of cartilage covering an epiphysis; reduces friction and acts
as a shock absorber
articulation
where two bone surfaces meet
canaliculi
(singular = canaliculus) channels within the bone matrix that house
one of an osteocyte’s many cytoplasmic extensions that it uses to
communicate and receive nutrients
central canal
longitudinal channel in the center of each osteon; contains blood
vessels, nerves, and lymphatic vessels; also known as the Haversian
canal
compact bone
dense osseous tissue that can withstand compressive forces
diaphysis
tubular shaft that runs between the proximal and distal ends of a long
bone
diploé
layer of spongy bone, that is sandwiched between two the layers of
compact bone found in flat bones
endosteum
delicate membranous lining of a bone’s medullary cavity
epiphyseal plate
(also, growth plate) sheet of hyaline cartilage in the metaphysis of an
immature bone; replaced by bone tissue as the organ grows in length
epiphysis
wide section at each end of a long bone; filled with spongy bone and
red marrow
hole
opening or depression in a bone
lacunae
(singular = lacuna) spaces in a bone that house an osteocyte
medullary cavity
hollow region of the diaphysis; filled with yellow marrow
nutrient foramen
small opening in the middle of the external surface of the diaphysis,
through which an artery enters the bone to provide nourishment
osteoblast
cell responsible for forming new bone
osteoclast
cell responsible for resorbing bone
osteocyte
primary cell in mature bone; responsible for maintaining the matrix
osteogenic cell
undifferentiated cell with high mitotic activity; the only bone cells that
divide; they differentiate and develop into osteoblasts
osteon
(also, Haversian system) basic structural unit of compact bone; made
of concentric layers of calcified matrix
perforating canal
(also, Volkmann’s canal) channel that branches off from the central
canal and houses vessels and nerves that extend to the periosteum and
endosteum
periosteum
fibrous membrane covering the outer surface of bone and continuous
with ligaments
projection
bone markings where part of the surface sticks out above the rest of the
surface, where tendons and ligaments attach
spongy bone
(also, cancellous bone) trabeculated osseous tissue that supports shifts
in weight distribution
trabeculae
(singular = trabecula) spikes or sections of the lattice-like matrix in
spongy bone
Bone Formation and Development
By the end of this section, you will be able to:
e Explain the function of cartilage
e List the steps of intramembranous ossification
e List the steps of endochondral ossification
e Explain the growth activity at the epiphyseal plate
e Compare and contrast the processes of modeling and remodeling
In the early stages of embryonic development, the embryo’s skeleton
consists of fibrous membranes and hyaline cartilage. By the sixth or seventh
week of embryonic life, the actual process of bone development,
ossification (osteogenesis), begins. There are two osteogenic pathways—
intramembranous ossification and endochondral ossification—but bone is
the same regardless of the pathway that produces it.
Cartilage Templates
Bone is a replacement tissue; that is, it uses a model tissue on which to lay
down its mineral matrix. For skeletal development, the most common
template is cartilage. During fetal development, a framework is laid down
that determines where bones will form. This framework is a flexible, semi-
solid matrix produced by chondroblasts and consists of hyaluronic acid,
chondroitin sulfate, collagen fibers, and water. As the matrix surrounds and
isolates chondroblasts, they are called chondrocytes. Unlike most
connective tissues, cartilage is avascular, meaning that it has no blood
vessels supplying nutrients and removing metabolic wastes. All of these
functions are carried on by diffusion through the matrix. This is why
damaged cartilage does not repair itself as readily as most tissues do.
Throughout fetal development and into childhood growth and development,
bone forms on the cartilaginous matrix. By the time a fetus is born, most of
the cartilage has been replaced with bone. Some additional cartilage will be
replaced throughout childhood, and some cartilage remains in the adult
skeleton.
Intramembranous Ossification
During intramembranous ossification, compact and spongy bone
develops directly from sheets of mesenchymal (undifferentiated) connective
tissue. The flat bones of the face, most of the cranial bones, and the
clavicles (collarbones) are formed via intramembranous ossification.
The process begins when mesenchymal cells in the embryonic skeleton
gather together and begin to differentiate into specialized cells ({link]a).
Some of these cells will differentiate into capillaries, while others will
become osteogenic cells and then osteoblasts. Although they will ultimately
be spread out by the formation of bone tissue, early osteoblasts appear in a
cluster called an ossification center.
The osteoblasts secrete osteoid, uncalcified matrix, which calcifies
(hardens) within a few days as mineral salts are deposited on it, thereby
entrapping the osteoblasts within. Once entrapped, the osteoblasts become
osteocytes ([{link]b). As osteoblasts transform into osteocytes, osteogenic
cells in the surrounding connective tissue differentiate into new osteoblasts.
Osteoid (unmineralized bone matrix) secreted around the capillaries results
in a trabecular matrix, while osteoblasts on the surface of the spongy bone
become the periosteum ([link]c). The periosteum then creates a protective
layer of compact bone superficial to the trabecular bone. The trabecular
bone crowds nearby blood vessels, which eventually condense into red
marrow ((link]d).
Intramembranous Ossification
Mesenchymal
cells
Osteoid
Osteoblast
Ossification
center
Osteocyte
New bone
matrix
Osteoid
Osteoblast
Mesenchyme —= SS
forms the ES ———
6. y
-——_ Fibrous
periosteum
periosteum
Osteoblast
Compact bone
Trabeculae
Spongy bone
(cavities contain
Zz = — = = red marrow)
Blood vessel
Intramembranous ossification follows four steps. (a)
Mesenchymal cells group into clusters, and ossification centers
form. (b) Secreted osteoid traps osteoblasts, which then
become osteocytes. (c) Trabecular matrix and periosteum form.
(d) Compact bone develops superficial to the trabecular bone,
and crowded blood vessels condense into red marrow.
Intramembranous ossification begins in utero during fetal development and
continues on into adolescence. At birth, the skull and clavicles are not fully
ossified nor are the sutures of the skull closed. This allows the skull and
shoulders to deform during passage through the birth canal. The last bones
to ossify via intramembranous ossification are the flat bones of the face,
which reach their adult size at the end of the adolescent growth spurt.
Endochondral Ossification
In endochondral ossification, bone develops by replacing hyaline
cartilage. Cartilage does not become bone. Instead, cartilage serves as a
template to be completely replaced by new bone. Endochondral ossification
takes much longer than intramembranous ossification. Bones at the base of
the skull and long bones form via endochondral ossification.
In a long bone, for example, at about 6 to 8 weeks after conception, some of
the mesenchymal cells differentiate into chondrocytes (cartilage cells) that
form the cartilaginous skeletal precursor of the bones ([link]a). Soon after,
the perichondrium, a membrane that covers the cartilage, appears [link]b).
Endochondral Ossification
Perichondrium
Primary
ossification
center
Hyaline
cartilage
Calcified
matrix
(c) Periosteum
(covers compact
bone)
Medullary
cavity
Artery and vein
(provide nutrients
to bone)
(d)
Secondary
ossification
center
Articular cartilage
_—— Artery
and vein
Artery and vein
(provide nutrients
to bone)
(e)
Endochondral ossification follows five steps. (a) Mesenchymal cells
differentiate into chondrocytes. (b) The cartilage model of the future
bony skeleton and the perichondrium form. (c) Capillaries penetrate
cartilage. Perichondrium transforms into periosteum. Periosteal collar
develops. Primary ossification center develops. (d) Cartilage and
chondrocytes continue to grow at ends of the bone. (e) Secondary
ossification centers develop. (f) Cartilage remains at epiphyseal
(growth) plate and at joint surface as articular cartilage.
As more matrix is produced, the chondrocytes in the center of the
cartilaginous model grow in size. As the matrix calcifies, nutrients can no
longer reach the chondrocytes. This results in their death and the
disintegration of the surrounding cartilage. Blood vessels invade the
resulting spaces, not only enlarging the cavities but also carrying osteogenic
cells with them, many of which will become osteoblasts. These enlarging
spaces eventually combine to become the medullary cavity.
As the cartilage grows, capillaries penetrate it. This penetration initiates the
transformation of the perichondrium into the bone-producing periosteum.
Here, the osteoblasts form a periosteal collar of compact bone around the
cartilage of the diaphysis. By the second or third month of fetal life, bone
cell development and ossification ramps up and creates the primary
ossification center, a region deep in the periosteal collar where ossification
begins ({link]c).
While these deep changes are occurring, chondrocytes and cartilage
continue to grow at the ends of the bone (the future epiphyses), which
increases the bone’s length at the same time bone is replacing cartilage in
the diaphyses. By the time the fetal skeleton is fully formed, cartilage only
remains at the joint surface as articular cartilage and between the diaphysis
and epiphysis as the epiphyseal plate, the latter of which is responsible for
the longitudinal growth of bones. After birth, this same sequence of events
(matrix mineralization, death of chondrocytes, invasion of blood vessels
from the periosteum, and seeding with osteogenic cells that become
osteoblasts) occurs in the epiphyseal regions, and each of these centers of
activity is referred to as a secondary ossification center ((link]e).
How Bones Grow in Length
The epiphyseal plate is the area of growth in a long bone. It is a layer of
hyaline cartilage where ossification occurs in immature bones. On the
epiphyseal side of the epiphyseal plate, cartilage is formed. On the
diaphyseal side, cartilage is ossified, and the diaphysis grows in length. The
epiphyseal plate is composed of four zones of cells and activity ((link]). The
reserve zone is the region closest to the epiphyseal end of the plate and
contains small chondrocytes within the matrix. These chondrocytes do not
participate in bone growth but secure the epiphyseal plate to the osseous
tissue of the epiphysis.
Longitudinal Bone Growth
Changes in
_—
Co Q
Reserve zone fo) (6) Matrix production
Growth plate zones ere
= |
Proliferative zone Mitosis
_ | Lipids, glycogen,
and alkaline
| phosphatase
accumulate;
matrix calcifies.
Maturation and nail
hypertrophy <P
Calcified matrix = \— Celldeath
=
Primary
spongiosa
Secondary
spongiosa
Zone of
ossification
Metaphysis
The epiphyseal plate is responsible for
longitudinal bone growth.
The proliferative zone is the next layer toward the diaphysis and contains
stacks of slightly larger chondrocytes. It makes new chondrocytes (via
mitosis) to replace those that die at the diaphyseal end of the plate.
Chondrocytes in the next layer, the zone of maturation and hypertrophy,
are older and larger than those in the proliferative zone. The more mature
cells are situated closer to the diaphyseal end of the plate. The longitudinal
growth of bone is a result of cellular division in the proliferative zone and
the maturation of cells in the zone of maturation and hypertrophy.
Most of the chondrocytes in the zone of calcified matrix, the zone closest
to the diaphysis, are dead because the matrix around them has calcified.
Capillaries and osteoblasts from the diaphysis penetrate this zone, and the
osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus,
the zone of calcified matrix connects the epiphyseal plate to the diaphysis.
A bone grows in length when osseous tissue is added to the diaphysis.
Bones continue to grow in length until early adulthood. The rate of growth
is controlled by hormones, which will be discussed later. When the
chondrocytes in the epiphyseal plate cease their proliferation and bone
replaces the cartilage, longitudinal growth stops. All that remains of the
epiphyseal plate is the epiphyseal line ([link]).
Progression from Epiphyseal Plate to Epiphyseal Line
Epiphysis ee
WV Metaphysis
Diaphysis
Epiphyseal
Epiphyseal line
plate
(growth
plate)
Ee Metaphysis —
J __ |} —— Eriptysis ——{__U
Femur
(thighbone)
(a) Growing long bone (b) Mature long bone
As a bone matures, the epiphyseal plate
progresses to an epiphyseal line. (a)
Epiphyseal plates are visible in a growing
bone. (b) Epiphyseal lines are the
remnants of epiphyseal plates in a mature
bone.
How Bones Grow in Diameter
While bones are increasing in length, they are also increasing in diameter;
growth in diameter can continue even after longitudinal growth ceases. This
is called appositional growth. Osteoclasts resorb old bone that lines the
medullary cavity, while osteoblasts, via intramembranous ossification,
produce new bone tissue beneath the periosteum. The erosion of old bone
along the medullary cavity and the deposition of new bone beneath the
periosteum not only increase the diameter of the diaphysis but also increase
the diameter of the medullary cavity. This process is called modeling.
Bone Remodeling
The process in which matrix is resorbed on one surface of a bone and
deposited on another is known as bone modeling. Modeling primarily takes
place during a bone’s growth. However, in adult life, bone undergoes
remodeling, in which resorption of old or damaged bone takes place on the
same surface where osteoblasts lay new bone to replace that which is
resorbed. Injury, exercise, and other activities lead to remodeling. Those
influences are discussed later in the chapter, but even without injury or
exercise, about 5 to 10 percent of the skeleton is remodeled annually just by
destroying old bone and renewing it with fresh bone.
Note:
Diseases of the...
Skeletal System
Osteogenesis imperfecta (OI) is a genetic disease in which bones do not
form properly and therefore are fragile and break easily. It is also called
brittle bone disease. The disease is present from birth and affects a person
throughout life.
The genetic mutation that causes OI affects the body’s production of
collagen, one of the critical components of bone matrix. The severity of the
disease can range from mild to severe. Those with the most severe forms of
the disease sustain many more fractures than those with a mild form.
Frequent and multiple fractures typically lead to bone deformities and short
stature. Bowing of the long bones and curvature of the spine are also
common in people afflicted with OI. Curvature of the spine makes
breathing difficult because the lungs are compressed.
Because collagen is such an important structural protein in many parts of
the body, people with OI may also experience fragile skin, weak muscles,
loose joints, easy bruising, frequent nosebleeds, brittle teeth, blue sclera,
and hearing loss. There is no known cure for OI. Treatment focuses on
helping the person retain as much independence as possible while
minimizing fractures and maximizing mobility. Toward that end, safe
exercises, like swimming, in which the body is less likely to experience
collisions or compressive forces, are recommended. Braces to support legs,
ankles, knees, and wrists are used as needed. Canes, walkers, or
wheelchairs can also help compensate for weaknesses.
When bones do break, casts, splints, or wraps are used. In some cases,
metal rods may be surgically implanted into the long bones of the arms and
legs. Research is currently being conducted on using bisphosphonates to
treat OI. Smoking and being overweight are especially risky in people with
OI, since smoking is known to weaken bones, and extra body weight puts
additional stress on the bones.
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Watch this video to see how a bone grows.
Chapter Review
All bone formation is a replacement process. Embryos develop a
cartilaginous skeleton and various membranes. During development, these
are replaced by bone during the ossification process. In intramembranous
ossification, bone develops directly from sheets of mesenchymal connective
tissue. In endochondral ossification, bone develops by replacing hyaline
cartilage. Activity in the epiphyseal plate enables bones to grow in length.
Modeling allows bones to grow in diameter. Remodeling occurs as bone is
resorbed and replaced by new bone. Osteogenesis imperfecta is a genetic
disease in which collagen production is altered, resulting in fragile, brittle
bones.
Review Questions
Exercise:
Problem: Why is cartilage slow to heal?
a. because it eventually develops into bone
b. because it is semi-solid and flexible
c. because it does not have a blood supply
d. because endochondral ossification replaces all cartilage with bone
Solution:
C
Exercise:
Problem: Why are osteocytes spread out in bone tissue?
a. They develop from mesenchymal cells.
b. They are surrounded by osteoid.
c. They travel through the capillaries.
d. Formation of osteoid spreads out the osteoblasts that formed the
ossification centers.
Solution:
D
Exercise:
Problem:
In endochondral ossification, what happens to the chondrocytes?
a. They develop into osteocytes.
b. They die in the calcified matrix that surrounds them and form the
medullary cavity.
c. They grow and form the periosteum.
d. They group together to form the primary ossification center.
Solution:
B
Exercise:
Problem:
Which of the following bones is (are) formed by intramembranous
ossification?
a. the metatarsals
b. the femur
c. the ribs
d. the flat bones of the cranium
Solution:
D
Exercise:
Problem:Bones grow in length due to activity in the
a. epiphyseal plate
b. perichondrium
Cc. periosteum
d. medullary cavity
Solution:
A
Exercise:
Problem:Bones grow in diameter due to bone formation
a. in the medullary cavity
b. beneath the periosteum
c. in the epiphyseal plate
d. within the metaphysis
Solution:
B
Exercise:
Problem:
Which of the following represents the correct sequence of zones in the
epiphyseal plate?
a. proliferation, reserved, maturation, calcification
b. maturation, proliferation, reserved, calcification
c. calcification, maturation, proliferation, reserved
d. calcification, reserved, proliferation, maturation
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
In what ways do intramembranous and endochondral ossification
differ?
Solution:
In intramembranous ossification, bone develops directly from sheets of
mesenchymal connective tissue, but in endochondral ossification, bone
develops by replacing hyaline cartilage. Intramembranous ossification
is complete by the end of the adolescent growth spurt, while
endochondral ossification lasts into young adulthood. The flat bones of
the face, most of the cranial bones, and a good deal of the clavicles
(collarbones) are formed via intramembranous ossification, while
bones at the base of the skull and the long bones form via
endochondral ossification.
Exercise:
Problem:
Considering how a long bone develops, what are the similarities and
differences between a primary and a secondary ossification center?
Solution:
A single primary ossification center is present, during endochondral
ossification, deep in the periosteal collar. Like the primary ossification
center, secondary ossification centers are present during endochondral
ossification, but they form later, and there are two of them, one in each
epiphysis.
Glossary
endochondral ossification
process in which bone forms by replacing hyaline cartilage
epiphyseal line
completely ossified remnant of the epiphyseal plate
intramembranous ossification
process by which bone forms directly from mesenchymal tissue
modeling
process, during bone growth, by which bone is resorbed on one surface
of a bone and deposited on another
ossification
(also, osteogenesis) bone formation
ossification center
cluster of osteoblasts found in the early stages of intramembranous
ossification
osteoid
uncalcified bone matrix secreted by osteoblasts
perichondrium
membrane that covers cartilage
primary ossification center
region, deep in the periosteal collar, where bone development starts
during endochondral ossification
proliferative zone
region of the epiphyseal plate that makes new chondrocytes to replace
those that die at the diaphyseal end of the plate and contributes to
longitudinal growth of the epiphyseal plate
remodeling
process by which osteoclasts resorb old or damaged bone at the same
time as and on the same surface where osteoblasts form new bone to
replace that which is resorbed
reserve zone
region of the epiphyseal plate that anchors the plate to the osseous
tissue of the epiphysis
secondary ossification center
region of bone development in the epiphyses
zone of calcified matrix
region of the epiphyseal plate closest to the diaphyseal end; functions
to connect the epiphyseal plate to the diaphysis
zone of maturation and hypertrophy
region of the epiphyseal plate where chondrocytes from the
proliferative zone grow and mature and contribute to the longitudinal
growth of the epiphyseal plate
Fractures: Bone Repair
By the end of this section, you will be able to:
e Differentiate among the different types of fractures
e Describe the steps involved in bone repair
A fracture is a broken bone. It will heal whether or not a physician resets it
in its anatomical position. If the bone is not reset correctly, the healing
process will keep the bone in its deformed position.
When a broken bone is manipulated and set into its natural position without
surgery, the procedure is called a closed reduction. Open reduction
requires surgery to expose the fracture and reset the bone. While some
fractures can be minor, others are quite severe and result in grave
complications. For example, a fractured diaphysis of the femur has the
potential to release fat globules into the bloodstream. These can become
lodged in the capillary beds of the lungs, leading to respiratory distress and
if not treated quickly, death.
Types of Fractures
Fractures are classified by their complexity, location, and other features
({link]). [link] outlines common types of fractures. Some fractures may be
described using more than one term because it may have the features of
more than one type (e.g., an open transverse fracture).
Types of Fractures
Open Transverse
Compare healthy bone with different types
of fractures: (a) closed fracture, (b) open
fracture, (c) transverse fracture, (d) spiral
fracture, (e) comminuted fracture, (f)
impacted fracture, (g) greenstick fracture,
and (h) oblique fracture.
Types of Fractures
Type of
fracture
Transverse
Oblique
Spiral
Comminuted
Impacted
Greenstick
Open (or
compound)
Description
Occurs straight across the long axis of the bone
Occurs at an angle that is not 90 degrees
Bone segments are pulled apart as a result of a
twisting motion
Several breaks result in many small pieces between
two large segments
One fragment is driven into the other, usually as a
result of compression
A partial fracture in which only one side of the
bone is broken
A fracture in which at least one end of the broken
bone tears through the skin; carries a high risk of
infection
Types of Fractures
Type of
fracture Description
oe kon A fracture in which the skin remains intact
simple)
Bone Repair
When a bone breaks, blood flows from any vessel torn by the fracture.
These vessels could be in the periosteum, osteons, and/or medullary cavity.
The blood begins to clot, and about six to eight hours after the fracture, the
clotting blood has formed a fracture hematoma ([link]a). The disruption
of blood flow to the bone results in the death of bone cells around the
fracture.
Stages in Fracture Repair
Hematoma New blood vessels
cae
eres
Bony callus
Spongy bone
trabecula of spongy bone
(b) (c) (d)
The healing of a bone fracture follows a series of
progressive steps: (a) A fracture hematoma forms. (b)
Internal and external calli form. (c) Cartilage of the
calli is replaced by trabecular bone. (d) Remodeling
occurs.
Within about 48 hours after the fracture, chondrocytes from the endosteum
have created an internal callus (plural = calli) by secreting a
fibrocartilaginous matrix between the two ends of the broken bone, while
the periosteal chondrocytes and osteoblasts create an external callus of
hyaline cartilage and bone, respectively, around the outside of the break
({link]b). This stabilizes the fracture.
Over the next several weeks, osteoclasts resorb the dead bone; osteogenic
cells become active, divide, and differentiate into osteoblasts. The cartilage
in the calli is replaced by trabecular bone via endochondral ossification
((link]c).
Eventually, the internal and external calli unite, compact bone replaces
spongy bone at the outer margins of the fracture, and healing is complete. A
slight swelling may remain on the outer surface of the bone, but quite often,
that region undergoes remodeling ([link]d), and no external evidence of the
fracture remains.
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Visit this website to review different types of fractures and then take a
short self-assessment quiz.
Chapter Review
Fractured bones may be repaired by closed reduction or open reduction.
Fractures are classified by their complexity, location, and other features.
Common types of fractures are transverse, oblique, spiral, comminuted,
impacted, greenstick, open (or compound), and closed (or simple). Healing
of fractures begins with the formation of a hematoma, followed by internal
and external calli. Osteoclasts resorb dead bone, while osteoblasts create
new bone that replaces the cartilage in the calli. The calli eventually unite,
remodeling occurs, and healing is complete.
Review Questions
Exercise:
Problem:A fracture can be both
a. open and closed
b. open and transverse
c. transverse and greenstick
d. greenstick and comminuted
Solution:
B
Exercise:
Problem:
How can a fractured diaphysis release fat globules into the
bloodstream?
a. The bone pierces fat stores in the skin.
b. The yellow marrow in the diaphysis is exposed and damaged.
c. The injury triggers the body to release fat from healthy bones.
d. The red marrow in the fractured bone releases fat to heal the
fracture.
Solution:
B
Exercise:
Problem:In a compound fracture,
a. the break occurs at an angle to the bone
b. the broken bone does not tear the skin
c. one fragment of broken bone is compressed into the other
d. broken bone pierces the skin
Solution:
D
Exercise:
Problem:The internal and external calli are replaced by
a. hyaline cartilage
b. trabecular bone
c. osteogenic cells
d. osteoclasts
Solution:
B
Exercise:
Problem:
The first type of bone to form during fracture repair is bone.
a. compact
b. lamellar
c. spongy
d. dense
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
What is the difference between closed reduction and open reduction?
In what type of fracture would closed reduction most likely occur? In
what type of fracture would open reduction most likely occur?
Solution:
In closed reduction, the broken ends of a fractured bone can be reset
without surgery. Open reduction requires surgery to return the broken
ends of the bone to their correct anatomical position. A partial fracture
would likely require closed reduction. A compound fracture would
require open reduction.
Exercise:
Problem:
In terms of origin and composition, what are the differences between
an internal callus and an external callus?
Solution:
The internal callus is produced by cells in the endosteum and is
composed of a fibrocartilaginous matrix. The external callus is
produced by cells in the periosteum and consists of hyaline cartilage
and bone.
Glossary
closed reduction
manual manipulation of a broken bone to set it into its natural position
without surgery
external callus
collar of hyaline cartilage and bone that forms around the outside of a
fracture
fracture
broken bone
fracture hematoma
blood clot that forms at the site of a broken bone
internal callus
fibrocartilaginous matrix, in the endosteal region, between the two
ends of a broken bone
open reduction
surgical exposure of a bone to reset a fracture
Divisions of the Skeletal System
By the end of this section, you will be able to:
e Discuss the functions of the skeletal system
e Distinguish between the axial skeleton and appendicular skeleton
¢ Define the axial skeleton and its components
¢ Define the appendicular skeleton and its components
The skeletal system includes all of the bones, cartilages, and ligaments of
the body that support and give shape to the body and body structures. The
skeleton consists of the bones of the body. For adults, there are 206 bones
in the skeleton. Younger individuals have higher numbers of bones because
some bones fuse together during childhood and adolescence to form an
adult bone. The primary functions of the skeleton are to provide a rigid,
internal structure that can support the weight of the body against the force
of gravity, and to provide a structure upon which muscles can act to
produce movements of the body. The lower portion of the skeleton is
specialized for stability during walking or running. In contrast, the upper
skeleton has greater mobility and ranges of motion, features that allow you
to lift and carry objects or turn your head and trunk.
In addition to providing for support and movements of the body, the
skeleton has protective and storage functions. It protects the internal organs,
including the brain, spinal cord, heart, lungs, and pelvic organs. The bones
of the skeleton serve as the primary storage site for important minerals such
as calcium and phosphate. The bone marrow found within bones stores fat
and houses the blood-cell producing tissue of the body.
The skeleton is subdivided into two major divisions—the axial and
appendicular.
The Axial Skeleton
The skeleton is subdivided into two major divisions—the axial and
appendicular. The axial skeleton forms the vertical, central axis of the body
and includes all bones of the head, neck, chest, and back ((link]). It serves
to protect the brain, spinal cord, heart, and lungs. It also serves as the
attachment site for muscles that move the head, neck, and back, and for
muscles that act across the shoulder and hip joints to move their
corresponding limbs.
The axial skeleton of the adult consists of 80 bones, including the skull, the
vertebral column, and the thoracic cage. The skull is formed by 22 bones.
Also associated with the head are an additional seven bones, including the
hyoid bone and the ear ossicles (three small bones found in each middle
ear). The vertebral column consists of 24 bones, each called a vertebra,
plus the sacrum and coccyx. The thoracic cage includes the 12 pairs of
ribs, and the sternum, the flattened bone of the anterior chest.
Axial and Appendicular Skeleton
Skull
Cranial portion
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column Gor . Humerus column
Pelvic J er. = | one fe. h Pelvic
girdle WZ | Radius wey, girdle
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Lower limb
———————- Femur
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Key
Axial skeleton
Appendicular
skeleton
Tarsals
\_}— Metatarsals
Phalanges
Anterior view Posterior view
The axial skeleton supports the head, neck, back, and
chest and thus forms the vertical axis of the body. It
consists of the skull, vertebral column (including the
sacrum and coccyx), and the thoracic cage, formed by
the ribs and sternum. The appendicular skeleton is made
up of all bones of the upper and lower limbs.
The Appendicular Skeleton
The appendicular skeleton includes all bones of the upper and lower
limbs, plus the bones that attach each limb to the axial skeleton. There are
126 bones in the appendicular skeleton of an adult. The bones of the
appendicular skeleton are covered in a separate chapter.
Chapter Review
The skeletal system includes all of the bones, cartilages, and ligaments of
the body. It serves to support the body, protect the brain and other internal
organs, and provides a rigid structure upon which muscles can pull to
generate body movements. It also stores fat and the tissue responsible for
the production of blood cells. The skeleton is subdivided into two parts. The
axial skeleton forms a vertical axis that includes the head, neck, back, and
chest. It has 80 bones and consists of the skull, vertebral column, and
thoracic cage. The adult vertebral column consists of 24 vertebrae plus the
sacrum and coccyx. The thoracic cage is formed by 12 pairs of ribs and the
sternum. The appendicular skeleton consists of 126 bones in the adult and
includes all of the bones of the upper and lower limbs plus the bones that
anchor each limb to the axial skeleton.
Review Questions
Exercise:
Problem: Which of the following is part of the axial skeleton?
a. shoulder bones
b. thigh bone
c. foot bones
d. vertebral column
Solution:
D
Exercise:
Problem: Which of the following is a function of the axial skeleton?
a. allows for movement of the wrist and hand
b. protects nerves and blood vessels at the elbow
c. supports trunk of body
d. allows for movements of the ankle and foot
Solution:
C
Exercise:
Problem:The axial skeleton
a. consists of 126 bones
b. forms the vertical axis of the body
c. includes all bones of the body trunk and limbs
d. includes only the bones of the lower limbs
Solution:
B
Critical Thinking Question
Exercise:
Problem: Define the two divisions of the skeleton.
Solution:
The axial skeleton forms the vertical axis of the body and includes the
bones of the head, neck, back, and chest of the body. It consists of 80
bones that include the skull, vertebral column, and thoracic cage. The
appendicular skeleton consists of 126 bones and includes all bones of
the upper and lower limbs.
Exercise:
Problem: Discuss the functions of the axial skeleton.
Solution:
The axial skeleton supports the head, neck, back, and chest of the body
and allows for movements of these body regions. It also gives bony
protections for the brain, spinal cord, heart, and lungs; stores fat and
minerals; and houses the blood-cell producing tissue.
Glossary
appendicular skeleton
all bones of the upper and lower limbs, plus the girdle bones that
attach each limb to the axial skeleton
axial skeleton
central, vertical axis of the body, including the skull, vertebral column,
and thoracic cage
coccyx
small bone located at inferior end of the adult vertebral column that is
formed by the fusion of four coccygeal vertebrae; also referred to as
the “tailbone”
ear ossicles
three small bones located in the middle ear cavity that serve to transmit
sound vibrations to the inner ear
hyoid bone
small, U-shaped bone located in upper neck that does not contact any
other bone
ribs
thin, curved bones of the chest wall
sacrum
single bone located near the inferior end of the adult vertebral column
that is formed by the fusion of five sacral vertebrae; forms the
posterior portion of the pelvis
skeleton
bones of the body
skull
bony structure that forms the head, face, and jaws, and protects the
brain; consists of 22 bones
sternum
flattened bone located at the center of the anterior chest
thoracic cage
consists of 12 pairs of ribs and sternum
vertebra
individual bone in the neck and back regions of the vertebral column
vertebral column
entire sequence of bones that extend from the skull to the tailbone
The Skull
By the end of this section, you will be able to:
List and identify the bones of the brain case and face
Locate the major suture lines of the skull and name the bones
associated with each
Locate and define the boundaries of the anterior, middle, and posterior
cranial fossae, the temporal fossa, and infratemporal fossa
Define the paranasal sinuses and identify the location of each
Name the bones that make up the walls of the orbit and identify the
openings associated with the orbit
Identify the bones and structures that form the nasal septum and nasal
conchae, and locate the hyoid bone
Identify the bony openings of the skull
The cranium (skull) is the skeletal structure of the head that supports the
face and protects the brain. It is subdivided into the facial bones and the
brain case, or cranial vault ([link]). The facial bones underlie the facial
structures, form the nasal cavity, enclose the eyeballs, and support the teeth
of the upper and lower jaws. The rounded brain case surrounds and protects
the brain and houses the middle and inner ear structures.
In the adult, the skull consists of 22 individual bones, 21 of which are
immobile and united into a single unit. The 22nd bone is the mandible
(lower jaw), which is the only moveable bone of the skull.
Parts of the Skull
Brain case
Facial bones
The skull consists of the rounded brain
case that houses the brain and the facial
bones that form the upper and lower jaws,
nose, orbits, and other facial structures.
Watch this video to view a rotating and exploded skull, with color-coded
bones. Which bone (yellow) is centrally located and joins with most of the
other bones of the skull?
Anterior View of Skull
The anterior skull consists of the facial bones and provides the bony support
for the eyes and structures of the face. This view of the skull is dominated
by the openings of the orbits and the nasal cavity. Also seen are the upper
and lower jaws, with their respective teeth ([link]).
The orbit is the bony socket that houses the eyeball and muscles that move
the eyeball or open the upper eyelid. The upper margin of the anterior orbit
is the supraorbital margin. Located near the midpoint of the supraorbital
margin is a small opening called the supraorbital foramen. This provides
for passage of a sensory nerve to the skin of the forehead. Below the orbit is
the infraorbital foramen, which is the point of emergence for a sensory
nerve that supplies the anterior face below the orbit.
Anterior View of Skull
Coronal suture
Glabella Frontal bone
Parietal bone
Supraorbital foramen
Supraorbital margin .
Orbit
Sphenoid bone :
Optic canal
Temporal bone
Superior orbital fissure
Ethmoid bone
Lacrimal bone
Nasal bone
. Inferior orbital fissure
Palatine bone
Zygomatic bone
Nasal septum:
Perpendicular plate Infraorbital foramen
of ethmoid bone
Vomer bone Middle nasal concha
Inferior nasal concha
Maxilla
Alveolar process of maxilla
Alveolar process
. / of mandible
Mental foramen ——————__>) € y
—_—_ Mandible
Anterior view
An anterior view of the skull shows the bones that
form the forehead, orbits (eye sockets), nasal cavity,
nasal septum, and upper and lower jaws.
Inside the nasal area of the skull, the nasal cavity is divided into halves by
the nasal septum. The upper portion of the nasal septum is formed by the
perpendicular plate of the ethmoid bone and the lower portion is the
vomer bone. Each side of the nasal cavity is triangular in shape, with a
broad inferior space that narrows superiorly. When looking into the nasal
cavity from the front of the skull, two bony plates are seen projecting from
each lateral wall. The larger of these is the inferior nasal concha, an
independent bone of the skull. Located just above the inferior concha is the
middle nasal concha, which is part of the ethmoid bone. A third bony
plate, also part of the ethmoid bone, is the superior nasal concha. It is
much smaller and out of sight, above the middle concha. The superior nasal
concha is located just lateral to the perpendicular plate, in the upper nasal
cavity.
Lateral View of Skull
A view of the lateral skull is dominated by the large, rounded brain case
above and the upper and lower jaws with their teeth below ([link]).
Separating these areas is the bridge of bone called the zygomatic arch. The
zygomatic arch is the bony arch on the side of skull that spans from the
area of the cheek to just above the ear canal. It is formed by the junction of
two bony processes: a short anterior component, the temporal process of
the zygomatic bone (the cheekbone) and a longer posterior portion, the
zygomatic process of the temporal bone, extending forward from the
temporal bone. Thus the temporal process (anteriorly) and the zygomatic
process (posteriorly) join together, like the two ends of a drawbridge, to
form the zygomatic arch. One of the major muscles that pulls the mandible
upward during biting and chewing arises from the zygomatic arch.
On the lateral side of the brain case, above the level of the zygomatic arch,
is a Shallow space called the temporal fossa. Below the level of the
zygomatic arch and deep to the vertical portion of the mandible is another
space called the infratemporal fossa. Both the temporal fossa and
infratemporal fossa contain muscles that act on the mandible during
chewing.
Lateral View of Skull
Zygomatic arch
Coronal suture
Parietal bone Frontal bone
Greater wing of
sphenoid bone
Si tl
quamous suture Ethmoid bone
Temporal bone Lacrimal bone
Squamous ‘
temporal Lacrimal fossa
Zygomatic process ~L_
External acoustic
meatus
Lambdoid suture
Nasal bone
Zygomatic bone
Mastoid portion Temporal process
Styloid process
Mastoid process
Maxilla
Articular tubercle
Occipital bone Mandibular fossa
Mandible Mental
protuberance
of mandible
Right lateral view
The lateral skull shows the large rounded brain case,
zygomatic arch, and the upper and lower jaws. The
zygomatic arch is formed jointly by the zygomatic
process of the temporal bone and the temporal process
of the zygomatic bone. The shallow space above the
zygomatic arch is the temporal fossa. The space
inferior to the zygomatic arch and deep to the
posterior mandible is the infratemporal fossa.
Bones of the Brain Case
The brain case contains and protects the brain. The interior space that is
almost completely occupied by the brain is called the cranial cavity. This
cavity is bounded superiorly by the rounded top of the skull, which is called
the calvaria (skullcap), and the lateral and posterior sides of the skull. The
bones that form the top and sides of the brain case are usually referred to as
the “flat” bones of the skull.
The floor of the brain case is referred to as the base of the skull. This is a
complex area that varies in depth and has numerous openings for the
passage of cranial nerves, blood vessels, and the spinal cord. Inside the
skull, the base is subdivided into three large spaces, called the anterior
cranial fossa, middle cranial fossa, and posterior cranial fossa (fossa =
“trench or ditch”) ([link]). From anterior to posterior, the fossae increase in
depth. The shape and depth of each fossa corresponds to the shape and size
of the brain region that each houses. The boundaries and openings of the
cranial fossae (singular = fossa) will be described in a later section.
Cranial Fossae
Anterior cranial
fossa
Middle cranial
fossa
Posterior cranial
fossa
Brain within —____.
cranial cavity ;
Lateral view
The bones of the brain case
surround and protect the brain,
which occupies the cranial cavity.
The base of the brain case, which
forms the floor of cranial cavity, is
subdivided into the shallow anterior
cranial fossa, the middle cranial
fossa, and the deep posterior cranial
fossa.
The brain case consists of eight bones. These include the paired parietal and
temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid
bones.
Parietal Bone
The parietal bone forms most of the upper lateral side of the skull (see
[link]). These are paired bones, with the right and left parietal bones joining
together at the top of the skull. Each parietal bone is also bounded
anteriorly by the frontal bone, inferiorly by the temporal bone, and
posteriorly by the occipital bone.
Temporal Bone
The temporal bone forms the lower lateral side of the skull (see [link]).
Common wisdom has it that the temporal bone (temporal = “time”’) is so
named because this area of the head (the temple) is where hair typically first
turns gray, indicating the passage of time.
The temporal bone is subdivided into several regions ({link]). The flattened,
upper portion is the squamous portion of the temporal bone. Below this area
and projecting anteriorly is the zygomatic process of the temporal bone,
which forms the posterior portion of the zygomatic arch. Posteriorly is the
mastoid portion of the temporal bone. Projecting inferiorly from this region
is a large prominence, the mastoid process, which serves as a muscle
attachment site. The mastoid process can easily be felt on the side of the
head just behind your earlobe. On the interior of the skull, the petrous
portion of each temporal bone forms the prominent, diagonally oriented
petrous ridge in the floor of the cranial cavity. Located inside each petrous
ridge are small cavities that house the structures of the middle and inner
ears.
Temporal Bone
External acoustic Squamous
meatus portion
Zygomatic
process
Articular
Masicid tubercle
pomiat Mandibular
fossa
Mastoid process Styloid process
A lateral view of the isolated temporal bone shows
the squamous, mastoid, and zygomatic portions of
the temporal bone.
Important landmarks of the temporal bone, as shown in [link], include the
following:
e External acoustic meatus (ear canal)—This is the large opening on
the lateral side of the skull that is associated with the ear.
¢ Internal acoustic meatus—This opening is located inside the cranial
cavity, on the medial side of the petrous ridge. It connects to the
middle and inner ear cavities of the temporal bone.
e Mandibular fossa—This is the deep, oval-shaped depression located
on the external base of the skull, just in front of the external acoustic
meatus. The mandible (lower jaw) joins with the skull at this site as
part of the temporomandibular joint, which allows for movements of
the mandible during opening and closing of the mouth.
e Articular tubercle—The smooth ridge located immediately anterior
to the mandibular fossa. Both the articular tubercle and mandibular
fossa contribute to the temporomandibular joint, the joint that provides
for movements between the temporal bone of the skull and the
mandible.
e Styloid process—Posterior to the mandibular fossa on the external
base of the skull is an elongated, downward bony projection called the
styloid process, so named because of its resemblance to a stylus (a pen
or writing tool). This structure serves as an attachment site for several
small muscles and for a ligament that supports the hyoid bone of the
neck. (See also [link].)
¢ Stylomastoid foramen—This small opening is located between the
styloid process and mastoid process. This is the point of exit for the
cranial nerve that supplies the facial muscles.
¢ Carotid canal—tThe carotid canal is a zig-zag shaped tunnel that
provides passage through the base of the skull for one of the major
arteries that supplies the brain. Its entrance is located on the outside
base of the skull, anteromedial to the styloid process. The canal then
runs anteromedially within the bony base of the skull, and then turns
upward to its exit in the floor of the middle cranial cavity, above the
foramen lacerum.
External and Internal Views of Base of Skull
Maxilla:
Palatine process
Zygomatic bone
Palatine bone
(horizontal plate)
Zygomatic arch
Medial and lateral
pterygoid plates
Articular tubercle
Sphenoid bone Foramen ovale
Foramen spinosum Mandibular fossa
Foramen lacerum External auditory meatus
Jugular foramen Mastoid process
Occipital condyle Styloid process
Temporal bone Stylomastoid foramen
Foramen magnum F
g Entrance to carotid canal
Occipital bone
Superior nuchal line External occipital
protuberance
(a) Inferior view
Transverse plane
Frontal bone
Ethmoid bone:
Crista galli
Superior orbital Cribriform plate
fissure Sphenoid bone:
Lesser wing
Hypophyseal eit eels
Foramen rotundum turcica
Foramen lacerum
and exit of carotid
canal
Foramen ovale
Foramen spinosum
Internal acoustic meatus
Temporal bone
Hypoglossal canal
Petrous portion
Foramen magnum (petrous ridge)
Occipital bone Jugular foramen
Parietal bone
(b) Superior view
(a) The hard palate is formed anteriorly by the palatine
processes of the maxilla bones and posteriorly by the
horizontal plate of the palatine bones. (b) The complex floor of
the cranial cavity is formed by the frontal, ethmoid, sphenoid,
temporal, and occipital bones. The lesser wing of the sphenoid
bone separates the anterior and middle cranial fossae. The
petrous ridge (petrous portion of temporal bone) separates the
middle and posterior cranial fossae.
Frontal Bone
The frontal bone is the single bone that forms the forehead. At its anterior
midline, between the eyebrows, there is a slight depression called the
glabella (see [link]). The frontal bone also forms the supraorbital margin of
the orbit. Near the middle of this margin, is the supraorbital foramen, the
opening that provides passage for a sensory nerve to the forehead. The
frontal bone is thickened just above each supraorbital margin, forming
rounded brow ridges. These are located just behind your eyebrows and vary
in size among individuals, although they are generally larger in males.
Inside the cranial cavity, the frontal bone extends posteriorly. This flattened
region forms both the roof of the orbit below and the floor of the anterior
cranial cavity above (see [link]b).
Occipital Bone
The occipital bone is the single bone that forms the posterior skull and
posterior base of the cranial cavity ([link]; see also [link]). On its outside
surface, at the posterior midline, is a small protrusion called the external
occipital protuberance, which serves as an attachment site for a ligament
of the posterior neck. Lateral to either side of this bump is a superior
nuchal line (nuchal = “nape” or “posterior neck”). The nuchal lines
represent the most superior point at which muscles of the neck attach to the
skull, with only the scalp covering the skull above these lines. On the base
of the skull, the occipital bone contains the large opening of the foramen
magnum, which allows for passage of the spinal cord as it exits the skull.
On either side of the foramen magnum is an oval-shaped occipital condyle.
These condyles form joints with the first cervical vertebra and thus support
the skull on top of the vertebral column.
Posterior View of Skull
Parietal bones Sagittal suture
Lambdoid
suture
Occipital bone External
occipital
protuberance
Temporal bone Superior
nuchal line
Mastoid process Occipital
condyle
Foramen magnum
Zygomatic
bone
Posterior view
This view of the posterior skull shows
attachment sites for muscles and joints that
support the skull.
Sphenoid Bone
The sphenoid bone is a single, complex bone of the central skull ([{link]). It
serves as a “keystone” bone, because it joins with almost every other bone
of the skull. The sphenoid forms much of the base of the central skull (see
[link]) and also extends laterally to contribute to the sides of the skull (see
[link]). Inside the cranial cavity, the right and left lesser wings of the
sphenoid bone, which resemble the wings of a flying bird, form the lip of a
prominent ridge that marks the boundary between the anterior and middle
cranial fossae. The sella turcica (“Turkish saddle”) is located at the midline
of the middle cranial fossa. This bony region of the sphenoid bone is named
for its resemblance to the horse saddles used by the Ottoman Turks, with a
high back and a tall front. The rounded depression in the floor of the sella
turcica is the hypophyseal (pituitary) fossa, which houses the pea-sized
pituitary (hypophyseal) gland. The greater wings of the sphenoid bone
extend laterally to either side away from the sella turcica, where they form
the anterior floor of the middle cranial fossa. The greater wing is best seen
on the outside of the lateral skull, where it forms a rectangular area
immediately anterior to the squamous portion of the temporal bone.
On the inferior aspect of the skull, each half of the sphenoid bone forms two
thin, vertically oriented bony plates. These are the medial pterygoid plate
and lateral pterygoid plate (pterygoid = “wing-shaped”). The right and left
medial pterygoid plates form the posterior, lateral walls of the nasal cavity.
The somewhat larger lateral pterygoid plates serve as attachment sites for
chewing muscles that fill the infratemporal space and act on the mandible.
Sphenoid Bone
Superior
orbital
fissure
Foramen
rotundum
< 'y Foramen
ovale
Ww
“| Foramen
spinosum
Hypophyseal
fossa of sella
turcica
Body of sphenoid
(a) Superior view
Body of sphenoid Lesser
wing
Superior
orbital
fissure
Pterygoid
plates
(b) Posterior view
Shown in isolation in (a) superior and (b)
posterior views, the sphenoid bone is a
single midline bone that forms the
anterior walls and floor of the middle
cranial fossa. It has a pair of lesser wings
and a pair of greater wings. The sella
turcica surrounds the hypophyseal fossa.
Projecting downward are the medial and
lateral pterygoid plates. The sphenoid has
multiple openings for the passage of
nerves and blood vessels, including the
optic canal, superior orbital fissure,
foramen rotundum, foramen ovale, and
foramen spinosum.
Ethmoid Bone
The ethmoid bone is a single, midline bone that forms the roof and lateral
walls of the upper nasal cavity, the upper portion of the nasal septum, and
contributes to the medial wall of the orbit ({link] and [link]). On the interior
of the skull, the ethmoid also forms a portion of the floor of the anterior
cranial cavity (see [link |b).
Within the nasal cavity, the perpendicular plate of the ethmoid bone forms
the upper portion of the nasal septum. The ethmoid bone also forms the
lateral walls of the upper nasal cavity. Extending from each lateral wall are
the superior nasal concha and middle nasal concha, which are thin, curved
projections that extend into the nasal cavity ({link]).
In the cranial cavity, the ethmoid bone forms a small area at the midline in
the floor of the anterior cranial fossa. This region also forms the narrow
roof of the underlying nasal cavity. This portion of the ethmoid bone
consists of two parts, the crista galli and cribriform plates. The crista galli
(“rooster’s comb or crest”) is a small upward bony projection located at the
midline. It functions as an anterior attachment point for one of the covering
layers of the brain. To either side of the crista galli is the cribriform plate
(cribrum = “sieve”), a small, flattened area with numerous small openings
termed olfactory foramina. Small nerve branches from the olfactory areas of
the nasal cavity pass through these openings to enter the brain.
The lateral portions of the ethmoid bone are located between the orbit and
upper nasal cavity, and thus form the lateral nasal cavity wall and a portion
of the medial orbit wall. Located inside this portion of the ethmoid bone are
several small, air-filled spaces that are part of the paranasal sinus system of
the skull.
Sagittal Section of Skull
Sella turcica:
Hypophyseal fossa
Parietal bone Crista galli
Frontal sinus
Cribriform plate
Perpendicular plate
Temporal bone Nasal bone
F Sphenoid bone
Internal acoustic
meatus Sphenoid sinus
Hypoglossal canal Inferior nasal
concha
Occipital bone
Vomer
Styloid process
bs a Maxilla
Medial and lateral
pterygoid plates Palatine bone
Mandibular foramen Mylohyoid line
Mandible
Hyoid bone
This midline view of the sagittally sectioned skull shows the
nasal septum.
Ethmoid Bone
Superior
Crista galli
Cribriform plate
Ethmoid
air cells
Superior nasal
; concha
Nasal cavity
Middle nasal concha
Medial wall
of orbit Perpendicular plate
Inferior
The unpaired ethmoid bone is located at the
midline within the central skull. It has an upward
projection, the crista galli, and a downward
projection, the perpendicular plate, which forms
the upper nasal septum. The cribriform plates form
both the roof of the nasal cavity and a portion of
the anterior cranial fossa floor. The lateral sides of
the ethmoid bone form the lateral walls of the
upper nasal cavity, part of the medial orbit wall,
and give rise to the superior and middle nasal
conchae. The ethmoid bone also contains the
ethmoid air cells.
Lateral Wall of Nasal Cavity
Ethmoid bone:
Superior nasal
concha
Middle nasal
concha
Inferior nasal
concha
Sphenoidal
sinus
Medial view
The three nasal conchae are curved bones that
project from the lateral walls of the nasal cavity.
The superior nasal concha and middle nasal
concha are parts of the ethmoid bone. The inferior
nasal concha is an independent bone of the skull.
Sutures of the Skull
A suture is an immobile joint between adjacent bones of the skull. The
narrow gap between the bones is filled with dense, fibrous connective tissue
that unites the bones. The long sutures located between the bones of the
brain case are not straight, but instead follow irregular, tightly twisting
paths. These twisting lines serve to tightly interlock the adjacent bones, thus
adding strength to the skull for brain protection.
The two suture lines seen on the top of the skull are the coronal and sagittal
sutures. The coronal suture runs from side to side across the skull, within
the coronal plane of section (see [link]). It joins the frontal bone to the right
and left parietal bones. The sagittal suture extends posteriorly from the
coronal suture, running along the midline at the top of the skull in the
Sagittal plane of section (see [link]). It unites the right and left parietal
bones. On the posterior skull, the sagittal suture terminates by joining the
lambdoid suture. The lambdoid suture extends downward and laterally to
either side away from its junction with the sagittal suture. The lambdoid
suture joins the occipital bone to the right and left parietal and temporal
bones. This suture is named for its upside-down "V" shape, which
resembles the capital letter version of the Greek letter lambda (A). The
squamous suture is located on the lateral skull. It unites the squamous
portion of the temporal bone with the parietal bone (see [link]). At the
intersection of four bones is the pterion, a small, capital-H-shaped suture
line region that unites the frontal bone, parietal bone, squamous portion of
the temporal bone, and greater wing of the sphenoid bone. It is the weakest
part of the skull. The pterion is located approximately two finger widths
above the zygomatic arch and a thumb’s width posterior to the upward
portion of the zygomatic bone.
Note:
Disorders of the...
Skeletal System
Head and traumatic brain injuries are major causes of immediate death and
disability, with bleeding and infections as possible additional
complications. According to the Centers for Disease Control and
Prevention (2010), approximately 30 percent of all injury-related deaths in
the United States are caused by head injuries. The majority of head injuries
involve falls. They are most common among young children (ages 0-4
years), adolescents (15-19 years), and the elderly (over 65 years).
Additional causes vary, but prominent among these are automobile and
motorcycle accidents.
Strong blows to the brain-case portion of the skull can produce fractures.
These may result in bleeding inside the skull with subsequent injury to the
brain. The most common is a linear skull fracture, in which fracture lines
radiate from the point of impact. Other fracture types include a
comminuted fracture, in which the bone is broken into several pieces at the
point of impact, or a depressed fracture, in which the fractured bone is
pushed inward. In a contrecoup (counterblow) fracture, the bone at the
point of impact is not broken, but instead a fracture occurs on the opposite
side of the skull. Fractures of the occipital bone at the base of the skull can
occur in this manner, producing a basilar fracture that can damage the
artery that passes through the carotid canal.
A blow to the lateral side of the head may fracture the bones of the pterion.
The pterion is an important clinical landmark because located immediately
deep to it on the inside of the skull is a major branch of an artery that
supplies the skull and covering layers of the brain. A strong blow to this
region can fracture the bones around the pterion. If the underlying artery is
damaged, bleeding can cause the formation of a hematoma (collection of
blood) between the brain and interior of the skull. As blood accumulates, it
will put pressure on the brain. Symptoms associated with a hematoma may
not be apparent immediately following the injury, but if untreated, blood
accumulation will exert increasing pressure on the brain and can result in
death within a few hours.
Note:
[= [=]
‘all
“eat a ee
5 openstax COLLEGE”
View this animation to see how a blow to the head may produce a
contrecoup (counterblow) fracture of the basilar portion of the occipital
bone on the base of the skull. Why may a basilar fracture be life
threatening?
Facial Bones of the Skull
The facial bones of the skull form the upper and lower jaws, the nose, nasal
cavity and nasal septum, and the orbit. The facial bones include 14 bones,
with six paired bones and two unpaired bones. The paired bones are the
maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae
bones. The unpaired bones are the vomer and mandible bones. Although
classified with the brain-case bones, the ethmoid bone also contributes to
the nasal septum and the walls of the nasal cavity and orbit.
Maxillary Bone
The maxillary bone, often referred to simply as the maxilla (plural =
maxillae), is one of a pair that together form the upper jaw, much of the
hard palate, the medial floor of the orbit, and the lateral base of the nose
(see [link]). The curved, inferior margin of the maxillary bone that forms
the upper jaw and contains the upper teeth is the alveolar process of the
maxilla ({link]). Each tooth is anchored into a deep socket called an
alveolus. On the anterior maxilla, just below the orbit, is the infraorbital
foramen. This is the point of exit for a sensory nerve that supplies the nose,
upper lip, and anterior cheek. On the inferior skull, the palatine process
from each maxillary bone can be seen joining together at the midline to
form the anterior three-quarters of the hard palate (see [link]a). The hard
palate is the bony plate that forms the roof of the mouth and floor of the
nasal cavity, separating the oral and nasal cavities.
Maxillary Bone
Articulates with
frontal bone
Infraorbital
foramen
Alveolar
process
Right lateral view
The maxillary bone forms the upper jaw and
supports the upper teeth. Each maxilla also
forms the lateral floor of each orbit and the
majority of the hard palate.
Palatine Bone
The palatine bone is one of a pair of irregularly shaped bones that
contribute small areas to the lateral walls of the nasal cavity and the medial
wall of each orbit. The largest region of each of the palatine bone is the
horizontal plate. The plates from the right and left palatine bones join
together at the midline to form the posterior quarter of the hard palate (see
[link]a). Thus, the palatine bones are best seen in an inferior view of the
skull and hard palate.
Note:
Homeostatic Imbalances
Cleft Lip and Cleft Palate
During embryonic development, the right and left maxilla bones come
together at the midline to form the upper jaw. At the same time, the muscle
and skin overlying these bones join together to form the upper lip. Inside
the mouth, the palatine processes of the maxilla bones, along with the
horizontal plates of the right and left palatine bones, join together to form
the hard palate. If an error occurs in these developmental processes, a birth
defect of cleft lip or cleft palate may result.
Cleft lip is a common development defect that affects approximately
1:1000 births, most of which are male. This defect involves a partial or
complete failure of the right and left portions of the upper lip to fuse
together, leaving a cleft (gap).
A more severe developmental defect is cleft palate, which affects the hard
palate. The hard palate is the bony structure that separates the nasal cavity
from the oral cavity. It is formed during embryonic development by the
midline fusion of the horizontal plates from the right and left palatine
bones and the palatine processes of the maxilla bones. Cleft palate affects
approximately 1:2500 births and is more common in females. It results
from a failure of the two halves of the hard palate to completely come
together and fuse at the midline, thus leaving a gap between them. This gap
allows for communication between the nasal and oral cavities. In severe
cases, the bony gap continues into the anterior upper jaw where the
alveolar processes of the maxilla bones also do not properly join together
above the front teeth. If this occurs, a cleft lip will also be seen. Because of
the communication between the oral and nasal cavities, a cleft palate
makes it very difficult for an infant to generate the suckling needed for
nursing, thus leaving the infant at risk for malnutrition. Surgical repair is
required to correct cleft palate defects.
Zygomatic Bone
The zygomatic bone is also known as the cheekbone. Each of the paired
zygomatic bones forms much of the lateral wall of the orbit and the lateral-
inferior margins of the anterior orbital opening (see [link]). The short
temporal process of the zygomatic bone projects posteriorly, where it forms
the anterior portion of the zygomatic arch (see [link]).
Nasal Bone
The nasal bone is one of two small bones that articulate (join) with each
other to form the bony base (bridge) of the nose. They also support the
cartilages that form the lateral walls of the nose (see [link]). These are the
bones that are damaged when the nose is broken.
Lacrimal Bone
Each lacrimal bone is a small, rectangular bone that forms the anterior,
medial wall of the orbit (see [link] and [link]). The anterior portion of the
lacrimal bone forms a shallow depression called the lacrimal fossa, and
extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid
(tears of the eye), which serves to maintain the moist surface of the eye,
drains at the medial corner of the eye into the nasolacrimal canal. This duct
then extends downward to open into the nasal cavity, behind the inferior
nasal concha. In the nasal cavity, the lacrimal fluid normally drains
posteriorly, but with an increased flow of tears due to crying or eye
irritation, some fluid will also drain anteriorly, thus causing a runny nose.
Inferior Nasal Conchae
The right and left inferior nasal conchae form a curved bony plate that
projects into the nasal cavity space from the lower lateral wall (see [link]).
The inferior concha is the largest of the nasal conchae and can easily be
seen when looking into the anterior opening of the nasal cavity.
Vomer Bone
The unpaired vomer bone, often referred to simply as the vomer, is
triangular-shaped and forms the posterior-inferior part of the nasal septum
(see [link]). The vomer is best seen when looking from behind into the
posterior openings of the nasal cavity (see [link]a). In this view, the vomer
is seen to form the entire height of the nasal septum. A much smaller
portion of the vomer can also be seen when looking into the anterior
opening of the nasal cavity.
Mandible
The mandible forms the lower jaw and is the only moveable bone of the
skull. At the time of birth, the mandible consists of paired right and left
bones, but these fuse together during the first year to form the single U-
shaped mandible of the adult skull. Each side of the mandible consists of a
horizontal body and posteriorly, a vertically oriented ramus of the
mandible (ramus = “branch’). The outside margin of the mandible, where
the body and ramus come together is called the angle of the mandible
({link]).
The ramus on each side of the mandible has two upward-going bony
projections. The more anterior projection is the flattened coronoid process
of the mandible, which provides attachment for one of the biting muscles.
The posterior projection is the condylar process of the mandible, which is
topped by the oval-shaped condyle. The condyle of the mandible articulates
(joins) with the mandibular fossa and articular tubercle of the temporal
bone. Together these articulations form the temporomandibular joint, which
allows for opening and closing of the mouth (see [link]). The broad U-
shaped curve located between the coronoid and condylar processes is the
mandibular notch.
Important landmarks for the mandible include the following:
e Alveolar process of the mandible—This is the upper border of the
mandibular body and serves to anchor the lower teeth.
¢ Mental protuberance—The forward projection from the inferior
margin of the anterior mandible that forms the chin (mental = “chin’”).
¢ Mental foramen—The opening located on each side of the anterior-
lateral mandible, which is the exit site for a sensory nerve that supplies
the chin.
e Mylohyoid line—This bony ridge extends along the inner aspect of
the mandibular body (see [link]). The muscle that forms the floor of
the oral cavity attaches to the mylohyoid lines on both sides of the
mandible.
e Mandibular foramen—This opening is located on the medial side of
the ramus of the mandible. The opening leads into a tunnel that runs
down the length of the mandibular body. The sensory nerve and blood
vessels that supply the lower teeth enter the mandibular foramen and
then follow this tunnel. Thus, to numb the lower teeth prior to dental
work, the dentist must inject anesthesia into the lateral wall of the oral
cavity at a point prior to where this sensory nerve enters the
mandibular foramen.
¢ Lingula—tThis small flap of bone is named for its shape (lingula =
“little tongue”). It is located immediately next to the mandibular
foramen, on the medial side of the ramus. A ligament that anchors the
mandible during opening and closing of the mouth extends down from
the base of the skull and attaches to the lingula.
Isolated Mandible
Condylar process ;
Coronoid process
Lingula
Mandibular
notch
Mandibular
condyle
Mandibular foramen
Mylohyoid line
Alveolar process
Ramus of
mandible
Mental
protuberance
Mental foramen
Mandibular
angle
Body of mandible
Right lateral view
The mandible is the only moveable bone of the
skull.
The Orbit
The orbit is the bony socket that houses the eyeball and contains the
muscles that move the eyeball or open the upper eyelid. Each orbit is cone-
shaped, with a narrow posterior region that widens toward the large anterior
opening. To help protect the eye, the bony margins of the anterior opening
are thickened and somewhat constricted. The medial walls of the two orbits
are parallel to each other but each lateral wall diverges away from the
midline at a 45° angle. This divergence provides greater lateral peripheral
vision.
The walls of each orbit include contributions from seven skull bones
({link]). The frontal bone forms the roof and the zygomatic bone forms the
lateral wall and lateral floor. The medial floor is primarily formed by the
maxilla, with a small contribution from the palatine bone. The ethmoid
bone and lacrimal bone make up much of the medial wall and the sphenoid
bone forms the posterior orbit.
At the posterior apex of the orbit is the opening of the optic canal, which
allows for passage of the optic nerve from the retina to the brain. Lateral to
this is the elongated and irregularly shaped superior orbital fissure, which
provides passage for the artery that supplies the eyeball, sensory nerves, and
the nerves that supply the muscles involved in eye movements.
Bones of the Orbit
Frontal bone Supraorbital foramen
Supraorbital margin
Sphenoid bone Nasal bone
Lacrimal bone
Ethmoid bone
Lacrimal fossa
Optic canal
Superior orbital fissure
Zygomatic bone Palatine bone
Infraorbital foramen
Seven skull bones contribute to the walls of the orbit. Opening
into the posterior orbit from the cranial cavity are the optic
canal and superior orbital fissure.
The Nasal Septum and Nasal Conchae
The nasal septum consists of both bone and cartilage components ([Link];
see also [link]). The upper portion of the septum is formed by the
perpendicular plate of the ethmoid bone. The lower and posterior parts of
the septum are formed by the triangular-shaped vomer bone. In an anterior
view of the skull, the perpendicular plate of the ethmoid bone is easily seen
inside the nasal opening as the upper nasal septum, but only a small portion
of the vomer is seen as the inferior septum. A better view of the vomer bone
is seen when looking into the posterior nasal cavity with an inferior view of
the skull, where the vomer forms the full height of the nasal septum. The
anterior nasal septum is formed by the septal cartilage, a flexible plate that
fills in the gap between the perpendicular plate of the ethmoid and vomer
bones. This cartilage also extends outward into the nose where it separates
the right and left nostrils. The septal cartilage is not found in the dry skull.
Attached to the lateral wall on each side of the nasal cavity are the superior,
middle, and inferior nasal conchae (singular = concha), which are named
for their positions (see [link]). These are bony plates that curve downward
as they project into the space of the nasal cavity. They serve to swirl the
incoming air, which helps to warm and moisturize it before the air moves
into the delicate air sacs of the lungs. This also allows mucus, secreted by
the tissue lining the nasal cavity, to trap incoming dust, pollen, bacteria, and
viruses. The largest of the conchae is the inferior nasal concha, which is an
independent bone of the skull. The middle concha and the superior conchae,
which is the smallest, are both formed by the ethmoid bone. When looking
into the anterior nasal opening of the skull, only the inferior and middle
conchae can be seen. The small superior nasal concha is well hidden above
and behind the middle concha.
Nasal Septum
Frontal bone
Crista galli
Frontal sinus
Sphenoid sinus Nasal bone
Nasal septum:
Perpendicular plate
of ethmoid bone
Vomer bone
Septal cartilage
Sphenoid
bone
Qy Palatine process
Horizontal plate — of maxilla
of palatine bone
Sagittal section
The nasal septum is formed by the perpendicular plate of the
ethmoid bone and the vomer bone. The septal cartilage fills the
gap between these bones and extends into the nose.
Cranial Fossae
Inside the skull, the floor of the cranial cavity is subdivided into three
cranial fossae (spaces), which increase in depth from anterior to posterior
(see [link], [link]b, and [link]). Since the brain occupies these areas, the
shape of each conforms to the shape of the brain regions that it contains.
Each cranial fossa has anterior and posterior boundaries and is divided at
the midline into right and left areas by a significant bony structure or
opening.
Anterior Cranial Fossa
The anterior cranial fossa is the most anterior and the shallowest of the
three cranial fossae. It overlies the orbits and contains the frontal lobes of
the brain. Anteriorly, the anterior fossa is bounded by the frontal bone,
which also forms the majority of the floor for this space. The lesser wings
of the sphenoid bone form the prominent ledge that marks the boundary
between the anterior and middle cranial fossae. Located in the floor of the
anterior cranial fossa at the midline is a portion of the ethmoid bone,
consisting of the upward projecting crista galli and to either side of this, the
cribriform plates.
Middle Cranial Fossa
The middle cranial fossa is deeper and situated posterior to the anterior
fossa. It extends from the lesser wings of the sphenoid bone anteriorly, to
the petrous ridges (petrous portion of the temporal bones) posteriorly. The
large, diagonally positioned petrous ridges give the middle cranial fossa a
butterfly shape, making it narrow at the midline and broad laterally. The
temporal lobes of the brain occupy this fossa. The middle cranial fossa is
divided at the midline by the upward bony prominence of the sella turcica, a
part of the sphenoid bone. The middle cranial fossa has several openings for
the passage of blood vessels and cranial nerves (see [link]).
Openings in the middle cranial fossa are as follows:
Optic canal—This opening is located at the anterior lateral comer of
the sella turcica. It provides for passage of the optic nerve into the
orbit.
Superior orbital fissure—This large, irregular opening into the
posterior orbit is located on the anterior wall of the middle cranial
fossa, lateral to the optic canal and under the projecting margin of the
lesser wing of the sphenoid bone. Nerves to the eyeball and associated
muscles, and sensory nerves to the forehead pass through this opening.
Foramen rotundum—This rounded opening (rotundum = “round”) is
located in the floor of the middle cranial fossa, just inferior to the
superior orbital fissure. It is the exit point for a major sensory nerve
that supplies the cheek, nose, and upper teeth.
Foramen ovale of the middle cranial fossa—This large, oval-shaped
opening in the floor of the middle cranial fossa provides passage for a
major sensory nerve to the lateral head, cheek, chin, and lower teeth.
Foramen spinosum—This small opening, located posterior-lateral to
the foramen ovale, is the entry point for an important artery that
supplies the covering layers surrounding the brain. The branching
pattern of this artery forms readily visible grooves on the internal
surface of the skull and these grooves can be traced back to their origin
at the foramen spinosum.
Carotid canal—This is the zig-zag passageway through which a
major artery to the brain enters the skull. The entrance to the carotid
canal is located on the inferior aspect of the skull, anteromedial to the
styloid process (see [link]a). From here, the canal runs anteromedially
within the bony base of the skull. Just above the foramen lacerum, the
carotid canal opens into the middle cranial cavity, near the posterior-
lateral base of the sella turcica.
Foramen lacerum—This irregular opening is located in the base of
the skull, immediately inferior to the exit of the carotid canal. This
opening is an artifact of the dry skull, because in life it is completely
filled with cartilage. All the openings of the skull that provide for
passage of nerves or blood vessels have smooth margins; the word
lacerum (“ragged” or “torn’”) tells us that this opening has ragged
edges and thus nothing passes through it.
Posterior Cranial Fossa
The posterior cranial fossa is the most posterior and deepest portion of the
cranial cavity. It contains the cerebellum of the brain. The posterior fossa is
bounded anteriorly by the petrous ridges, while the occipital bone forms the
floor and posterior wall. It is divided at the midline by the large foramen
magnum (“great aperture”), the opening that provides for passage of the
spinal cord.
Located on the medial wall of the petrous ridge in the posterior cranial fossa
is the internal acoustic meatus (see [link]). This opening provides for
passage of the nerve from the hearing and equilibrium organs of the inner
ear, and the nerve that supplies the muscles of the face. Located at the
anterior-lateral margin of the foramen magnum is the hypoglossal canal.
These emerge on the inferior aspect of the skull at the base of the occipital
condyle and provide passage for an important nerve to the tongue.
Immediately inferior to the internal acoustic meatus is the large, irregularly
shaped jugular foramen (see [link]a). Several cranial nerves from the brain
exit the skull via this opening. It is also the exit point through the base of
the skull for all the venous return blood leaving the brain. The venous
structures that carry blood inside the skull form large, curved grooves on
the inner walls of the posterior cranial fossa, which terminate at each
jugular foramen.
Paranasal Sinuses
The paranasal sinuses are hollow, air-filled spaces located within certain
bones of the skull ([link]). All of the sinuses communicate with the nasal
cavity (paranasal = “next to nasal cavity”) and are lined with nasal mucosa.
They serve to reduce bone mass and thus lighten the skull, and they also
add resonance to the voice. This second feature is most obvious when you
have a cold or sinus congestion. These produce swelling of the mucosa and
excess mucus production, which can obstruct the narrow passageways
between the sinuses and the nasal cavity, causing your voice to sound
different to yourself and others. This blockage can also allow the sinuses to
fill with fluid, with the resulting pressure producing pain and discomfort.
The paranasal sinuses are named for the skull bone that each occupies. The
frontal sinus is located just above the eyebrows, within the frontal bone
(see [link]). This irregular space may be divided at the midline into bilateral
spaces, or these may be fused into a single sinus space. The frontal sinus is
the most anterior of the paranasal sinuses. The largest sinus is the maxillary
sinus. These are paired and located within the right and left maxillary
bones, where they occupy the area just below the orbits. The maxillary
sinuses are most commonly involved during sinus infections. Because their
connection to the nasal cavity is located high on their medial wall, they are
difficult to drain. The sphenoid sinus is a single, midline sinus. It is located
within the body of the sphenoid bone, just anterior and inferior to the sella
turcica, thus making it the most posterior of the paranasal sinuses. The
lateral aspects of the ethmoid bone contain multiple small spaces separated
by very thin bony walls. Each of these spaces is called an ethmoid air cell.
These are located on both sides of the ethmoid bone, between the upper
nasal cavity and medial orbit, just behind the superior nasal conchae.
Paranasal Sinuses
Anterior Lateral
The paranasal sinuses are hollow, air-filled spaces
named for the skull bone that each occupies. The most
anterior is the frontal sinus, located in the frontal bone
above the eyebrows. The largest are the maxillary
sinuses, located in the right and left maxillary bones
below the orbits. The most posterior is the sphenoid
sinus, located in the body of the sphenoid bone, under
the sella turcica. The ethmoid air cells are multiple
small spaces located in the right and left sides of the
ethmoid bone, between the medial wall of the orbit
and lateral wall of the upper nasal cavity.
Hyoid Bone
The hyoid bone is an independent bone that does not contact any other bone
and thus is not part of the skull ({link]). It is a small U-shaped bone located
in the upper neck near the level of the inferior mandible, with the tips of the
“U” pointing posteriorly. The hyoid serves as the base for the tongue above,
and is attached to the larynx below and the pharynx posteriorly. The hyoid
is held in position by a series of small muscles that attach to it either from
above or below. These muscles act to move the hyoid up/down or
forward/back. Movements of the hyoid are coordinated with movements of
the tongue, larynx, and pharynx during swallowing and speaking.
Hyoid Bone
Mandible
Hyoid bone
Larynx
Greater horn
Lesser horn
Body
Greater horn
Lesser horn
Right lateral view
The hyoid bone is located in the
upper neck and does not join
with any other bone. It provides
attachments for muscles that act
on the tongue, larynx, and
pharynx.
Chapter Review
The skull consists of the brain case and the facial bones. The brain case
surrounds and protects the brain, which occupies the cranial cavity inside
the skull. It consists of the rounded calvaria and a complex base. The brain
case is formed by eight bones, the paired parietal and temporal bones plus
the unpaired frontal, occipital, sphenoid, and ethmoid bones. The narrow
gap between the bones is filled with dense, fibrous connective tissue that
unites the bones. The sagittal suture joins the right and left parietal bones.
The coronal suture joins the parietal bones to the frontal bone, the lamboid
suture joins them to the occipital bone, and the squamous suture joins them
to the temporal bone.
The facial bones support the facial structures and form the upper and lower
jaws. These consist of 14 bones, with the paired maxillary, palatine,
zygomatic, nasal, lacrimal, and inferior conchae bones and the unpaired
vomer and mandible bones. The ethmoid bone also contributes to the
formation of facial structures. The maxilla forms the upper jaw and the
mandible forms the lower jaw. The maxilla also forms the larger anterior
portion of the hard palate, which is completed by the smaller palatine bones
that form the posterior portion of the hard palate.
The floor of the cranial cavity increases in depth from front to back and is
divided into three cranial fossae. The anterior cranial fossa is located
between the frontal bone and lesser wing of the sphenoid bone. A small
area of the ethmoid bone, consisting of the crista galli and cribriform plates,
is located at the midline of this fossa. The middle cranial fossa extends from
the lesser wing of the sphenoid bone to the petrous ridge (petrous portion of
temporal bone). The right and left sides are separated at the midline by the
sella turcica, which surrounds the shallow hypophyseal fossa. Openings
through the skull in the floor of the middle fossa include the optic canal and
superior orbital fissure, which open into the posterior orbit, the foramen
rotundum, foramen ovale, and foramen spinosum, and the exit of the carotid
canal with its underlying foramen lacerum. The deep posterior cranial fossa
extends from the petrous ridge to the occipital bone. Openings here include
the large foramen magnum, plus the internal acoustic meatus, jugular
foramina, and hypoglossal canals. Additional openings located on the
external base of the skull include the stylomastoid foramen and the entrance
to the carotid canal.
The anterior skull has the orbits that house the eyeballs and associated
muscles. The walls of the orbit are formed by contributions from seven
bones: the frontal, zygomatic, maxillary, palatine, ethmoid, lacrimal, and
sphenoid. Located at the superior margin of the orbit is the supraorbital
foramen, and below the orbit is the infraorbital foramen. The mandible has
two openings, the mandibular foramen on its inner surface and the mental
foramen on its external surface near the chin. The nasal conchae are bony
projections from the lateral walls of the nasal cavity. The large inferior
nasal concha is an independent bone, while the middle and superior
conchae are parts of the ethmoid bone. The nasal septum is formed by the
perpendicular plate of the ethmoid bone, the vomer bone, and the septal
cartilage. The paranasal sinuses are air-filled spaces located within the
frontal, maxillary, sphenoid, and ethmoid bones.
On the lateral skull, the zygomatic arch consists of two parts, the temporal
process of the zygomatic bone anteriorly and the zygomatic process of the
temporal bone posteriorly. The temporal fossa is the shallow space located
on the lateral skull above the level of the zygomatic arch. The infratemporal
fossa is located below the zygomatic arch and deep to the ramus of the
mandible.
The hyoid bone is located in the upper neck and does not join with any
other bone. It is held in position by muscles and serves to support the
tongue above, the larynx below, and the pharynx posteriorly.
Interactive Link Questions
Exercise:
Problem:
Watch this video to view a rotating and exploded skull with color-
coded bones. Which bone (yellow) is centrally located and joins with
most of the other bones of the skull?
Solution:
The sphenoid bone joins with most other bones of the skull. It is
centrally located, where it forms portions of the rounded brain case
and cranial base.
Exercise:
Problem:
View this animation to see how a blow to the head may produce a
contrecoup (counterblow) fracture of the basilar portion of the
occipital bone on the base of the skull. Why may a basilar fracture be
life threatening?
Solution:
A basilar fracture may damage an artery entering the skull, causing
bleeding in the brain.
Review Questions
Exercise:
Problem: Which of the following is a bone of the brain case?
a. parietal bone
b. zygomatic bone
c. maxillary bone
d. lacrimal bone
Solution:
A
Exercise:
Problem:The lambdoid suture joins the parietal bone to the
a. frontal bone
b. occipital bone
c. other parietal bone
d. temporal bone
Solution:
B
Exercise:
Problem:The middle cranial fossa
a. is bounded anteriorly by the petrous ridge
b. is bounded posteriorly by the lesser wing of the sphenoid bone
c. is divided at the midline by a small area of the ethmoid bone
d. has the foramen rotundum, foramen ovale, and foramen spinosum
Solution:
D
Exercise:
Problem:The paranasal sinuses are
a. air-filled spaces found within the frontal, maxilla, sphenoid, and
ethmoid bones only
b. air-filled spaces found within all bones of the skull
c. not connected to the nasal cavity
d. divided at the midline by the nasal septum
Solution:
A
Exercise:
Problem: Parts of the sphenoid bone include the
a. sella turcica
b. squamous portion
c. glabella
d. zygomatic process
Solution:
A
Exercise:
Problem:The bony openings of the skull include the
a. carotid canal, which is located in the anterior cranial fossa
b. superior orbital fissure, which is located at the superior margin of
the anterior orbit
c. mental foramen, which is located just below the orbit
d. hypoglossal canal, which is located in the posterior cranial fossa
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Define and list the bones that form the brain case or support the facial
structures.
Solution:
The brain case is that portion of the skull that surrounds and protects
the brain. It is subdivided into the rounded top of the skull, called the
calvaria, and the base of the skull. There are eight bones that form the
brain case. These are the paired parietal and temporal bones, plus the
unpaired frontal, occipital, sphenoid, and ethmoid bones. The facial
bones support the facial structures, and form the upper and lower jaws,
nasal cavity, nasal septum, and orbit. There are 14 facial bones. These
are the paired maxillary, palatine, zygomatic, nasal, lacrimal, and
inferior nasal conchae bones, and the unpaired vomer and mandible
bones.
Exercise:
Problem:
Identify the major sutures of the skull, their locations, and the bones
united by each.
Solution:
The coronal suture passes across the top of the anterior skull. It unites
the frontal bone anteriorly with the right and left parietal bones. The
sagittal suture runs at the midline on the top of the skull. It unites the
right and left parietal bones with each other. The squamous suture is a
curved suture located on the lateral side of the skull. It unites the
squamous portion of the temporal bone to the parietal bone. The
lambdoid suture is located on the posterior skull and has an inverted V-
shape. It unites the occipital bone with the right and left parietal bones.
Exercise:
Problem:
Describe the anterior, middle, and posterior cranial fossae and their
boundaries, and give the midline structure that divides each into right
and left areas.
Solution:
The anterior cranial fossa is the shallowest of the three cranial fossae.
It extends from the frontal bone anteriorly to the lesser wing of the
sphenoid bone posteriorly. It is divided at the midline by the crista galli
and cribriform plates of the ethmoid bone. The middle cranial fossa is
located in the central skull, and is deeper than the anterior fossa. The
middle fossa extends from the lesser wing of the sphenoid bone
anteriorly to the petrous ridge posteriorly. It is divided at the midline
by the sella turcica. The posterior cranial fossa is the deepest fossa. It
extends from the petrous ridge anteriorly to the occipital bone
posteriorly. The large foramen magnum is located at the midline of the
posterior fossa.
Exercise:
Problem:
Describe the parts of the nasal septum in both the dry and living skull.
Solution:
There are two bony parts of the nasal septum in the dry skull. The
perpendicular plate of the ethmoid bone forms the superior part of the
septum. The vomer bone forms the inferior and posterior parts of the
septum. In the living skull, the septal cartilage completes the septum
by filling in the anterior area between the bony components and
extending outward into the nose.
References
Centers for Disease Control and Prevention (US). Injury prevention and
control: traumatic brain injury [Internet]. Atlanta, GA; [cited 2013 Mar 18].
Glossary
alveolar process of the mandible
upper border of mandibular body that contains the lower teeth
alveolar process of the maxilla
curved, inferior margin of the maxilla that supports and anchors the
upper teeth
angle of the mandible
rounded corner located at outside margin of the body and ramus
junction
anterior cranial fossa
shallowest and most anterior cranial fossa of the cranial base that
extends from the frontal bone to the lesser wing of the sphenoid bone
articular tubercle
smooth ridge located on the inferior skull, immediately anterior to the
mandibular fossa
brain case
portion of the skull that contains and protects the brain, consisting of
the eight bones that form the cranial base and rounded upper skull
calvaria
(also, skullcap) rounded top of the skull
carotid canal
zig-zag tunnel providing passage through the base of the skull for the
internal carotid artery to the brain; begins anteromedial to the styloid
process and terminates in the middle cranial cavity, near the posterior-
lateral base of the sella turcica
condylar process of the mandible
thickened upward projection from posterior margin of mandibular
ramus
condyle
oval-shaped process located at the top of the condylar process of the
mandible
coronal suture
joint that unites the frontal bone to the right and left parietal bones
across the top of the skull
coronoid process of the mandible
flattened upward projection from the anterior margin of the mandibular
ramus
cranial cavity
interior space of the skull that houses the brain
cranium
skull
cribriform plate
small, flattened areas with numerous small openings, located to either
side of the midline in the floor of the anterior cranial fossa; formed by
the ethmoid bone
crista galli
small upward projection located at the midline in the floor of the
anterior cranial fossa; formed by the ethmoid bone
ethmoid air cell
one of several small, air-filled spaces located within the lateral sides of
the ethmoid bone, between the orbit and upper nasal cavity
ethmoid bone
unpaired bone that forms the roof and upper, lateral walls of the nasal
cavity, portions of the floor of the anterior cranial fossa and medial
wall of orbit, and the upper portion of the nasal septum
external acoustic meatus
ear canal opening located on the lateral side of the skull
external occipital protuberance
small bump located at the midline on the posterior skull
facial bones
fourteen bones that support the facial structures and form the upper
and lower jaws and the hard palate
foramen lacerum
irregular opening in the base of the skull, located inferior to the exit of
carotid canal
foramen magnum
large opening in the occipital bone of the skull through which the
spinal cord emerges and the vertebral arteries enter the cranium
foramen ovale of the middle cranial fossa
oval-shaped opening in the floor of the middle cranial fossa
foramen rotundum
round opening in the floor of the middle cranial fossa, located between
the superior orbital fissure and foramen ovale
foramen spinosum
small opening in the floor of the middle cranial fossa, located lateral to
the foramen ovale
frontal bone
unpaired bone that forms forehead, roof of orbit, and floor of anterior
cranial fossa
frontal sinus
air-filled space within the frontal bone; most anterior of the paranasal
sinuses
glabella
slight depression of frontal bone, located at the midline between the
eyebrows
greater wings of sphenoid bone
lateral projections of the sphenoid bone that form the anterior wall of
the middle cranial fossa and an area of the lateral skull
hard palate
bony structure that forms the roof of the mouth and floor of the nasal
cavity, formed by the palatine process of the maxillary bones and the
horizontal plate of the palatine bones
horizontal plate
medial extension from the palatine bone that forms the posterior
quarter of the hard palate
hypoglossal canal
paired openings that pass anteriorly from the anterior-lateral margins
of the foramen magnum deep to the occipital condyles
hypophyseal (pituitary) fossa
shallow depression on top of the sella turcica that houses the pituitary
(hypophyseal) gland
inferior nasal concha
one of the paired bones that project from the lateral walls of the nasal
cavity to form the largest and most inferior of the nasal conchae
infraorbital foramen
opening located on anterior skull, below the orbit
infratemporal fossa
space on lateral side of skull, below the level of the zygomatic arch
and deep (medial) to the ramus of the mandible
internal acoustic meatus
opening into petrous ridge, located on the lateral wall of the posterior
cranial fossa
jugular foramen
irregularly shaped opening located in the lateral floor of the posterior
cranial cavity
lacrimal bone
paired bones that contribute to the anterior-medial wall of each orbit
lacrimal fossa
shallow depression in the anterior-medial wall of the orbit, formed by
the lacrimal bone that gives rise to the nasolacrimal canal
lambdoid suture
inverted V-shaped joint that unites the occipital bone to the right and
left parietal bones on the posterior skull
lateral pterygoid plate
paired, flattened bony projections of the sphenoid bone located on the
inferior skull, lateral to the medial pterygoid plate
lesser wings of the sphenoid bone
lateral extensions of the sphenoid bone that form the bony lip
separating the anterior and middle cranial fossae
lingula
small flap of bone located on the inner (medial) surface of mandibular
ramus, next to the mandibular foramen
mandible
unpaired bone that forms the lower jaw bone; the only moveable bone
of the skull
mandibular foramen
opening located on the inner (medial) surface of the mandibular ramus
mandibular fossa
oval depression located on the inferior surface of the skull
mandibular notch
large U-shaped notch located between the condylar process and
coronoid process of the mandible
mastoid process
large bony prominence on the inferior, lateral skull, just behind the
earlobe
maxillary bone
(also, maxilla) paired bones that form the upper jaw and anterior
portion of the hard palate
maxillary sinus
air-filled space located with each maxillary bone; largest of the
paranasal sinuses
medial pterygoid plate
paired, flattened bony projections of the sphenoid bone located on the
inferior skull medial to the lateral pterygoid plate; form the posterior
portion of the nasal cavity lateral wall
mental foramen
opening located on the anterior-lateral side of the mandibular body
mental protuberance
inferior margin of anterior mandible that forms the chin
middle cranial fossa
centrally located cranial fossa that extends from the lesser wings of the
sphenoid bone to the petrous ridge
middle nasal concha
nasal concha formed by the ethmoid bone that is located between the
superior and inferior conchae
mylohyoid line
bony ridge located along the inner (medial) surface of the mandibular
body
nasal bone
paired bones that form the base of the nose
nasal cavity
opening through skull for passage of air
nasal conchae
curved bony plates that project from the lateral walls of the nasal
cavity; include the superior and middle nasal conchae, which are parts
of the ethmoid bone, and the independent inferior nasal conchae bone
nasal septum
flat, midline structure that divides the nasal cavity into halves, formed
by the perpendicular plate of the ethmoid bone, vomer bone, and septal
cartilage
nasolacrimal canal
passage for drainage of tears that extends downward from the medial-
anterior orbit to the nasal cavity, terminating behind the inferior nasal
conchae
occipital bone
unpaired bone that forms the posterior portions of the brain case and
base of the skull
occipital condyle
paired, oval-shaped bony knobs located on the inferior skull, to either
side of the foramen magnum
optic canal
opening spanning between middle cranial fossa and posterior orbit
orbit
bony socket that contains the eyeball and associated muscles
palatine bone
paired bones that form the posterior quarter of the hard palate and a
small area in floor of the orbit
palatine process
medial projection from the maxilla bone that forms the anterior three
quarters of the hard palate
paranasal sinuses
cavities within the skull that are connected to the conchae that serve to
warm and humidify incoming air, produce mucus, and lighten the
weight of the skull; consist of frontal, maxillary, sphenoidal, and
ethmoidal sinuses
parietal bone
paired bones that form the upper, lateral sides of the skull
perpendicular plate of the ethmoid bone
downward, midline extension of the ethmoid bone that forms the
superior portion of the nasal septum
petrous ridge
petrous portion of the temporal bone that forms a large, triangular
ridge in the floor of the cranial cavity, separating the middle and
posterior cranial fossae; houses the middle and inner ear structures
posterior cranial fossa
deepest and most posterior cranial fossa; extends from the petrous
ridge to the occipital bone
pterion
H-shaped suture junction region that unites the frontal, parietal,
temporal, and sphenoid bones on the lateral side of the skull
ramus of the mandible
vertical portion of the mandible
Sagittal suture
joint that unites the right and left parietal bones at the midline along
the top of the skull
sella turcica
elevated area of sphenoid bone located at midline of the middle cranial
fossa
septal cartilage
flat cartilage structure that forms the anterior portion of the nasal
septum
sphenoid bone
unpaired bone that forms the central base of skull
sphenoid sinus
air-filled space located within the sphenoid bone; most posterior of the
paranasal sinuses
squamous suture
joint that unites the parietal bone to the squamous portion of the
temporal bone on the lateral side of the skull
styloid process
downward projecting, elongated bony process located on the inferior
aspect of the skull
stylomastoid foramen
opening located on inferior skull, between the styloid process and
mastoid process
superior nasal concha
smallest and most superiorly located of the nasal conchae; formed by
the ethmoid bone
superior nuchal line
paired bony lines on the posterior skull that extend laterally from the
external occipital protuberance
superior orbital fissure
irregularly shaped opening between the middle cranial fossa and the
posterior orbit
supraorbital foramen
opening located on anterior skull, at the superior margin of the orbit
supraorbital margin
superior margin of the orbit
suture
junction line at which adjacent bones of the skull are united by fibrous
connective tissue
temporal bone
paired bones that form the lateral, inferior portions of the skull, with
Squamous, mastoid, and petrous portions
temporal fossa
shallow space on the lateral side of the skull, above the level of the
zygomatic arch
temporal process of the zygomatic bone
short extension from the zygomatic bone that forms the anterior
portion of the zygomatic arch
vomer bone
unpaired bone that forms the inferior and posterior portions of the
nasal septum
zygomatic arch
elongated, free-standing arch on the lateral skull, formed anteriorly by
the temporal process of the zygomatic bone and posteriorly by the
zygomatic process of the temporal bone
zygomatic bone
cheekbone; paired bones that contribute to the lateral orbit and anterior
zygomatic arch
zygomatic process of the temporal bone
extension from the temporal bone that forms the posterior portion of
the zygomatic arch
The Vertebral Column
By the end of this section, you will be able to:
Describe each region of the vertebral column and the number of bones
in each region
Discuss the curves of the vertebral column and how these change after
birth
Describe a typical vertebra and determine the distinguishing
characteristics for vertebrae in each vertebral region and features of the
sacrum and the coccyx
Define the structure of an intervertebral disc
Determine the location of the ligaments that provide support for the
vertebral column
The vertebral column is also known as the spinal column or spine ([link]). It
consists of a sequence of vertebrae (singular = vertebra), each of which is
separated and united by an intervertebral disc. Together, the vertebrae and
intervertebral discs form the vertebral column. It is a flexible column that
supports the head, neck, and body and allows for their movements. It also
protects the spinal cord, which passes down the back through openings in
the vertebrae.
Vertebral Column
7 Cervical vertebrae
(C1-C7) form cervical curve
12 Thoracic vertbrae
(T1-T12) form thoracic curve
5 Lumbar vertebrae (L1—L5)
form lumbar curve
Fused vertebrae of sacrum
and coccyx form
sacrococcygeal curve
The adult vertebral column consists of 24 vertebrae,
plus the sacrum and coccyx. The vertebrae are divided
into three regions: cervical C1—C7 vertebrae, thoracic
T1—-T12 vertebrae, and lumbar L1—L5 vertebrae. The
vertebral column is curved, with two primary
curvatures (thoracic and sacrococcygeal curves) and
two secondary curvatures (cervical and lumbar
curves).
Regions of the Vertebral Column
The vertebral column originally develops as a series of 33 vertebrae, but
this number is eventually reduced to 24 vertebrae, plus the sacrum and
coccyx. The vertebral column is subdivided into five regions, with the
vertebrae in each area named for that region and numbered in descending
order. In the neck, there are seven cervical vertebrae, each designated with
the letter “C” followed by its number. Superiorly, the C1 vertebra articulates
(forms a joint) with the occipital condyles of the skull. Inferiorly, C1
articulates with the C2 vertebra, and so on. Below these are the 12 thoracic
vertebrae, designated T1—T12. The lower back contains the L1—L5 lumbar
vertebrae. The single sacrum, which is also part of the pelvis, is formed by
the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results
from the fusion of four small coccygeal vertebrae. However, the sacral and
coccygeal fusions do not start until age 20 and are not completed until
middle age.
An interesting anatomical fact is that almost all mammals have seven
cervical vertebrae, regardless of body size. This means that there are large
variations in the size of cervical vertebrae, ranging from the very small
cervical vertebrae of a shrew to the greatly elongated vertebrae in the neck
of a giraffe. In a full-grown giraffe, each cervical vertebra is 11 inches tall.
Curvatures of the Vertebral Column
The adult vertebral column does not form a straight line, but instead has
four curvatures along its length (see [link]). These curves increase the
vertebral column’s strength, flexibility, and ability to absorb shock. When
the load on the spine is increased, by carrying a heavy backpack for
example, the curvatures increase in depth (become more curved) to
accommodate the extra weight. They then spring back when the weight is
removed. The four adult curvatures are classified as either primary or
secondary curvatures. Primary curves are retained from the original fetal
curvature, while secondary curvatures develop after birth.
During fetal development, the body is flexed anteriorly into the fetal
position, giving the entire vertebral column a single curvature that is
concave anteriorly. In the adult, this fetal curvature is retained in two
regions of the vertebral column as the thoracic curve, which involves the
thoracic vertebrae, and the sacrococcygeal curve, formed by the sacrum
and coccyx. Each of these is thus called a primary curve because they are
retained from the original fetal curvature of the vertebral column.
A secondary curve develops gradually after birth as the child learns to sit
upright, stand, and walk. Secondary curves are concave posteriorly,
opposite in direction to the original fetal curvature. The cervical curve of
the neck region develops as the infant begins to hold their head upright
when sitting. Later, as the child begins to stand and then to walk, the
lumbar curve of the lower back develops. In adults, the lumbar curve is
generally deeper in females.
Disorders associated with the curvature of the spine include kyphosis (an
excessive posterior curvature of the thoracic region), lordosis (an excessive
anterior curvature of the lumbar region), and scoliosis (an abnormal, lateral
curvature, accompanied by twisting of the vertebral column).
Note:
Disorders of the...
Vertebral Column
Developmental anomalies, pathological changes, or obesity can enhance
the normal vertebral column curves, resulting in the development of
abnormal or excessive curvatures ({link]). Kyphosis, also referred to as
humpback or hunchback, is an excessive posterior curvature of the thoracic
region. This can develop when osteoporosis causes weakening and erosion
of the anterior portions of the upper thoracic vertebrae, resulting in their
gradual collapse ({link]). Lordosis, or swayback, is an excessive anterior
curvature of the lumbar region and is most commonly associated with
obesity or late pregnancy. The accumulation of body weight in the
abdominal region results an anterior shift in the line of gravity that carries
the weight of the body. This causes in an anterior tilt of the pelvis and a
pronounced enhancement of the lumbar curve.
Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the
vertebral column. Compensatory curves may also develop in other areas of
the vertebral column to help maintain the head positioned over the feet.
Scoliosis is the most common vertebral abnormality among girls. The
cause is usually unknown, but it may result from weakness of the back
muscles, defects such as differential growth rates in the right and left sides
of the vertebral column, or differences in the length of the lower limbs.
When present, scoliosis tends to get worse during adolescent growth
spurts. Although most individuals do not require treatment, a back brace
may be recommended for growing children. In extreme cases, surgery may
be required.
Excessive vertebral curves can be identified while an individual stands in
the anatomical position. Observe the vertebral profile from the side and
then from behind to check for kyphosis or lordosis. Then have the person
bend forward. If scoliosis is present, an individual will have difficulty in
bending directly forward, and the right and left sides of the back will not
be level with each other in the bent position.
Abnormal Curvatures of the Vertebral Column
(a) Scoliosis (b) Kyphosis (c) Lordosis
(a) Scoliosis is an abnormal lateral bending of the vertebral
column. (b) An excessive curvature of the upper thoracic
vertebral column is called kyphosis. (c) Lordosis is an
excessive curvature in the lumbar region of the vertebral
column.
Osteoporosis
Normal Bone loss
vertebrae amplifies curvature
Osteoporosis is an age-related
disorder that causes the gradual
loss of bone density and strength.
When the thoracic vertebrae are
affected, there can be a gradual
collapse of the vertebrae. This
results in kyphosis, an excessive
curvature of the thoracic region.
Note:
—
meee OPENStAX COLLEGE
Osteoporosis is a common age-related bone disease in which bone density
and strength is decreased. Watch this video to get a better understanding of
how thoracic vertebrae may become weakened and may fracture due to this
disease. How may vertebral osteoporosis contribute to kyphosis?
General Structure of a Vertebra
Within the different regions of the vertebral column, vertebrae vary in size
and shape, but they all follow a similar structural pattern. A typical vertebra
will consist of a body, a vertebral arch, and seven processes ({link]).
The body is the anterior portion of each vertebra and is the part that
supports the body weight. Because of this, the vertebral bodies
progressively increase in size and thickness going down the vertebral
column. The bodies of adjacent vertebrae are separated and strongly united
by an intervertebral disc.
The vertebral arch forms the posterior portion of each vertebra. It consists
of four parts, the right and left pedicles and the right and left laminae. Each
pedicle forms one of the lateral sides of the vertebral arch. The pedicles are
anchored to the posterior side of the vertebral body. Each lamina forms part
of the posterior roof of the vertebral arch. The large opening between the
vertebral arch and body is the vertebral foramen, which contains the
spinal cord. In the intact vertebral column, the vertebral foramina of all of
the vertebrae align to form the vertebral (spinal) canal, which serves as
the bony protection and passageway for the spinal cord down the back.
When the vertebrae are aligned together in the vertebral column, notches in
the margins of the pedicles of adjacent vertebrae together form an
intervertebral foramen, the opening through which a spinal nerve exits
from the vertebral column ([link]).
Seven processes arise from the vertebral arch. Each paired transverse
process projects laterally and arises from the junction point between the
pedicle and lamina. The single spinous process (vertebral spine) projects
posteriorly at the midline of the back. The vertebral spines can easily be felt
as a Series of bumps just under the skin down the middle of the back. The
transverse and spinous processes serve as important muscle attachment
sites. A superior articular process extends or faces upward, and an
inferior articular process faces or projects downward on each side of a
vertebrae. The paired superior articular processes of one vertebra join with
the corresponding paired inferior articular processes from the next higher
vertebra. These junctions form slightly moveable joints between the
adjacent vertebrae. The shape and orientation of the articular processes vary
in different regions of the vertebral column and play a major role in
determining the type and range of motion available in each region.
Parts of a Typical Vertebra
Anterior Posterior
Posterior
Spinal cord
Spinal cord
Spinous
process
Transverse
process
Facet of superior
articular process
Vertebral
foramen
Intervetebral disc
Facet for head
Vertebral arch: of rib
Lamina
Facet of
superior
articular
process
Inferior articular
process
Facet for
head of rib Spinal nerve exiting
Anterior through intervertebral
foramen Spinous process
Superior view Left posterolateral view
of articulated vertebrae
A typical vertebra consists of a body and a vertebral arch. The
arch is formed by the paired pedicles and paired laminae.
Arising from the vertebral arch are the transverse, spinous,
superior articular, and inferior articular processes. The
vertebral foramen provides for passage of the spinal cord. Each
spinal nerve exits through an intervertebral foramen, located
between adjacent vertebrae. Intervertebral discs unite the
bodies of adjacent vertebrae.
Intervertebral Disc
Vertebral body
Intervertebral foramen
o> Anulus fibrosus ~~
Nucleus pulposus
Lateral view Superior view
The bodies of adjacent vertebrae are separated and
united by an intervertebral disc, which provides
padding and allows for movements between adjacent
vertebrae. The disc consists of a fibrous outer layer
called the anulus fibrosus and a gel-like center called
the nucleus pulposus. The intervertebral foramen is the
opening formed between adjacent vertebrae for the
exit of a spinal nerve.
Regional Modifications of Vertebrae
In addition to the general characteristics of a typical vertebra described
above, vertebrae also display characteristic size and structural features that
vary between the different vertebral column regions. Thus, cervical
vertebrae are smaller than lumbar vertebrae due to differences in the
proportion of body weight that each supports. Thoracic vertebrae have sites
for rib attachment, and the vertebrae that give rise to the sacrum and coccyx
have fused together into single bones.
Cervical Vertebrae
Typical cervical vertebrae, such as C4 or C5, have several characteristic
features that differentiate them from thoracic or lumbar vertebrae ([link]).
Cervical vertebrae have a small body, reflecting the fact that they carry the
least amount of body weight. Cervical vertebrae usually have a bifid (Y-
shaped) spinous process. The spinous processes of the C3—C6 vertebrae are
short, but the spine of C7 is much longer. You can find these vertebrae by
running your finger down the midline of the posterior neck until you
encounter the prominent C7 spine located at the base of the neck. The
transverse processes of the cervical vertebrae are sharply curved (U-shaped)
to allow for passage of the cervical spinal nerves. Each transverse process
also has an opening called the transverse foramen. An important artery
that supplies the brain ascends up the neck by passing through these
openings. The superior and inferior articular processes of the cervical
vertebrae are flattened and largely face upward or downward, respectively.
The first and second cervical vertebrae are further modified, giving each a
distinctive appearance. The first cervical (C1) vertebra is also called the
atlas, because this is the vertebra that supports the skull on top of the
vertebral column (in Greek mythology, Atlas was the god who supported
the heavens on his shoulders). The C1 vertebra does not have a body or
spinous process. Instead, it is ring-shaped, consisting of an anterior arch
and a posterior arch. The transverse processes of the atlas are longer and
extend more laterally than do the transverse processes of any other cervical
vertebrae. The superior articular processes face upward and are deeply
curved for articulation with the occipital condyles on the base of the skull.
The inferior articular processes are flat and face downward to join with the
superior articular processes of the C2 vertebra.
The second cervical (C2) vertebra is called the axis, because it serves as the
axis for rotation when turning the head toward the right or left. The axis
resembles typical cervical vertebrae in most respects, but is easily
distinguished by the dens (odontoid process), a bony projection that extends
upward from the vertebral body. The dens joins with the inner aspect of the
anterior arch of the atlas, where it is held in place by transverse ligament.
Cervical Vertebrae
Dens of axis
Transverse
ligament s
Spinous process (bifid) & C4 f
SS ers
Vertebral foramen C; (atlas) “> '
Cz (axis) —— >
Pedicle C3 :
Lamina Inferior
articular
process
Superior
articular
process Bifid spinous
process
ieee? Transverse
process
process
Transverse C; (vertebra
foramen Body Groove prominens)
for spinal
nerve
Structure of a typical cervical vertebra
Dens
Superior articular Transverse
facet fie process
Dens -
Superior articular Anterior arch Transverse
foramen
facet
Transverse
Lamina
process
Transverse
foramen
Posterior arch Spinous process
Ligament
Superior view of atlas Superior view of axis
Dens
Transverse
process
Inferior articular Body
process
Anterior view of axis
A typical cervical vertebra has a small body, a bifid spinous
process, transverse processes that have a transverse foramen
and are curved for spinal nerve passage. The atlas (C1
vertebra) does not have a body or spinous process. It consists
of an anterior and a posterior arch and elongated transverse
processes. The axis (C2 vertebra) has the upward projecting
dens, which articulates with the anterior arch of the atlas.
Thoracic Vertebrae
The bodies of the thoracic vertebrae are larger than those of cervical
vertebrae ({link]). The characteristic feature for a typical midthoracic
vertebra is the spinous process, which is long and has a pronounced
downward angle that causes it to overlap the next inferior vertebra. The
superior articular processes of thoracic vertebrae face anteriorly and the
inferior processes face posteriorly. These orientations are important
determinants for the type and range of movements available to the thoracic
region of the vertebral column.
Thoracic vertebrae have several additional articulation sites, each of which
is called a facet, where a rib is attached. Most thoracic vertebrae have two
facets located on the lateral sides of the body, each of which is called a
costal facet (costal = “rib”). These are for articulation with the head (end)
of arib. An additional facet is located on the transverse process for
articulation with the tubercle of a rib.
Thoracic Vertebrae
Superior articular
process
Articular facet
for tubercle of rib
Transverse
process
Pedicle
Intervertebral
Lamina disc
Body
Spinous
rocess '
P Superior costal
facet
fe
vy] // \nferior Inferior costal
iff articular facet
process
A typical thoracic vertebra is distinguished by the
spinous process, which is long and projects
downward to overlap the next inferior vertebra. It
also has articulation sites (facets) on the vertebral
body and a transverse process for rib attachment.
Rib Articulation in Thoracic Vertebrae
Superior L
articular facets U a
Superior costal facet
7
fi)
Crea ZY
\ Z -
Facet for —_ WAN eS
; \\ YYY
tubercle of rib
Ay Body of vertebra
|
we tl Head of rib
Zl >: b
Ns Intervertebral disc
Tranverse
r ;
processes Fit— Neck of rib
él
{ !
Se Body of vertebra
; 7
= Tubercle of rib
BS .
. Inferior costal facet
Spinous
process
Thoracic vertebrae have superior and
inferior articular facets on the vertebral
body for articulation with the head of a
rib, and a transverse process facet for
articulation with the rib tubercle.
Lumbar Vertebrae
Lumbar vertebrae carry the greatest amount of body weight and are thus
characterized by the large size and thickness of the vertebral body ((link]).
They have short transverse processes and a short, blunt spinous process that
projects posteriorly. The articular processes are large, with the superior
process facing backward and the inferior facing forward.
Lumbar Vertebrae
Superior articular
Transverse
process
Inferior
articular
process
; go ST,
Spinous ————4—“
: [7
process
Intervertebral
disc
Inferior articular process
Lumbar vertebrae are characterized by
having a large, thick body and a short,
rounded spinous process.
Sacrum and Coccyx
The sacrum is a triangular-shaped bone that is thick and wide across its
superior base where it is weight bearing and then tapers down to an inferior,
non-weight bearing apex ([link]). It is formed by the fusion of five sacral
vertebrae, a process that does not begin until after the age of 20. On the
anterior surface of the older adult sacrum, the lines of vertebral fusion can
be seen as four transverse ridges. On the posterior surface, running down
the midline, is the median sacral crest, a bumpy ridge that is the remnant
of the fused spinous processes (median = “midline”; while medial =
“toward, but not necessarily at, the midline”). Similarly, the fused
transverse processes of the sacral vertebrae form the lateral sacral crest.
The sacral promontory is the anterior lip of the superior base of the
sacrum. Lateral to this is the roughened auricular surface, which joins with
the ilium portion of the hipbone to form the immobile sacroiliac joints of
the pelvis. Passing inferiorly through the sacrum is a bony tunnel called the
sacral canal, which terminates at the sacral hiatus near the inferior tip of
the sacrum. The anterior and posterior surfaces of the sacrum have a series
of paired openings called sacral foramina (singular = foramen) that
connect to the sacral canal. Each of these openings is called a posterior
(dorsal) sacral foramen or anterior (ventral) sacral foramen. These
openings allow for the anterior and posterior branches of the sacral spinal
nerves to exit the sacrum. The superior articular process of the sacrum,
one of which is found on either side of the superior opening of the sacral
canal, articulates with the inferior articular processes from the L5 vertebra.
The coccyx, or tailbone, is derived from the fusion of four very small
coccygeal vertebrae (see [link]). It articulates with the inferior tip of the
sacrum. It is not weight bearing in the standing position, but may receive
some body weight when sitting.
Sacrum and Coccyx
Sacral Body Facet of superior
Sacral promontory
articular process
Body of first
sacral vertebra wi
T
Transverse ridges
(sites of vertebral
fusion)
Auricular
surface
Lateral
sacral
crest
Posterior
sacral
foramina
Anterior sacral
foramina A Sacral hiatus
- Coccyx
Anterior view Posterior view
Apex
Coccyx
The sacrum is formed from the fusion of five sacral vertebrae,
whose lines of fusion are indicated by the transverse ridges.
The fused spinous processes form the median sacral crest,
while the lateral sacral crest arises from the fused transverse
processes. The coccyx is formed by the fusion of four small
coccygeal vertebrae.
Intervertebral Discs and Ligaments of the Vertebral Column
The bodies of adjacent vertebrae are strongly anchored to each other by an
intervertebral disc. This structure provides padding between the bones
during weight bearing, and because it can change shape, also allows for
movement between the vertebrae. Although the total amount of movement
available between any two adjacent vertebrae is small, when these
movements are summed together along the entire length of the vertebral
column, large body movements can be produced. Ligaments that extend
along the length of the vertebral column also contribute to its overall
support and stability.
Intervertebral Disc
An intervertebral disc is a fibrocartilaginous pad that fills the gap between
adjacent vertebral bodies (see [link]). Each disc is anchored to the bodies of
its adjacent vertebrae, thus strongly uniting these. The discs also provide
padding between vertebrae during weight bearing. Because of this,
intervertebral discs are thin in the cervical region and thickest in the lumbar
region, which carries the most body weight. In total, the intervertebral discs
account for approximately 25 percent of your body height between the top
of the pelvis and the base of the skull. Intervertebral discs are also flexible
and can change shape to allow for movements of the vertebral column.
Each intervertebral disc consists of two parts. The anulus fibrosus is the
tough, fibrous outer layer of the disc. It forms a circle (anulus = “ring” or
“circle”) and is firmly anchored to the outer margins of the adjacent
vertebral bodies. Inside is the nucleus pulposus, consisting of a softer,
more gel-like material. It has a high water content that serves to resist
compression and thus is important for weight bearing. With increasing age,
the water content of the nucleus pulposus gradually declines. This causes
the disc to become thinner, decreasing total body height somewhat, and
reduces the flexibility and range of motion of the disc, making bending
more difficult.
The gel-like nature of the nucleus pulposus also allows the intervertebral
disc to change shape as one vertebra rocks side to side or forward and back
in relation to its neighbors during movements of the vertebral column.
Thus, bending forward causes compression of the anterior portion of the
disc but expansion of the posterior disc. If the posterior anulus fibrosus is
weakened due to injury or increasing age, the pressure exerted on the disc
when bending forward and lifting a heavy object can cause the nucleus
pulposus to protrude posteriorly through the anulus fibrosus, resulting in a
herniated disc (“ruptured” or “slipped” disc) ({link]). The posterior bulging
of the nucleus pulposus can cause compression of a spinal nerve at the point
where it exits through the intervertebral foramen, with resulting pain and/or
muscle weakness in those body regions supplied by that nerve. The most
common sites for disc herniation are the L4/L5 or L5/S1 intervertebral
discs, which can cause sciatica, a widespread pain that radiates from the
lower back down the thigh and into the leg. Similar injuries of the C5/C6 or
C6/C7 intervertebral discs, following forcible hyperflexion of the neck from
a collision accident or football injury, can produce pain in the neck,
shoulder, and upper limb.
Herniated Intervertebral Disc
Spinal cord within
vertebral canal
Herniated disc
compresses
nerve in
intervertebral
foramen
Nucleus
pulposus \ -
Anulus
fibrosus
Herniated
portion of disc
Superior view
Weakening of the anulus fibrosus can result in
herniation (protrusion) of the nucleus pulposus and
compression of a spinal nerve, resulting in pain
and/or muscle weakness in the body regions
supplied by that nerve.
Note:
os
1 ‘all
at t
4 openstax COLLEGE
er
[my: i C
Watch this animation to see what it means to “slip” a disk. Watch this
second animation to see one possible treatment for a herniated disc,
removing and replacing the damaged disc with an artificial one that allows
for movement between the adjacent certebrae. How could lifting a heavy
object produce pain in a lower limb?
Ligaments of the Vertebral Column
Adjacent vertebrae are united by ligaments that run the length of the
vertebral column along both its posterior and anterior aspects ([link]). These
serve to resist excess forward or backward bending movements of the
vertebral column, respectively.
The anterior longitudinal ligament runs down the anterior side of the
entire vertebral column, uniting the vertebral bodies. It serves to resist
excess backward bending of the vertebral column. Protection against this
movement is particularly important in the neck, where extreme posterior
bending of the head and neck can stretch or tear this ligament, resulting in a
painful whiplash injury. Prior to the mandatory installation of seat
headrests, whiplash injuries were common for passengers involved in a
rear-end automobile collision.
The supraspinous ligament is located on the posterior side of the vertebral
column, where it interconnects the spinous processes of the thoracic and
lumbar vertebrae. This strong ligament supports the vertebral column
during forward bending motions. In the posterior neck, where the cervical
spinous processes are short, the supraspinous ligament expands to become
the nuchal ligament (nuchae = “nape” or “back of the neck”). The nuchal
ligament is attached to the cervical spinous processes and extends upward
and posteriorly to attach to the midline base of the skull, out to the external
occipital protuberance. It supports the skull and prevents it from falling
forward. This ligament is much larger and stronger in four-legged animals
such as cows, where the large skull hangs off the front end of the vertebral
column. You can easily feel this ligament by first extending your head
backward and pressing down on the posterior midline of your neck. Then
tilt your head forward and you will fill the nuchal ligament popping out as it
tightens to limit anterior bending of the head and neck.
Additional ligaments are located inside the vertebral canal, next to the
spinal cord, along the length of the vertebral column. The posterior
longitudinal ligament is found anterior to the spinal cord, where it is
attached to the posterior sides of the vertebral bodies. Posterior to the spinal
cord is the ligamentum flavum (“yellow ligament”). This consists of a
series of short, paired ligaments, each of which interconnects the lamina
regions of adjacent vertebrae. The ligamentum flavum has large numbers of
elastic fibers, which have a yellowish color, allowing it to stretch and then
pull back. Both of these ligaments provide important support for the
vertebral column when bending forward.
Ligaments of Vertebral Column
External
occipital
protuberance
Nuchal
ligament
Spinous process
of T1 vertebra
Anterior
longitudinal
Supraspinous ligament
ligament
The anterior longitudinal ligament
runs the length of the vertebral
column, uniting the anterior sides
of the vertebral bodies. The
supraspinous ligament connects
the spinous processes of the
thoracic and lumbar vertebrae. In
the posterior neck, the
supraspinous ligament enlarges to
form the nuchal ligament, which
attaches to the cervical spinous
processes and to the base of the
skull.
Note:
oe
openstax COLLEGE
Use this tool to identify the bones, intervertebral discs, and ligaments of
the vertebral column. The thickest portions of the anterior longitudinal
ligament and the supraspinous ligament are found in which regions of the
vertebral column?
Note:
Career Connections
Chiropractor
Chiropractors are health professionals who use nonsurgical techniques to
help patients with musculoskeletal system problems that involve the bones,
muscles, ligaments, tendons, or nervous system. They treat problems such
as neck pain, back pain, joint pain, or headaches. Chiropractors focus on
the patient’s overall health and can also provide counseling related to
lifestyle issues, such as diet, exercise, or sleep problems. If needed, they
will refer the patient to other medical specialists.
Chiropractors use a drug-free, hands-on approach for patient diagnosis and
treatment. They will perform a physical exam, assess the patient’s posture
and spine, and may perform additional diagnostic tests, including taking X-
ray images. They primarily use manual techniques, such as spinal
manipulation, to adjust the patient’s spine or other joints. They can
recommend therapeutic or rehabilitative exercises, and some also include
acupuncture, massage therapy, or ultrasound as part of the treatment
program. In addition to those in general practice, some chiropractors
specialize in sport injuries, neurology, orthopaedics, pediatrics, nutrition,
internal disorders, or diagnostic imaging.
To become a chiropractor, students must have 3—4 years of undergraduate
education, attend an accredited, four-year Doctor of Chiropractic (D.C.)
degree program, and pass a licensure examination to be licensed for
practice in their state. With the aging of the baby-boom generation,
employment for chiropractors is expected to increase.
Chapter Review
The vertebral column forms the neck and back. The vertebral column
originally develops as 33 vertebrae, but is eventually reduced to 24
vertebrae, plus the sacrum and coccyx. The vertebrae are divided into the
cervical region (C1—C7 vertebrae), the thoracic region (T1—T12 vertebrae),
and the lumbar region (L1—L5 vertebrae). The sacrum arises from the
fusion of five sacral vertebrae and the coccyx from the fusion of four small
coccygeal vertebrae. The vertebral column has four curvatures, the cervical,
thoracic, lumbar, and sacrococcygeal curves. The thoracic and
sacrococcygeal curves are primary curves retained from the original fetal
curvature. The cervical and lumbar curves develop after birth and thus are
secondary curves. The cervical curve develops as the infant begins to hold
up the head, and the lumbar curve appears with standing and walking.
A typical vertebra consists of an enlarged anterior portion called the body,
which provides weight-bearing support. Attached posteriorly to the body is
a vertebral arch, which surrounds and defines the vertebral foramen for
passage of the spinal cord. The vertebral arch consists of the pedicles,
which attach to the vertebral body, and the laminae, which come together to
form the roof of the arch. Arising from the vertebral arch are the laterally
projecting transverse processes and the posteriorly oriented spinous process.
The superior articular processes project upward, where they articulate with
the downward projecting inferior articular processes of the next higher
vertebrae.
A typical cervical vertebra has a small body, a bifid (Y-shaped) spinous
process, and U-shaped transverse processes with a transverse foramen. In
addition to these characteristics, the axis (C2 vertebra) also has the dens
projecting upward from the vertebral body. The atlas (C1 vertebra) differs
from the other cervical vertebrae in that it does not have a body, but instead
consists of bony ring formed by the anterior and posterior arches. The atlas
articulates with the dens from the axis. A typical thoracic vertebra is
distinguished by its long, downward projecting spinous process. Thoracic
vertebrae also have articulation facets on the body and transverse processes
for attachment of the ribs. Lumbar vertebrae support the greatest amount of
body weight and thus have a large, thick body. They also have a short, blunt
spinous process. The sacrum is triangular in shape. The median sacral crest
is formed by the fused vertebral spinous processes and the lateral sacral
crest is derived from the fused transverse processes. Anterior (ventral) and
posterior (dorsal) sacral foramina allow branches of the sacral spinal nerves
to exit the sacrum. The auricular surfaces are articulation sites on the lateral
sacrum that anchor the sacrum to the hipbones to form the pelvis. The
coccyx is small and derived from the fusion of four small vertebrae.
The intervertebral discs fill in the gaps between the bodies of adjacent
vertebrae. They provide strong attachments and padding between the
vertebrae. The outer, fibrous layer of a disc is called the anulus fibrosus.
The gel-like interior is called the nucleus pulposus. The disc can change
shape to allow for movement between vertebrae. If the anulus fibrosus is
weakened or damaged, the nucleus pulposus can protrude outward,
resulting in a herniated disc.
The anterior longitudinal ligament runs along the full length of the anterior
vertebral column, uniting the vertebral bodies. The supraspinous ligament is
located posteriorly and interconnects the spinous processes of the thoracic
and lumbar vertebrae. In the neck, this ligament expands to become the
nuchal ligament. The nuchal ligament is attached to the cervical spinous
processes and superiorly to the base of the skull, out to the external
occipital protuberance. The posterior longitudinal ligament runs within the
vertebral canal and unites the posterior sides of the vertebral bodies. The
ligamentum flavum unites the lamina of adjacent vertebrae.
Interactive Link Questions
Exercise:
Problem:
Osteoporosis is a common age-related bone disease in which bone
density and strength is decreased. Watch this video to get a better
understanding of how thoracic vertebrae may become weakened and
may fractured due to this disease. How may vertebral osteoporosis
contribute to kyphosis?
Solution:
Osteoporosis causes thinning and weakening of the vertebral bodies.
When this occurs in thoracic vertebrae, the bodies may collapse
producing kyphosis, an enhanced anterior curvature of the thoracic
vertebral column.
Exercise:
Problem:
Watch this animation to see what it means to “slip” a disk. Watch this
second animation to see one possible treatment for a herniated disc,
removing and replacing the damaged disc with an artificial one that
allows for movement between the adjacent certebrae. How could
lifting a heavy object produce pain in a lower limb?
Solution:
Lifting a heavy object can cause an intervertebral disc in the lower
back to bulge and compress a spinal nerve as it exits through the
intervertebral foramen, thus producing pain in those regions of the
lower limb supplied by that nerve.
Exercise:
Problem:
Use this tool to identify the bones, intervertebral discs, and ligaments
of the vertebral column. The thickest portions of the anterior
longitudinal ligament and the supraspinous ligament are found in
which regions of the vertebral column?
Solution:
The anterior longitudinal ligament is thickest in the thoracic region of
the vertebral column, while the supraspinous ligament is thickest in the
lumbar region.
Review Questions
Exercise:
Problem:
The cervical region of the vertebral column consists of
a. seven vertebrae
b. 12 vertebrae
c. five vertebrae
d. a single bone derived from the fusion of five vertebrae
Solution:
A
Exercise:
Problem:The primary curvatures of the vertebral column
a. include the lumbar curve
b. are remnants of the original fetal curvature
c. include the cervical curve
d. develop after the time of birth
Solution:
B
Exercise:
Problem: A typical vertebra has
a. a vertebral foramen that passes through the body
b. a superior articular process that projects downward to articulate
with the superior portion of the next lower vertebra
c. lamina that spans between the transverse process and spinous
process
d. a pair of laterally projecting spinous processes
Solution:
C
Exercise:
Problem:A typical lumbar vertebra has
a. a Short, rounded spinous process
b. a bifid spinous process
c. articulation sites for ribs
d. a transverse foramen
Solution:
A
Exercise:
Problem:
Which is found only in the cervical region of the vertebral column?
a. nuchal ligament
b. ligamentum flavum
c. supraspinous ligament
d. anterior longitudinal ligament
Solution:
A
Critical Thinking Questions
Exercise:
Problem: Describe the vertebral column and define each region.
Solution:
Answer: The adult vertebral column consists of 24 vertebrae, plus the
sacrum and coccyx. The vertebrae are subdivided into cervical,
thoracic, and lumbar regions. There are seven cervical vertebrae (C1—
C7), 12 thoracic vertebrae (T1—T12), and five lumbar vertebrae (L1—
L5). The sacrum is derived from the fusion of five sacral vertebrae and
the coccyx is formed by the fusion of four small coccygeal vertebrae.
Exercise:
Problem: Describe a typical vertebra.
Solution:
A typical vertebra consists of an anterior body and a posterior vertebral
arch. The body serves for weight bearing. The vertebral arch surrounds
and protects the spinal cord. The vertebral arch is formed by the
pedicles, which are attached to the posterior side of the vertebral body,
and the lamina, which come together to form the top of the arch. A
pair of transverse processes extends laterally from the vertebral arch, at
the junction between each pedicle and lamina. The spinous process
extends posteriorly from the top of the arch. A pair of superior
articular processes project upward and a pair of inferior articular
processes project downward. Together, the notches found in the
margins of the pedicles of adjacent vertebrae form an intervertebral
foramen.
Exercise:
Problem: Describe the sacrum.
Solution:
The sacrum is a single, triangular-shaped bone formed by the fusion of
five sacral vertebrae. On the posterior sacrum, the median sacral crest
is derived from the fused spinous processes, and the lateral sacral crest
results from the fused transverse processes. The sacral canal contains
the sacral spinal nerves, which exit via the anterior (ventral) and
posterior (dorsal) sacral foramina. The sacral promontory is the
anterior lip. The sacrum also forms the posterior portion of the pelvis.
Exercise:
Problem: Describe the structure and function of an intervertebral disc.
Solution:
An intervertebral disc fills in the space between adjacent vertebrae,
where it provides padding and weight-bearing ability, and allows for
movements between the vertebrae. It consists of an outer anulus
fibrosus and an inner nucleus pulposus. The anulus fibrosus strongly
anchors the adjacent vertebrae to each other, and the high water
content of the nucleus pulposus resists compression for weight bearing
and can change shape to allow for vertebral column movements.
Exercise:
Problem: Define the ligaments of the vertebral column.
Solution:
The anterior longitudinal ligament is attached to the vertebral bodies
on the anterior side of the vertebral column. The supraspinous
ligament is located on the posterior side, where it interconnects the
thoracic and lumbar spinous processes. In the posterior neck, this
ligament expands to become the nuchal ligament, which attaches to the
cervical spinous processes and the base of the skull. The posterior
longitudinal ligament and ligamentum flavum are located inside the
vertebral canal. The posterior longitudinal ligament unites the posterior
sides of the vertebral bodies. The ligamentum flavum unites the lamina
of adjacent vertebrae.
Glossary
anterior arch
anterior portion of the ring-like C1 (atlas) vertebra
anterior longitudinal ligament
ligament that runs the length of the vertebral column, uniting the
anterior aspects of the vertebral bodies
anterior (ventral) sacral foramen
one of the series of paired openings located on the anterior (ventral)
side of the sacrum
anulus fibrosus
tough, fibrous outer portion of an intervertebral disc, which is strongly
anchored to the bodies of the adjacent vertebrae
atlas
first cervical (C1) vertebra
axis
second cervical (C2) vertebra
cervical curve
posteriorly concave curvature of the cervical vertebral column region;
a secondary curve of the vertebral column
cervical vertebrae
seven vertebrae numbered as C1—C7 that are located in the neck region
of the vertebral column
costal facet
site on the lateral sides of a thoracic vertebra for articulation with the
head of a rib
dens
bony projection (odontoid process) that extends upward from the body
of the C2 (axis) vertebra
facet
small, flattened area on a bone for an articulation (joint) with another
bone, or for muscle attachment
inferior articular process
bony process that extends downward from the vertebral arch of a
vertebra that articulates with the superior articular process of the next
lower vertebra
intervertebral disc
structure located between the bodies of adjacent vertebrae that strongly
joins the vertebrae; provides padding, weight bearing ability, and
enables vertebral column movements
intervertebral foramen
opening located between adjacent vertebrae for exit of a spinal nerve
kyphosis
(also, humpback or hunchback) excessive posterior curvature of the
thoracic vertebral column region
lamina
portion of the vertebral arch on each vertebra that extends between the
transverse and spinous process
lateral sacral crest
paired irregular ridges running down the lateral sides of the posterior
sacrum that was formed by the fusion of the transverse processes from
the five sacral vertebrae
ligamentum flavum
series of short ligaments that unite the lamina of adjacent vertebrae
lordosis
(also, swayback) excessive anterior curvature of the lumbar vertebral
column region
lumbar curve
posteriorly concave curvature of the lumbar vertebral column region; a
secondary curve of the vertebral column
lumbar vertebrae
five vertebrae numbered as L1—L5 that are located in lumbar region
(lower back) of the vertebral column
median sacral crest
irregular ridge running down the midline of the posterior sacrum that
was formed from the fusion of the spinous processes of the five sacral
vertebrae
nuchal ligament
expanded portion of the supraspinous ligament within the posterior
neck; interconnects the spinous processes of the cervical vertebrae and
attaches to the base of the skull
nucleus pulposus
gel-like central region of an intervertebral disc; provides for padding,
weight-bearing, and movement between adjacent vertebrae
pedicle
portion of the vertebral arch that extends from the vertebral body to the
transverse process
posterior arch
posterior portion of the ring-like C1 (atlas) vertebra
posterior longitudinal ligament
ligament that runs the length of the vertebral column, uniting the
posterior sides of the vertebral bodies
posterior (dorsal) sacral foramen
one of the series of paired openings located on the posterior (dorsal)
side of the sacrum
primary curve
anteriorly concave curvatures of the thoracic and sacrococcygeal
regions that are retained from the original fetal curvature of the
vertebral column
sacral canal
bony tunnel that runs through the sacrum
sacral foramina
series of paired openings for nerve exit located on both the anterior
(ventral) and posterior (dorsal) aspects of the sacrum
sacral hiatus
inferior opening and termination of the sacral canal
sacral promontory
anterior lip of the base (superior end) of the sacrum
sacrococcygeal curve
anteriorly concave curvature formed by the sacrum and coccyx; a
primary curve of the vertebral column
scoliosis
abnormal lateral curvature of the vertebral column
secondary curve
posteriorly concave curvatures of the cervical and lumbar regions of
the vertebral column that develop after the time of birth
spinous process
unpaired bony process that extends posteriorly from the vertebral arch
of a vertebra
superior articular process
bony process that extends upward from the vertebral arch of a vertebra
that articulates with the inferior articular process of the next higher
vertebra
superior articular process of the sacrum
paired processes that extend upward from the sacrum to articulate
(join) with the inferior articular processes from the L5 vertebra
supraspinous ligament
ligament that interconnects the spinous processes of the thoracic and
lumbar vertebrae
thoracic curve
anteriorly concave curvature of the thoracic vertebral column region; a
primary curve of the vertebral column
thoracic vertebrae
twelve vertebrae numbered as T1—T12 that are located in the thoracic
region (upper back) of the vertebral column
transverse foramen
opening found only in the transverse processes of cervical vertebrae
transverse process
paired bony processes that extends laterally from the vertebral arch of
a vertebra
vertebral arch
bony arch formed by the posterior portion of each vertebra that
surrounds and protects the spinal cord
vertebral (spinal) canal
bony passageway within the vertebral column for the spinal cord that
is formed by the series of individual vertebral foramina
vertebral foramen
opening associated with each vertebra defined by the vertebral arch
that provides passage for the spinal cord
The Thoracic Cage
By the end of this section, you will be able to:
e Discuss the components that make up the thoracic cage
e Identify the parts of the sternum and define the sternal angle
e Discuss the parts of a rib and rib classifications
The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It
consists of the 12 pairs of ribs with their costal cartilages and the sternum
({link]). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1—
T12). The thoracic cage protects the heart and lungs.
Thoracic Cage
Superior Superior
Jugular
Clavicular a notch
notch or ——
Jugular notch Clavicular notch
Manubrium Clavicle
Sternal 4
angle Sternum:
Manubrium Scapula
Body
§
Sternal angle
Body —_
) Xiphoid
: process
Costal cartilages
Intercostal space
Xiphoid aw
process
Inferior Inferior
(a) Anterior view of sternum (b) Anterior view of skeleton of thorax
The thoracic cage is formed by the (a) sternum and (b) 12 pairs
of ribs with their costal cartilages. The ribs are anchored
posteriorly to the 12 thoracic vertebrae. The sternum consists
of the manubrium, body, and xiphoid process. The ribs are
classified as true ribs (1—7) and false ribs (8-12). The last two
pairs of false ribs are also known as floating ribs (11-12).
Sternum
The sternum is the elongated bony structure that anchors the anterior
thoracic cage. It consists of three parts: the manubrium, body, and xiphoid
process. The manubrium is the wider, superior portion of the sternum. The
top of the manubrium has a shallow, U-shaped border called the jugular
(suprasternal) notch. This can be easily felt at the anterior base of the
neck, between the medial ends of the clavicles. The clavicular notch is the
shallow depression located on either side at the superior-lateral margins of
the manubrium. This is the site of the sternoclavicular joint, between the
sternum and clavicle. The first ribs also attach to the manubrium.
The elongated, central portion of the sternum is the body. The manubrium
and body join together at the sternal angle, so called because the junction
between these two components is not flat, but forms a slight bend. The
second rib attaches to the sternum at the sternal angle. Since the first rib is
hidden behind the clavicle, the second rib is the highest rib that can be
identified by palpation. Thus, the sternal angle and second rib are important
landmarks for the identification and counting of the lower ribs. Ribs 3—7
attach to the sternal body.
The inferior tip of the sternum is the xiphoid process. This small structure
is cartilaginous early in life, but gradually becomes ossified starting during
middle age.
Ribs
Each rib is a curved, flattened bone that contributes to the wall of the
thorax. The ribs articulate posteriorly with the T1—-T12 thoracic vertebrae,
and most attach anteriorly via their costal cartilages to the sternum. There
are 12 pairs of ribs. The ribs are numbered 1—12 in accordance with the
thoracic vertebrae.
Parts of a Typical Rib
The posterior end of a typical rib is called the head of the rib (see [link]).
This region articulates primarily with the costal facet located on the body of
the same numbered thoracic vertebra and to a lesser degree, with the costal
facet located on the body of the next higher vertebra. Lateral to the head is
the narrowed neck of the rib. A small bump on the posterior rib surface is
the tubercle of the rib, which articulates with the facet located on the
transverse process of the same numbered vertebra. The remainder of the rib
is the body of the rib (shaft). Just lateral to the tubercle is the angle of the
rib, the point at which the rib has its greatest degree of curvature. The
angles of the ribs form the most posterior extent of the thoracic cage. In the
anatomical position, the angles align with the medial border of the scapula.
A shallow costal groove for the passage of blood vessels and a nerve is
found along the inferior margin of each rib.
Rib Classifications
The bony ribs do not extend anteriorly completely around to the sternum.
Instead, each rib ends in a costal cartilage. These cartilages are made of
hyaline cartilage and can extend for several inches. Most ribs are then
attached, either directly or indirectly, to the sternum via their costal
cartilage (see [link]). The ribs are classified into three groups based on their
relationship to the sternum.
Ribs 1—7 are classified as true ribs (vertebrosternal ribs). The costal
cartilage from each of these ribs attaches directly to the sternum. Ribs 8-12
are called false ribs (vertebrochondral ribs). The costal cartilages from
these ribs do not attach directly to the sternum. For ribs 8—10, the costal
cartilages are attached to the cartilage of the next higher rib. Thus, the
cartilage of rib 10 attaches to the cartilage of rib 9, rib 9 then attaches to rib
8, and rib 8 is attached to rib 7. The last two false ribs (11-12) are also
called floating ribs (vertebral ribs). These are short ribs that do not attach
to the sternum at all. Instead, their small costal cartilages terminate within
the musculature of the lateral abdominal wall.
Chapter Review
The thoracic cage protects the heart and lungs. It is composed of 12 pairs of
ribs with their costal cartilages and the sternum. The ribs are anchored
posteriorly to the 12 thoracic vertebrae. The sternum consists of the
manubrium, body, and xiphoid process. The manubrium and body are
joined at the sternal angle, which is also the site for attachment of the
second ribs.
Ribs are flattened, curved bones and are numbered 1-12. Posteriorly, the
head of the rib articulates with the costal facets located on the bodies of
thoracic vertebrae and the rib tubercle articulates with the facet located on
the vertebral transverse process. The angle of the ribs forms the most
posterior portion of the thoracic cage. The costal groove in the inferior
margin of each rib carries blood vessels and a nerve. Anteriorly, each rib
ends in a costal cartilage. True ribs (1—7) attach directly to the sternum via
their costal cartilage. The false ribs (8-12) either attach to the sternum
indirectly or not at all. Ribs 8—10 have their costal cartilages attached to the
cartilage of the next higher rib. The floating ribs (11-12) are short and do
not attach to the sternum or to another rib.
Review Questions
Exercise:
Problem: The sternum
a. consists of only two parts, the manubrium and xiphoid process
b. has the sternal angle located between the manubrium and body
c. receives direct attachments from the costal cartilages of all 12
pairs of ribs
d. articulates directly with the thoracic vertebrae
Solution:
B
Exercise:
Problem:The sternal angle is the
a. junction between the body and xiphoid process
b. site for attachment of the clavicle
c. site for attachment of the floating ribs
d. junction between the manubrium and body
Solution:
D
Exercise:
Problem:The tubercle of a rib
a. is for articulation with the transverse process of a thoracic
vertebra
b. is for articulation with the body of a thoracic vertebra
c. provides for passage of blood vessels and a nerve
d. is the area of greatest rib curvature
Solution:
A
Exercise:
Problem: True ribs are
a. ribs 8-12
b. attached via their costal cartilage to the next higher rib
c. made entirely of bone, and thus do not have a costal cartilage
d. attached via their costal cartilage directly to the sternum
Solution:
D
Critical Thinking Questions
Exercise:
Problem: Define the parts and functions of the thoracic cage.
Solution:
The thoracic cage is formed by the 12 pairs of ribs with their costal
cartilages and the sternum. The ribs are attached posteriorly to the 12
thoracic vertebrae and most are anchored anteriorly either directly or
indirectly to the sternum. The thoracic cage functions to protect the
heart and lungs.
Exercise:
Problem: Describe the parts of the sternum.
Solution:
The sternum consists of the manubrium, body, and xiphoid process.
The manubrium forms the expanded, superior end of the sternum. It
has a jugular (suprasternal) notch, a pair of clavicular notches for
articulation with the clavicles, and receives the costal cartilage of the
first rib. The manubrium is joined to the body of the sternum at the
sternal angle, which is also the site for attachment of the second rib
costal cartilages. The body receives the costal cartilage attachments for
ribs 3-7. The small xiphoid process forms the inferior tip of the
sternum.
Exercise:
Problem: Discuss the parts of a typical rib.
Solution:
A typical rib is a flattened, curved bone. The head of a rib is attached
posteriorly to the costal facets of the thoracic vertebrae. The rib
tubercle articulates with the transverse process of a thoracic vertebra.
The angle is the area of greatest rib curvature and forms the largest
portion of the thoracic cage. The body (shaft) of a rib extends
anteriorly and terminates at the attachment to its costal cartilage. The
shallow costal groove runs along the inferior margin of a rib and
carries blood vessels and a nerve.
Exercise:
Problem: Define the classes of ribs.
Solution:
Ribs are classified based on if and how their costal cartilages attach to
the sternum. True (vertebrosternal) ribs are ribs 1—7. The costal
cartilage for each of these attaches directly to the sternum. False
(vertebrochondral) ribs, 8—12, are attached either indirectly or not at all
to the sternum. Ribs 8—10 are attached indirectly to the sternum. For
these ribs, the costal cartilage of each attaches to the cartilage of the
next higher rib. The last false ribs (11-12) are also called floating
(vertebral) ribs, because these ribs do not attach to the sternum at all.
Instead, the ribs and their small costal cartilages terminate within the
muscles of the lateral abdominal wall.
Glossary
angle of the rib
portion of rib with greatest curvature; together, the rib angles form the
most posterior extent of the thoracic cage
body of the rib
shaft portion of a rib
clavicular notch
paired notches located on the superior-lateral sides of the sternal
manubrium, for articulation with the clavicle
costal cartilage
hyaline cartilage structure attached to the anterior end of each rib that
provides for either direct or indirect attachment of most ribs to the
sternum
costal groove
shallow groove along the inferior margin of a rib that provides passage
for blood vessels and a nerve
false ribs
vertebrochondral ribs 8-12 whose costal cartilage either attaches
indirectly to the sternum via the costal cartilage of the next higher rib
or does not attach to the sternum at all
floating ribs
vertebral ribs 11—12 that do not attach to the sternum or to the costal
cartilage of another rib
head of the rib
posterior end of a rib that articulates with the bodies of thoracic
vertebrae
jugular (suprasternal) notch
shallow notch located on superior surface of sternal manubrium
manubrium
expanded, superior portion of the sternum
neck of the rib
narrowed region of a rib, next to the rib head
sternal angle
junction line between manubrium and body of the sternum and the site
for attachment of the second rib to the sternum
true ribs
vertebrosternal ribs 1—7 that attach via their costal cartilage directly to
the sternum
tubercle of the rib
small bump on the posterior side of a rib for articulation with the
transverse process of a thoracic vertebra
xiphoid process
small process that forms the inferior tip of the sternum
The Pectoral Girdle
By the end of this section, you will be able to:
¢ Describe the bones that form the pectoral girdle
e List the functions of the pectoral girdle
The appendicular skeleton includes all of the limb bones, plus the bones
that unite each limb with the axial skeleton ({link]). The bones that attach
each upper limb to the axial skeleton form the pectoral girdle (shoulder
girdle). This consists of two bones, the scapula and clavicle ([link]). The
clavicle (collarbone) is an S-shaped bone located on the anterior side of the
shoulder. It is attached on its medial end to the sternum of the thoracic cage,
which is part of the axial skeleton. The lateral end of the clavicle articulates
(joins) with the scapula just above the shoulder joint. You can easily
palpate, or feel with your fingers, the entire length of your clavicle.
Axial and Appendicular Skeletons
Vertebral
column
Pelvic
girdle
(hip bones)
w
i
Anterior view
Skull
Cranial portion
Facial portion
Pectoral (shoulder) girdle
Clavicle
Scapula
Thoracic cage
Sternum
Ribs
Upper limb Vertebral
Humerus column
vine Pelvic
Radius girdle
Carpals (hip bones)
Metacarpals
A Ny
Phalanges
Lower limb
Femur
Patella
| Key
Tee © Axial skeleton
Fibula | Appendicular
skeleton
Tarsals
Metatarsals |
Phalanges Hy)
Posterior view
The axial skeleton forms the central axis of the body and consists of
the skull, vertebral column, and thoracic cage. The appendicular
skeleton consists of the pectoral and pelvic girdles, the limb bones,
and the bones of the hands and feet.
Pectoral Girdle
Coracoclavicular Costoclavicular
ligament ligament
Clavicle Clavicle
Acromioclavicular
joint
Anterior view of pectoral girdle Posterior view of pectoral girdle
Posterior
Lateral
Anterior
Superior view of clavicle
Acromial end Sternal end
Anterior
Posterior
Inferior view of clavicle
The pectoral girdle consists of the clavicle and the
scapula, which serve to attach the upper limb to
the sternum of the axial skeleton.
The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It
is supported by the clavicle, which also articulates with the humerus (arm
bone) to form the shoulder joint. The scapula is a flat, triangular-shaped
bone with a prominent ridge running across its posterior surface. This ridge
extends out laterally, where it forms the bony tip of the shoulder and joins
with the lateral end of the clavicle. By following along the clavicle, you can
palpate out to the bony tip of the shoulder, and from there, you can move
back across your posterior shoulder to follow the ridge of the scapula. Move
your shoulder around and feel how the clavicle and scapula move together
as a unit. Both of these bones serve as important attachment sites for
muscles that aid with movements of the shoulder and arm.
The right and left pectoral girdles are not joined to each other, allowing
each to operate independently. In addition, the clavicle of each pectoral
girdle is anchored to the axial skeleton by a single, highly mobile joint.
This allows for the extensive mobility of the entire pectoral girdle, which in
turn enhances movements of the shoulder and upper limb.
Clavicle
The clavicle is the only long bone that lies in a horizontal position in the
body (see [link]). The clavicle has several important functions. First,
anchored by muscles from above, it serves as a strut that extends laterally to
support the scapula. This in turn holds the shoulder joint superiorly and
laterally from the body trunk, allowing for maximal freedom of motion for
the upper limb. The clavicle also transmits forces acting on the upper limb
to the sternum and axial skeleton. Finally, it serves to protect the underlying
nerves and blood vessels as they pass between the trunk of the body and the
upper limb.
The clavicle has three regions: the medial end, the lateral end, and the shaft.
The medial end, known as the sternal end of the clavicle, has a triangular
shape and articulates with the manubrium portion of the sternum. This
forms the sternoclavicular joint, which is the only bony articulation
between the pectoral girdle of the upper limb and the axial skeleton. This
joint allows considerable mobility, enabling the clavicle and scapula to
move in upward/downward and anterior/posterior directions during
shoulder movements. The sternoclavicular joint is indirectly supported by
the costoclavicular ligament (costo- = “rib”), which spans the sternal end
of the clavicle and the underlying first rib. The lateral or acromial end of
the clavicle articulates with the acromion of the scapula, the portion of the
scapula that forms the bony tip of the shoulder. There are some sex
differences in the morphology of the clavicle. In women, the clavicle tends
to be shorter, thinner, and less curved. In men, the clavicle is heavier and
longer, and has a greater curvature and rougher surfaces where muscles
attach, features that are more pronounced in manual workers.
The clavicle is the most commonly fractured bone in the body. Such breaks
often occur because of the force exerted on the clavicle when a person falls
onto his or her outstretched arms, or when the lateral shoulder receives a
strong blow. Because the sternoclavicular joint is strong and rarely
dislocated, excessive force results in the breaking of the clavicle, usually
between the middle and lateral portions of the bone. If the fracture is
complete, the shoulder and lateral clavicle fragment will drop due to the
weight of the upper limb, causing the person to support the sagging limb
with their other hand. Muscles acting across the shoulder will also pull the
shoulder and lateral clavicle anteriorly and medially, causing the clavicle
fragments to override. The clavicle overlies many important blood vessels
and nerves for the upper limb, but fortunately, due to the anterior
displacement of a broken clavicle, these structures are rarely affected when
the clavicle is fractured.
Scapula
The scapula is also part of the pectoral girdle and thus plays an important
role in anchoring the upper limb to the body. The scapula is located on the
posterior side of the shoulder. It is surrounded by muscles on both its
anterior (deep) and posterior (superficial) sides, and thus does not articulate
with the ribs of the thoracic cage.
The scapula has several important landmarks ([link]). The three margins or
borders of the scapula, named for their positions within the body, are the
superior border of the scapula, the medial border of the scapula, and
the lateral border of the scapula. The suprascapular notch is located
lateral to the midpoint of the superior border. The corners of the triangular
scapula, at either end of the medial border, are the superior angle of the
scapula, located between the medial and superior borders, and the inferior
angle of the scapula, located between the medial and lateral borders. The
inferior angle is the most inferior portion of the scapula, and is particularly
important because it serves as the attachment point for several powerful
muscles involved in shoulder and upper limb movements. The remaining
corner of the scapula, between the superior and lateral borders, is the
location of the glenoid cavity (glenoid fossa). This shallow depression
articulates with the humerus bone of the arm to form the glenohumeral
joint (shoulder joint). The small bony bumps located immediately above
and below the glenoid cavity are the supraglenoid tubercle and the
infraglenoid tubercle, respectively. These provide attachments for muscles
of the arm.
Scapula
oS Pectoral girdle:
Acromion Suprascapular Superior border Coracoid process
Suprascapular notch
Coracoid F
process Acromion
Glenoid
. Glenoid
cavity
cavity
Superior
Sy ee angle ee -
¥ _~—=——_> Supraspinous l-—~\ b
. \ fossa Te
;
\ - ] fae
ea la
foss:
\ ee
fossa
\ | ey
Lateral border \7 Medial border
=4_—_——_ inferior angle———
Lateral border
Right scapula, anterior aspect Right scapula, posterior aspect
The isolated scapula is shown here from its
anterior (deep) side and its posterior (superficial)
side.
The scapula also has two prominent projections. Toward the lateral end of
the superior border, between the suprascapular notch and glenoid cavity, is
the hook-like coracoid process (coracoid = “shaped like a crow’s beak”).
This process projects anteriorly and curves laterally. At the shoulder, the
coracoid process is located inferior to the lateral end of the clavicle. It is
anchored to the clavicle by a strong ligament, and serves as the attachment
site for muscles of the anterior chest and arm. On the posterior aspect, the
spine of the scapula is a long and prominent ridge that runs across its
upper portion. Extending laterally from the spine is a flattened and
expanded region called the acromion or acromial process. The acromion
forms the bony tip of the superior shoulder region and articulates with the
lateral end of the clavicle, forming the acromioclavicular joint (see [link]).
Together, the clavicle, acromion, and spine of the scapula form a V-shaped
bony line that provides for the attachment of neck and back muscles that act
on the shoulder, as well as muscles that pass across the shoulder joint to act
on the arm.
The scapula has three depressions, each of which is called a fossa (plural =
fossae). Two of these are found on the posterior scapula, above and below
the scapular spine. Superior to the spine is the narrow supraspinous fossa,
and inferior to the spine is the broad infraspinous fossa. The anterior
(deep) surface of the scapula forms the broad subscapular fossa. All of
these fossae provide large surface areas for the attachment of muscles that
cross the shoulder joint to act on the humerus.
The acromioclavicular joint transmits forces from the upper limb to the
clavicle. The ligaments around this joint are relatively weak. A hard fall
onto the elbow or outstretched hand can stretch or tear the
acromioclavicular ligaments, resulting in a moderate injury to the joint.
However, the primary support for the acromioclavicular joint comes from a
very strong ligament called the coracoclavicular ligament (see [link]).
This connective tissue band anchors the coracoid process of the scapula to
the inferior surface of the acromial end of the clavicle and thus provides
important indirect support for the acromioclavicular joint. Following a
strong blow to the lateral shoulder, such as when a hockey player is driven
into the boards, a complete dislocation of the acromioclavicular joint can
result. In this case, the acromion is thrust under the acromial end of the
clavicle, resulting in ruptures of both the acromioclavicular and
coracoclavicular ligaments. The scapula then separates from the clavicle,
with the weight of the upper limb pulling the shoulder downward. This
dislocation injury of the acromioclavicular joint is known as a “shoulder
separation” and is common in contact sports such as hockey, football, or
martial arts.
Chapter Review
The pectoral girdle, consisting of the clavicle and the scapula, attaches each
upper limb to the axial skeleton. The clavicle is an anterior bone whose
sternal end articulates with the manubrium of the sternum at the
sternoclavicular joint. The sternal end is also anchored to the first rib by the
costoclavicular ligament. The acromial end of the clavicle articulates with
the acromion of the scapula at the acromioclavicular joint. This end is also
anchored to the coracoid process of the scapula by the coracoclavicular
ligament, which provides indirect support for the acromioclavicular joint.
The clavicle supports the scapula, transmits the weight and forces from the
upper limb to the body trunk, and protects the underlying nerves and blood
vessels.
The scapula lies on the posterior aspect of the pectoral girdle. It mediates
the attachment of the upper limb to the clavicle, and contributes to the
formation of the glenohumeral (shoulder) joint. This triangular bone has
three sides called the medial, lateral, and superior borders. The
suprascapular notch is located on the superior border. The scapula also has
three corners, two of which are the superior and inferior angles. The third
corner is occupied by the glenoid cavity. Posteriorly, the spine separates the
supraspinous and infraspinous fossae, and then extends laterally as the
acromion. The subscapular fossa is located on the anterior surface of the
scapula. The coracoid process projects anteriorly, passing inferior to the
lateral end of the clavicle.
Review Questions
Exercise:
Problem: Which part of the clavicle articulates with the manubrium?
a. shaft
b. sternal end
c. acromial end
d. coracoid process
Solution:
B
Exercise:
Problem:A shoulder separation results from injury to the
a. glenohumeral joint
b. costoclavicular joint
c. acromioclavicular joint
d. sternoclavicular joint
Solution:
C
Exercise:
Problem:
Which feature lies between the spine and superior border of the
scapula?
a. suprascapular notch
b. glenoid cavity
c. superior angle
d. supraspinous fossa
Solution:
D
Exercise:
Problem: What structure is an extension of the spine of the scapula?
a. acromion
b. coracoid process
c. supraglenoid tubercle
d. glenoid cavity
Solution:
A
Exercise:
Problem:
Name the short, hook-like bony process of the scapula that projects
anteriorly.
a. acromial process
b. clavicle
c. coracoid process
d. glenoid fossa
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Describe the shape and palpable line formed by the clavicle and
scapula.
Solution:
The clavicle extends laterally across the anterior shoulder and can be
palpated along its entire length. At its lateral end, the clavicle
articulates with the acromion of the scapula, which forms the bony tip
of the shoulder. The acromion is continuous with the spine of the
scapula, which can be palpated medially and posteriorly along its
length. Together, the clavicle, acromion, and spine of the scapula form
a V-shaped line that serves as an important area for muscle attachment.
Exercise:
Problem:
Discuss two possible injuries of the pectoral girdle that may occur
following a strong blow to the shoulder or a hard fall onto an
outstretched hand.
Solution:
A blow to the shoulder or falling onto an outstretched hand passes
strong forces through the scapula to the clavicle and sternum. A hard
fall may thus cause a fracture of the clavicle (broken collarbone) or
may injure the ligaments of the acromioclavicular joint. In a severe
case, the coracoclavicular ligament may also rupture, resulting in
complete dislocation of the acromioclavicular joint (a “shoulder
separation”).
Glossary
acromial end of the clavicle
lateral end of the clavicle that articulates with the acromion of the
scapula
acromial process
acromion of the scapula
acromioclavicular joint
articulation between the acromion of the scapula and the acromial end
of the clavicle
acromion
flattened bony process that extends laterally from the scapular spine to
form the bony tip of the shoulder
clavicle
collarbone; elongated bone that articulates with the manubrium of the
sternum medially and the acromion of the scapula laterally
coracoclavicular ligament
strong band of connective tissue that anchors the coracoid process of
the scapula to the lateral clavicle; provides important indirect support
for the acromioclavicular joint
coracoid process
short, hook-like process that projects anteriorly and laterally from the
superior margin of the scapula
costoclavicular ligament
band of connective tissue that unites the medial clavicle with the first
rib
fossa
(plural = fossae) shallow depression on the surface of a bone
glenohumeral joint
shoulder joint; formed by the articulation between the glenoid cavity
of the scapula and the head of the humerus
glenoid cavity
(also, glenoid fossa) shallow depression located on the lateral scapula,
between the superior and lateral borders
inferior angle of the scapula
inferior corner of the scapula located where the medial and lateral
borders meet
infraglenoid tubercle
small bump or roughened area located on the lateral border of the
scapula, near the inferior margin of the glenoid cavity
infraspinous fossa
broad depression located on the posterior scapula, inferior to the spine
lateral border of the scapula
diagonally oriented lateral margin of the scapula
medial border of the scapula
elongated, medial margin of the scapula
pectoral girdle
shoulder girdle; the set of bones, consisting of the scapula and clavicle,
which attaches each upper limb to the axial skeleton
scapula
shoulder blade bone located on the posterior side of the shoulder
spine of the scapula
prominent ridge passing mediolaterally across the upper portion of the
posterior scapular surface
sternal end of the clavicle
medial end of the clavicle that articulates with the manubrium of the
sternum
sternoclavicular joint
articulation between the manubrium of the sternum and the sternal end
of the clavicle; forms the only bony attachment between the pectoral
girdle of the upper limb and the axial skeleton
subscapular fossa
broad depression located on the anterior (deep) surface of the scapula
superior angle of the scapula
comer of the scapula between the superior and medial borders of the
scapula
superior border of the scapula
superior margin of the scapula
supraglenoid tubercle
small bump located at the superior margin of the glenoid cavity
suprascapular notch
small notch located along the superior border of the scapula, medial to
the coracoid process
supraspinous fossa
narrow depression located on the posterior scapula, superior to the
spine
Bones of the Upper Limb
By the end of this section, you will be able to:
e Identify the divisions of the upper limb and describe the bones in each
region
e List the bones and bony landmarks that articulate at each joint of the
upper limb
The upper limb is divided into three regions. These consist of the arm,
located between the shoulder and elbow joints; the forearm, which is
between the elbow and wrist joints; and the hand, which is located distal to
the wrist. There are 30 bones in each upper limb (see [link]). The humerus
is the single bone of the upper arm, and the ulna (medially) and the radius
(laterally) are the paired bones of the forearm. The base of the hand
contains eight bones, each called a carpal bone, and the palm of the hand is
formed by five bones, each called a metacarpal bone. The fingers and
thumb contain a total of 14 bones, each of which is a phalanx bone of the
hand.
Humerus
The humerus is the single bone of the upper arm region ([link]). At its
proximal end is the head of the humerus. This is the large, round, smooth
region that faces medially. The head articulates with the glenoid cavity of
the scapula to form the glenohumeral (shoulder) joint. The margin of the
smooth area of the head is the anatomical neck of the humerus. Located on
the lateral side of the proximal humerus is an expanded bony area called the
greater tubercle. The smaller lesser tubercle of the humerus is found on
the anterior aspect of the humerus. Both the greater and lesser tubercles
serve as attachment sites for muscles that act across the shoulder joint.
Passing between the greater and lesser tubercles is the narrow
intertubercular groove (sulcus), which is also known as the bicipital
groove because it provides passage for a tendon of the biceps brachii
muscle. The surgical neck is located at the base of the expanded, proximal
end of the humerus, where it joins the narrow shaft of the humerus. The
surgical neck is a common site of arm fractures. The deltoid tuberosity is a
roughened, V-shaped region located on the lateral side in the middle of the
humerus shaft. As its name indicates, it is the site of attachment for the
deltoid muscle.
Humerus and Elbow Joint
Humerus
Anatomical
Greater (Oy, Gin Greater
tubercle 1 \ ) Head tubercle
Lesser
tubercle
Intertubercular
groove (sulcus)
Deltoid
| tuberosity
| Body (shaft)
Lateral
supracondylar
ridge
Radial
fossa
Olecranon fossa Lateral
Coronoid fossa epicondyle
Olecranon
of ulna
Head of
radius
Capitulum —+~—— Medial
Head of epicondyle
radius Trochlea
Coronoid process
of ulna
Anterior view Posterior view
The humerus is the single bone of the
upper arm region. It articulates with
the radius and ulna bones of the
forearm to form the elbow joint.
Distally, the humerus becomes flattened. The prominent bony projection on
the medial side is the medial epicondyle of the humerus. The much
smaller lateral epicondyle of the humerus is found on the lateral side of
the distal humerus. The roughened ridge of bone above the lateral
epicondyle is the lateral supracondylar ridge. All of these areas are
attachment points for muscles that act on the forearm, wrist, and hand. The
powerful grasping muscles of the anterior forearm arise from the medial
epicondyle, which is thus larger and more robust than the lateral epicondyle
that gives rise to the weaker posterior forearm muscles.
The distal end of the humerus has two articulation areas, which join the
ulna and radius bones of the forearm to form the elbow joint. The more
medial of these areas is the trochlea, a spindle- or pulley-shaped region
(trochlea = “pulley”), which articulates with the ulna bone. Immediately
lateral to the trochlea is the capitulum (“small head”), a knob-like structure
located on the anterior surface of the distal humerus. The capitulum
articulates with the radius bone of the forearm. Just above these bony areas
are two small depressions. These spaces accommodate the forearm bones
when the elbow is fully bent (flexed). Superior to the trochlea is the
coronoid fossa, which receives the coronoid process of the ulna, and above
the capitulum is the radial fossa, which receives the head of the radius
when the elbow is flexed. Similarly, the posterior humerus has the
olecranon fossa, a larger depression that receives the olecranon process of
the ulna when the forearm is fully extended.
Ulna
The ulna is the medial bone of the forearm. It runs parallel to the radius,
which is the lateral bone of the forearm ([link]). The proximal end of the
ulna resembles a crescent wrench with its large, C-shaped trochlear notch.
This region articulates with the trochlea of the humerus as part of the elbow
joint. The inferior margin of the trochlear notch is formed by a prominent
lip of bone called the coronoid process of the ulna. Just below this on the
anterior ulna is a roughened area called the ulnar tuberosity. To the lateral
side and slightly inferior to the trochlear notch is a small, smooth area
called the radial notch of the ulna. This area is the site of articulation
between the proximal radius and the ulna, forming the proximal
radioulnar joint. The posterior and superior portions of the proximal ulna
make up the olecranon process, which forms the bony tip of the elbow.
Ulna and Radius
er a ie Olecranon
Radial notch on process
of the ulna 1S
Trochlear Head of
Head of notch radius
radius
Neck of Coronoid
di process Neck of
radius radius
Radial Proximal
tuberosity radioulnar
} joint
| Interosseous
\\ | membrane
\ \ \ Uina
Radius S
Ulnar notch
of the radius
: Head of ulna Radius
Distal
radioulnar
joint
Styloid process
of ulna
Styloid process
of radius
The ulna is located on the medial side
of the forearm, and the radius is on the
lateral side. These bones are attached
to each other by an interosseous
membrane.
More distal is the shaft of the ulna. The lateral side of the shaft forms a
ridge called the interosseous border of the ulna. This is the line of
attachment for the interosseous membrane of the forearm, a sheet of
dense connective tissue that unites the ulna and radius bones. The small,
rounded area that forms the distal end is the head of the ulna. Projecting
from the posterior side of the ulnar head is the styloid process of the ulna,
a short bony projection. This serves as an attachment point for a connective
tissue structure that unites the distal ends of the ulna and radius.
In the anatomical position, with the elbow fully extended and the palms
facing forward, the arm and forearm do not form a straight line. Instead, the
forearm deviates laterally by 5-15 degrees from the line of the arm. This
deviation is called the carrying angle. It allows the forearm and hand to
swing freely or to carry an object without hitting the hip. The carrying angle
is larger in females to accommodate their wider pelvis.
Radius
The radius runs parallel to the ulna, on the lateral (thumb) side of the
forearm (see [link]). The head of the radius is a disc-shaped structure that
forms the proximal end. The small depression on the surface of the head
articulates with the capitulum of the humerus as part of the elbow joint,
whereas the smooth, outer margin of the head articulates with the radial
notch of the ulna at the proximal radioulnar joint. The neck of the radius is
the narrowed region immediately below the expanded head. Inferior to this
point on the medial side is the radial tuberosity, an oval-shaped, bony
protuberance that serves as a muscle attachment point. The shaft of the
radius is slightly curved and has a small ridge along its medial side. This
ridge forms the interosseous border of the radius, which, like the similar
border of the ulna, is the line of attachment for the interosseous membrane
that unites the two forearm bones. The distal end of the radius has a smooth
surface for articulation with two carpal bones to form the radiocarpal joint
or wrist joint ({link] and [link]). On the medial side of the distal radius is
the ulnar notch of the radius. This shallow depression articulates with the
head of the ulna, which together form the distal radioulnar joint. The
lateral end of the radius has a pointed projection called the styloid process
of the radius. This provides attachment for ligaments that support the
lateral side of the wrist joint. Compared to the styloid process of the ulna,
the styloid process of the radius projects more distally, thereby limiting the
range of movement for lateral deviations of the hand at the wrist joint.
Note:
Watch this video to see how fractures of the distal radius bone can affect
the wrist joint. Explain the problems that may occur if a fracture of the
distal radius involves the joint surface of the radiocarpal joint of the wrist.
Carpal Bones
The wrist and base of the hand are formed by a series of eight small carpal
bones (see [link]). The carpal bones are arranged in two rows, forming a
proximal row of four carpal bones and a distal row of four carpal bones.
The bones in the proximal row, running from the lateral (thumb) side to the
medial side, are the scaphoid (“boat-shaped”), lunate (“moon-shaped”),
triquetrum (“three-cornered”), and pisiform (“pea-shaped”) bones. The
small, rounded pisiform bone articulates with the anterior surface of the
triquetrum bone. The pisiform thus projects anteriorly, where it forms the
bony bump that can be felt at the medial base of your hand. The distal
bones (lateral to medial) are the trapezium (“table”), trapezoid
(“resembles a table”), capitate (“head-shaped”), and hamate (“hooked
bone”) bones. The hamate bone is characterized by a prominent bony
extension on its anterior side called the hook of the hamate bone.
A helpful mnemonic for remembering the arrangement of the carpal bones
is “So Long To Pinky, Here Comes The Thumb.” This mnemonic starts on
the lateral side and names the proximal bones from lateral to medial
(scaphoid, lunate, triquetrum, pisiform), then makes a U-turn to name the
distal bones from medial to lateral (hamate, capitate, trapezoid, trapezium).
Thus, it starts and finishes on the lateral side.
Bones of the Wrist and Hand
| Carpals
S i\\ \ ~;Metacarpals
yyy ib Phalanges
Middle finger
Index finger Ring finger
4 Distal
Little finger .
Miceie Phalanges
Thumb f
(pollex)
Phalanges: /f ie
Distal Head —¥~q —
Proximal a Shaft \
Base
Head
Metacarpals v)
(1-5) J Shaft
Carpals: \ y . :
Trapezium - a on Va j Base
Trapezoid t ) — ; _=—_ :
f - Capitate : Carpals:
Scaphoid x ‘< Pisiform ) : Trapezium
VW y> Triquetrum p =: Trapeziod
" \ f Lunate - Scaphoid
7 Ulna
| Radius
Anterior view Posterior view
The eight carpal bones form the base of the hand.
These are arranged into proximal and distal rows of
four bones each. The metacarpal bones form the palm
of the hand. The thumb and fingers consist of the
phalanx bones.
The carpal bones form the base of the hand. This can be seen in the
radiograph (X-ray image) of the hand that shows the relationships of the
hand bones to the skin creases of the hand (see [link]). Within the carpal
bones, the four proximal bones are united to each other by ligaments to
form a unit. Only three of these bones, the scaphoid, lunate, and triquetrum,
contribute to the radiocarpal joint. The scaphoid and lunate bones articulate
directly with the distal end of the radius, whereas the triquetrum bone
articulates with a fibrocartilaginous pad that spans the radius and styloid
process of the ulna. The distal end of the ulna thus does not directly
articulate with any of the carpal bones.
The four distal carpal bones are also held together as a group by ligaments.
The proximal and distal rows of carpal bones articulate with each other to
form the midcarpal joint (see [link]). Together, the radiocarpal and
midcarpal joints are responsible for all movements of the hand at the wrist.
The distal carpal bones also articulate with the metacarpal bones of the
hand.
Bones of the Hand
Metacarpophalangeal
joints Interphalangeal joints
Carpometacarpal
joints
Midcarpal joint
Radiocarpal (wrist)
fs :
This radiograph shows the position of the bones within
the hand. Note the carpal bones that form the base of
the hand. (credit: modification of work by Trace
Meek)
In the articulated hand, the carpal bones form a U-shaped grouping. A
strong ligament called the flexor retinaculum spans the top of this U-
shaped area to maintain this grouping of the carpal bones. The flexor
retinaculum is attached laterally to the trapezium and scaphoid bones, and
medially to the hamate and pisiform bones. Together, the carpal bones and
the flexor retinaculum form a passageway called the carpal tunnel, with
the carpal bones forming the walls and floor, and the flexor retinaculum
forming the roof of this space ({link]). The tendons of nine muscles of the
anterior forearm and an important nerve pass through this narrow tunnel to
enter the hand. Overuse of the muscle tendons or wrist injury can produce
inflammation and swelling within this space. This produces compression of
the nerve, resulting in carpal tunnel syndrome, which is characterized by
pain or numbness, and muscle weakness in those areas of the hand supplied
by this nerve.
Carpal Tunnel
Carpal
tunnel
Muscle
tendons
— Flexor
\
rpal bones: — i
Carpal bones retinaculum
Hamate
Trapezoid
Trapezium
Capitate
Nerve
The carpal tunnel is the passageway
by which nine muscle tendons and a
major nerve enter the hand from the
anterior forearm. The walls and floor
of the carpal tunnel are formed by the
U-shaped grouping of the carpal
bones, and the roof is formed by the
flexor retinaculum, a strong ligament
that anteriorly unites the bones.
Metacarpal Bones
The palm of the hand contains five elongated metacarpal bones. These
bones lie between the carpal bones of the wrist and the bones of the fingers
and thumb (see [link]). The proximal end of each metacarpal bone
articulates with one of the distal carpal bones. Each of these articulations is
a Carpometacarpal joint (see [link]). The expanded distal end of each
metacarpal bone articulates at the metacarpophalangeal joint with the
proximal phalanx bone of the thumb or one of the fingers. The distal end
also forms the knuckles of the hand, at the base of the fingers. The
metacarpal bones are numbered 1-5, beginning at the thumb.
The first metacarpal bone, at the base of the thumb, is separated from the
other metacarpal bones. This allows it a freedom of motion that is
independent of the other metacarpal bones, which is very important for
thumb mobility. The remaining metacarpal bones are united together to
form the palm of the hand. The second and third metacarpal bones are
firmly anchored in place and are immobile. However, the fourth and fifth
metacarpal bones have limited anterior-posterior mobility, a motion that is
greater for the fifth bone. This mobility is important during power gripping
with the hand ([link]). The anterior movement of these bones, particularly
the fifth metacarpal bone, increases the strength of contact for the medial
hand during gripping actions.
Hand During Gripping
(a) Loosely held (b) Firmly gripped
During tight gripping—compare (b) to (a)—the fourth
and, particularly, the fifth metatarsal bones are pulled
anteriorly. This increases the contact between the
object and the medial side of the hand, thus improving
the firmness of the grip.
Phalanx Bones
The fingers and thumb contain 14 bones, each of which is called a phalanx
bone (plural = phalanges), named after the ancient Greek phalanx (a
rectangular block of soldiers). The thumb (pollex) is digit number 1 and has
two phalanges, a proximal phalanx, and a distal phalanx bone (see [link]).
Digits 2 (index finger) through 5 (little finger) have three phalanges each,
called the proximal, middle, and distal phalanx bones. An interphalangeal
joint is one of the articulations between adjacent phalanges of the digits
(see [Link]).
Note:
ORs sao
at ;
— .
=——:
wm OPENSTAX COLLEGE
Visit this site to explore the bones and joints of the hand. What are the
three arches of the hand, and what is the importance of these during the
sripping of an object?
Note:
Disorders of the...
Appendicular System: Fractures of Upper Limb Bones
Due to our constant use of the hands and the rest of our upper limbs, an
injury to any of these areas will cause a significant loss of functional
ability. Many fractures result from a hard fall onto an outstretched hand.
The resulting transmission of force up the limb may result in a fracture of
the humerus, radius, or scaphoid bones. These injuries are especially
common in elderly people whose bones are weakened due to osteoporosis.
Falls onto the hand or elbow, or direct blows to the arm, can result in
fractures of the humerus ((link]). Following a fall, fractures at the surgical
neck, the region at which the expanded proximal end of the humerus joins
with the shaft, can result in an impacted fracture, in which the distal
portion of the humerus is driven into the proximal portion. Falls or blows
to the arm can also produce transverse or spiral fractures of the humeral
shaft.
In children, a fall onto the tip of the elbow frequently results in a distal
humerus fracture. In these, the olecranon of the ulna is driven upward,
resulting in a fracture across the distal humerus, above both epicondyles
(supracondylar fracture), or a fracture between the epicondyles, thus
separating one or both of the epicondyles from the body of the humerus
(intercondylar fracture). With these injuries, the immediate concern is
possible compression of the artery to the forearm due to swelling of the
surrounding tissues. If compression occurs, the resulting ischemia (lack of
oxygen) due to reduced blood flow can quickly produce irreparable
damage to the forearm muscles. In addition, four major nerves for shoulder
and upper limb muscles are closely associated with different regions of the
humerus, and thus, humeral fractures may also damage these nerves.
Another frequent injury following a fall onto an outstretched hand is a
Colles fracture (“col-lees”’) of the distal radius (see [link]). This involves a
complete transverse fracture across the distal radius that drives the
separated distal fragment of the radius posteriorly and superiorly. This
injury results in a characteristic “dinner fork” bend of the forearm just
above the wrist due to the posterior displacement of the hand. This is the
most frequent forearm fracture and is a common injury in persons over the
age of 50, particularly in older women with osteoporosis. It also commonly
occurs following a high-speed fall onto the hand during activities such as
snowboarding or skating.
The most commonly fractured carpal bone is the scaphoid, often resulting
from a fall onto the hand. Deep pain at the lateral wrist may yield an initial
diagnosis of a wrist sprain, but a radiograph taken several weeks after the
injury, after tissue swelling has subsided, will reveal the fracture. Due to
the poor blood supply to the scaphoid bone, healing will be slow and there
is the danger of bone necrosis and subsequent degenerative joint disease of
the wrist.
Fractures of the Humerus and Radius
Surgical neck fracture
Transverse humeral fracture
Fractures of the Humerus
Supracondylar fracture
KL
i
Eas
Normal
Normal
Colles Fracture of the Distal Radius
Falls or direct blows can result in fractures of
the surgical neck or shaft of the humerus. Falls
onto the elbow can fracture the distal humerus.
A Colles fracture of the distal radius is the
most common forearm fracture.
Note:
[=]
[eae
1
openstax COLLEGE”
Watch this video to learn about a Colles fracture, a break of the distal
radius, usually caused by falling onto an outstretched hand. When would
surgery be required and how would the fracture be repaired in this case?
Chapter Review
Each upper limb is divided into three regions and contains a total of 30
bones. The upper arm is the region located between the shoulder and elbow
joints. This area contains the humerus. The proximal humerus consists of
the head, which articulates with the scapula at the glenohumeral joint, the
greater and lesser tubercles separated by the intertubercular (bicipital)
groove, and the anatomical and surgical necks. The humeral shaft has the
roughened area of the deltoid tuberosity on its lateral side. The distal
humerus is flattened, forming a lateral supracondylar ridge that terminates
at the small lateral epicondyle. The medial side of the distal humerus has
the large, medial epicondyle. The articulating surfaces of the distal humerus
consist of the trochlea medially and the capitulum laterally. Depressions on
the humerus that accommodate the forearm bones during bending (flexing)
and straightening (extending) of the elbow include the coronoid fossa, the
radial fossa, and the olecranon fossa.
The forearm is the region of the upper limb located between the elbow and
wrist joints. This region contains two bones, the ulna medially and the
radius on the lateral (thumb) side. The elbow joint is formed by the
articulation between the trochlea of the humerus and the trochlear notch of
the ulna, plus the articulation between the capitulum of the humerus and the
head of the radius. The proximal radioulnar joint is the articulation between
the head of the radius and the radial notch of the ulna. The proximal ulna
also has the olecranon process, forming an expanded posterior region, and
the coronoid process and ulnar tuberosity on its anterior aspect. On the
proximal radius, the narrowed region below the head is the neck; distal to
this is the radial tuberosity. The shaft portions of both the ulna and radius
have an interosseous border, whereas the distal ends of each bone have a
pointed styloid process. The distal radioulnar joint is found between the
head of the ulna and the ulnar notch of the radius. The distal end of the
radius articulates with the proximal carpal bones, but the ulna does not.
The base of the hand is formed by eight carpal bones. The carpal bones are
united into two rows of bones. The proximal row contains (from lateral to
medial) the scaphoid, lunate, triquetrum, and pisiform bones. The scaphoid,
lunate, and triquetrum bones contribute to the formation of the radiocarpal
joint. The distal row of carpal bones contains (from medial to lateral) the
hamate, capitate, trapezoid, and trapezium bones (“So Long To Pinky, Here
Comes The Thumb”). The anterior hamate has a prominent bony hook. The
proximal and distal carpal rows articulate with each other at the midcarpal
joint. The carpal bones, together with the flexor retinaculum, also form the
carpal tunnel of the wrist.
The five metacarpal bones form the palm of the hand. The metacarpal bones
are numbered 1-5, starting with the thumb side. The first metacarpal bone
is freely mobile, but the other bones are united as a group. The digits are
also numbered 1—5, with the thumb being number 1. The fingers and thumb
contain a total of 14 phalanges (phalanx bones). The thumb contains a
proximal and a distal phalanx, whereas the remaining digits each contain
proximal, middle, and distal phalanges.
Interactive Link Questions
Exercise:
Problem:
Watch this video to see how fractures of the distal radius bone can
affect the wrist joint. Explain the problems that may occur if a fracture
of the distal radius involves the joint surface of the radiocarpal joint of
the wrist.
Solution:
A fracture through the joint surface of the distal radius may make the
articulating surface of the radius rough or jagged. This can then cause
painful movements involving this joint and the early development of
arthritis. Surgery can return the joint surface to its original smoothness,
thus allowing for the return of normal function.
Exercise:
Problem:
Visit this site to explore the bones and joints of the hand. What are the
three arches of the hand, and what is the importance of these during
the gripping of an object?
Solution:
The hand has a proximal transverse arch, a distal transverse arch, and a
longitudinal arch. These allow the hand to conform to objects being
held. These arches maximize the amount of surface contact between
the hand and object, which enhances stability and increases sensory
input.
Exercise:
Problem:
Watch this video to learn about a Colles fracture, a break of the distal
radius, usually caused by falling onto an outstretched hand. When
would surgery be required and how would the fracture be repaired in
this case?
Solution:
Surgery may be required if the fracture is unstable, meaning that the
broken ends of the radius won’t stay in place to allow for proper
healing. In this case, metal plates and screws can be used to stabilize
the fractured bone.
Review Questions
Exercise:
Problem:How many bones are there in the upper limbs combined?
a. 20
b. 30
c. 40
d. 60
Solution:
D
Exercise:
Problem:
Which bony landmark is located on the lateral side of the proximal
humerus?
a. greater tubercle
b. trochlea
c. lateral epicondyle
d. lesser tubercle
Solution:
A
Exercise:
Problem:
Which region of the humerus articulates with the radius as part of the
elbow joint?
a. trochlea
b. styloid process
c. capitulum
d. olecranon process
Solution:
C
Exercise:
Problem: Which is the lateral-most carpal bone of the proximal row?
a. trapezium
b. hamate
c. pisiform
d. scaphoid
Solution:
D
Exercise:
Problem: The radius bone
a. is found on the medial side of the forearm
b. has a head that articulates with the radial notch of the ulna
c. does not articulate with any of the carpal bones
d. has the radial tuberosity located near its distal end
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Your friend runs out of gas and you have to help push his car. Discuss
the sequence of bones and joints that convey the forces passing from
your hand, through your upper limb and your pectoral girdle, and to
your axial skeleton.
Solution:
As you push against the car, forces will pass from the metacarpal
bones of your hand into the carpal bones at the base of your hand.
Forces will then pass through the midcarpal and radiocarpal joints into
the radius and ulna bones of the forearm. These will pass the force
through the elbow joint into the humerus of the arm, and then through
the glenohumeral joint into the scapula. The force will travel through
the acromioclavicular joint into the clavicle, and then through the
sternoclavicular joint into the sternum, which is part of the axial
skeleton.
Exercise:
Problem:
Name the bones in the wrist and hand, and describe or sketch out their
locations and articulations.
Solution:
The base of the hand is formed by the eight carpal bones arranged in
two rows (distal and proximal) of four bones each. The proximal row
contains (from lateral to medial) the scaphoid, lunate, triquetrum, and
pisiform bones. The distal row contains (from medial to lateral) the
hamate, capitate, trapezoid, and trapezium bones. (Use the mnemonic
“So Long To Pinky, Here Comes The Thumb” to remember this
sequence). The rows of the proximal and distal carpal bones articulate
with each other at the midcarpal joint. The palm of the hand contains
the five metacarpal bones, which are numbered 1-5 starting on the
thumb side. The proximal ends of the metacarpal bones articulate with
the distal row of the carpal bones. The distal ends of the metacarpal
bones articulate with the proximal phalanx bones of the thumb and
fingers. The thumb (digit 1) has both a proximal and distal phalanx
bone. The fingers (digits 2—5) all contain proximal, middle, and distal
phalanges.
Glossary
anatomical neck
line on the humerus located around the outside margin of the humeral
head
arm
region of the upper limb located between the shoulder and elbow
joints; contains the humerus bone
bicipital groove
intertubercular groove; narrow groove located between the greater and
lesser tubercles of the humerus
capitate
from the lateral side, the third of the four distal carpal bones;
articulates with the scaphoid and lunate proximally, the trapezoid
laterally, the hamate medially, and primarily with the third metacarpal
distally
capitulum
knob-like bony structure located anteriorly on the lateral, distal end of
the humerus
carpal bone
one of the eight small bones that form the wrist and base of the hand;
these are grouped as a proximal row consisting of (from lateral to
medial) the scaphoid, lunate, triquetrum, and pisiform bones, and a
distal row containing (from lateral to medial) the trapezium, trapezoid,
capitate, and hamate bones
carpal tunnel
passageway between the anterior forearm and hand formed by the
carpal bones and flexor retinaculum
Carpometacarpal joint
articulation between one of the carpal bones in the distal row and a
metacarpal bone of the hand
coronoid fossa
depression on the anterior surface of the humerus above the trochlea;
this space receives the coronoid process of the ulna when the elbow is
maximally flexed
coronoid process of the ulna
projecting bony lip located on the anterior, proximal ulna; forms the
inferior margin of the trochlear notch
deltoid tuberosity
roughened, V-shaped region located laterally on the mid-shaft of the
humerus
distal radioulnar joint
articulation between the head of the ulna and the ulnar notch of the
radius
elbow joint
joint located between the upper arm and forearm regions of the upper
limb; formed by the articulations between the trochlea of the humerus
and the trochlear notch of the ulna, and the capitulum of the humerus
and the head of the radius
flexor retinaculum
strong band of connective tissue at the anterior wrist that spans the top
of the U-shaped grouping of the carpal bones to form the roof of the
carpal tunnel
forearm
region of the upper limb located between the elbow and wrist joints;
contains the radius and ulna bones
greater tubercle
enlarged prominence located on the lateral side of the proximal
humerus
hamate
from the lateral side, the fourth of the four distal carpal bones;
articulates with the lunate and triquetrum proximally, the fourth and
fifth metacarpals distally, and the capitate laterally
hand
region of the upper limb distal to the wrist joint
head of the humerus
smooth, rounded region on the medial side of the proximal humerus;
articulates with the glenoid fossa of the scapula to form the
glenohumeral (shoulder) joint
head of the radius
disc-shaped structure that forms the proximal end of the radius;
articulates with the capitulum of the humerus as part of the elbow
joint, and with the radial notch of the ulna as part of the proximal
radioulnar joint
head of the ulna
small, rounded distal end of the ulna; articulates with the ulnar notch
of the distal radius, forming the distal radioulnar joint
hook of the hamate bone
bony extension located on the anterior side of the hamate carpal bone
humerus
single bone of the upper arm
interosseous border of the radius
narrow ridge located on the medial side of the radial shaft; for
attachment of the interosseous membrane between the ulna and radius
bones
interosseous border of the ulna
narrow ridge located on the lateral side of the ulnar shaft; for
attachment of the interosseous membrane between the ulna and radius
interosseous membrane of the forearm
sheet of dense connective tissue that unites the radius and ulna bones
interphalangeal joint
articulation between adjacent phalanx bones of the hand or foot digits
intertubercular groove (sulcus)
bicipital groove; narrow groove located between the greater and lesser
tubercles of the humerus
lateral epicondyle of the humerus
small projection located on the lateral side of the distal humerus
lateral supracondylar ridge
narrow, bony ridge located along the lateral side of the distal humerus,
superior to the lateral epicondyle
lesser tubercle
small, bony prominence located on anterior side of the proximal
humerus
lunate
from the lateral side, the second of the four proximal carpal bones;
articulates with the radius proximally, the capitate and hamate distally,
the scaphoid laterally, and the triquetrum medially
medial epicondyle of the humerus
enlarged projection located on the medial side of the distal humerus
metacarpal bone
one of the five long bones that form the palm of the hand; numbered
1—5, starting on the lateral (thumb) side of the hand
metacarpophalangeal joint
articulation between the distal end of a metacarpal bone of the hand
and a proximal phalanx bone of the thumb or a finger
midcarpal joint
articulation between the proximal and distal rows of the carpal bones;
contributes to movements of the hand at the wrist
neck of the radius
narrowed region immediately distal to the head of the radius
olecranon fossa
large depression located on the posterior side of the distal humerus;
this space receives the olecranon process of the ulna when the elbow is
fully extended
olecranon process
expanded posterior and superior portions of the proximal ulna; forms
the bony tip of the elbow
phalanx bone of the hand
(plural = phalanges) one of the 14 bones that form the thumb and
fingers; these include the proximal and distal phalanges of the thumb,
and the proximal, middle, and distal phalanx bones of the fingers two
through five
pisiform
from the lateral side, the fourth of the four proximal carpal bones;
articulates with the anterior surface of the triquetrum
pollex
(also, thumb) digit 1 of the hand
proximal radioulnar joint
articulation formed by the radial notch of the ulna and the head of the
radius
radial fossa
small depression located on the anterior humerus above the capitulum;
this space receives the head of the radius when the elbow is maximally
flexed
radial notch of the ulna
small, smooth area on the lateral side of the proximal ulna; articulates
with the head of the radius as part of the proximal radioulnar joint
radial tuberosity
oval-shaped, roughened protuberance located on the medial side of the
proximal radius
radiocarpal joint
wrist joint, located between the forearm and hand regions of the upper
limb; articulation formed proximally by the distal end of the radius and
the fibrocartilaginous pad that unites the distal radius and ulna bone,
and distally by the scaphoid, lunate, and triquetrum carpal bones
radius
bone located on the lateral side of the forearm
scaphoid
from the lateral side, the first of the four proximal carpal bones;
articulates with the radius proximally, the trapezoid, trapezium, and
capitate distally, and the lunate medially
shaft of the humerus
narrow, elongated, central region of the humerus
shaft of the radius
narrow, elongated, central region of the radius
shaft of the ulna
narrow, elongated, central region of the ulna
styloid process of the radius
pointed projection located on the lateral end of the distal radius
styloid process of the ulna
short, bony projection located on the medial end of the distal ulna
surgical neck
region of the humerus where the expanded, proximal end joins with
the narrower shaft
trapezium
from the lateral side, the first of the four distal carpal bones; articulates
with the scaphoid proximally, the first and second metacarpals distally,
and the trapezoid medially
trapezoid
from the lateral side, the second of the four distal carpal bones;
articulates with the scaphoid proximally, the second metacarpal
distally, the trapezium laterally, and the capitate medially
triquetrum
from the lateral side, the third of the four proximal carpal bones;
articulates with the lunate laterally, the hamate distally, and has a facet
for the pisiform
trochlea
pulley-shaped region located medially at the distal end of the humerus;
articulates at the elbow with the trochlear notch of the ulna
trochlear notch
large, C-shaped depression located on the anterior side of the proximal
ulna; articulates at the elbow with the trochlea of the humerus
ulna
bone located on the medial side of the forearm
ulnar notch of the radius
shallow, smooth area located on the medial side of the distal radius;
articulates with the head of the ulna at the distal radioulnar joint
ulnar tuberosity
roughened area located on the anterior, proximal ulna inferior to the
coronoid process
The Pelvic Girdle and Pelvis
By the end of this section, you will be able to:
¢ Define the pelvic girdle and describe the bones and ligaments of the
pelvis
e Explain the three regions of the hip bone and identify their bony
landmarks
e Describe the openings of the pelvis and the boundaries of the greater
and lesser pelvis
The pelvic girdle (hip girdle) is formed by a single bone, the hip bone or
coxal bone (coxal = “hip”), which serves as the attachment point for each
lower limb. Each hip bone, in turn, is firmly joined to the axial skeleton via
its attachment to the sacrum of the vertebral column. The right and left hip
bones also converge anteriorly to attach to each other. The bony pelvis is
the entire structure formed by the two hip bones, the sacrum, and, attached
inferiorly to the sacrum, the coccyx ([link]).
Unlike the bones of the pectoral girdle, which are highly mobile to enhance
the range of upper limb movements, the bones of the pelvis are strongly
united to each other to form a largely immobile, weight-bearing structure.
This is important for stability because it enables the weight of the body to
be easily transferred laterally from the vertebral column, through the pelvic
girdle and hip joints, and into either lower limb whenever the other limb is
not bearing weight. Thus, the immobility of the pelvis provides a strong
foundation for the upper body as it rests on top of the mobile lower limbs.
Pelvis
Sacroiliac joint
Sacral promonitory
Sacrum
Pelvic brim
Acetabulum
Coccyx
Obturator f <i
ical ieti \ ax Pubic symphysis
Ischial tuberosity
ae FH ~~
Ischiopubic ramus
The pelvic girdle is formed by a single hip bone. The
hip bone attaches the lower limb to the axial skeleton
through its articulation with the sacrum. The right and
left hip bones, plus the sacrum and the coccyx,
together form the pelvis.
Hip Bone
The hip bone, or coxal bone, forms the pelvic girdle portion of the pelvis.
The paired hip bones are the large, curved bones that form the lateral and
anterior aspects of the pelvis. Each adult hip bone is formed by three
separate bones that fuse together during the late teenage years. These bony
components are the ilium, ischium, and pubis ([link]). These names are
retained and used to define the three regions of the adult hip bone.
The Hip Bone
Ilium
lliac fossa
lliac crest
- Posterior
Posterior Anterior superior
‘ superior = ‘4
superior iliac spine iliac spine
ilac spine a Posterior
Posterior oe inferior
inferior weet iliac spine
iliac spine Acetabulum Auricul rf
J : uricular surface
Greater sciatic Arcuate line Greater sciatic
notch : :
: Superior ramus of pubis notch
Ischial body Pubic tubercle Ischial spine
Ischial spine Lesser sciatic
Lesser sciatic ont
notch ; Obturator
Articular surface foramen
Ischium of pubis (at pubic Ischium
symphysis
Obturator ymphysie) Ischial ramus
foramen
Inferior ramus a
of pubis
Ischiopubic ramus Ischiopubic ramus
Ischial tuberosity
Ischial ramus
Lateral view, right hip bone Medial view, right hip bone
The adult hip bone consists of three regions. The ilium
forms the large, fan-shaped superior portion, the
ischium forms the posteroinferior portion, and the
pubis forms the anteromedial portion.
The ilium is the fan-like, superior region that forms the largest part of the
hip bone. It is firmly united to the sacrum at the largely immobile sacroiliac
joint (see [link]). The ischium forms the posteroinferior region of each hip
bone. It supports the body when sitting. The pubis forms the anterior
portion of the hip bone. The pubis curves medially, where it joins to the
pubis of the opposite hip bone at a specialized joint called the pubic
symphysis.
Tlium
When you place your hands on your waist, you can feel the arching,
superior margin of the ilium along your waistline (see [link]). This curved,
superior margin of the ilium is the iliac crest. The rounded, anterior
termination of the iliac crest is the anterior superior iliac spine. This
important bony landmark can be felt at your anterolateral hip. Inferior to the
anterior superior iliac spine is a rounded protuberance called the anterior
inferior iliac spine. Both of these iliac spines serve as attachment points for
muscles of the thigh. Posteriorly, the iliac crest curves downward to
terminate as the posterior superior iliac spine. Muscles and ligaments
surround but do not cover this bony landmark, thus sometimes producing a
depression seen as a “dimple” located on the lower back. More inferiorly is
the posterior inferior iliac spine. This is located at the inferior end of a
large, roughened area called the auricular surface of the ilium. The
auricular surface articulates with the auricular surface of the sacrum to form
the sacroiliac joint. Both the posterior superior and posterior inferior iliac
spines serve as attachment points for the muscles and very strong ligaments
that support the sacroiliac joint.
The shallow depression located on the anteromedial (internal) surface of the
upper ilium is called the iliac fossa. The inferior margin of this space is
formed by the arcuate line of the ilium, the ridge formed by the
pronounced change in curvature between the upper and lower portions of
the ilium. The large, inverted U-shaped indentation located on the posterior
margin of the lower ilium is called the greater sciatic notch.
Ischium
The ischium forms the posterolateral portion of the hip bone (see [link]).
The large, roughened area of the inferior ischium is the ischial tuberosity.
This serves as the attachment for the posterior thigh muscles and also
carries the weight of the body when sitting. You can feel the ischial
tuberosity if you wiggle your pelvis against the seat of a chair. Projecting
superiorly and anteriorly from the ischial tuberosity is a narrow segment of
bone called the ischial ramus. The slightly curved posterior margin of the
ischium above the ischial tuberosity is the lesser sciatic notch. The bony
projection separating the lesser sciatic notch and greater sciatic notch is the
ischial spine.
Pubis
The pubis forms the anterior portion of the hip bone (see [link]). The
enlarged medial portion of the pubis is the pubic body. Located superiorly
on the pubic body is a small bump called the pubic tubercle. The superior
pubic ramus is the segment of bone that passes laterally from the pubic
body to join the ilium. The narrow ridge running along the superior margin
of the superior pubic ramus is the pectineal line of the pubis.
The pubic body is joined to the pubic body of the opposite hip bone by the
pubic symphysis. Extending downward and laterally from the body is the
inferior pubic ramus. The pubic arch is the bony structure formed by the
pubic symphysis, and the bodies and inferior pubic rami of the adjacent
pubic bones. The inferior pubic ramus extends downward to join the ischial
ramus. Together, these form the single ischiopubic ramus, which extends
from the pubic body to the ischial tuberosity. The inverted V-shape formed
as the ischiopubic rami from both sides come together at the pubic
symphysis is called the subpubic angle.
Pelvis
The pelvis consists of four bones: the right and left hip bones, the sacrum,
and the coccyx (see [link]). The pelvis has several important functions. Its
primary role is to support the weight of the upper body when sitting and to
transfer this weight to the lower limbs when standing. It serves as an
attachment point for trunk and lower limb muscles, and also protects the
internal pelvic organs. When standing in the anatomical position, the pelvis
is tilted anteriorly. In this position, the anterior superior iliac spines and the
pubic tubercles lie in the same vertical plane, and the anterior (internal)
surface of the sacrum faces forward and downward.
The three areas of each hip bone, the ilium, pubis, and ischium, converge
centrally to form a deep, cup-shaped cavity called the acetabulum. This is
located on the lateral side of the hip bone and is part of the hip joint. The
large opening in the anteroinferior hip bone between the ischium and pubis
is the obturator foramen. This space is largely filled in by a layer of
connective tissue and serves for the attachment of muscles on both its
internal and external surfaces.
Several ligaments unite the bones of the pelvis ([link]). The largely
immobile sacroiliac joint is supported by a pair of strong ligaments that are
attached between the sacrum and ilium portions of the hip bone. These are
the anterior sacroiliac ligament on the anterior side of the joint and the
posterior sacroiliac ligament on the posterior side. Also spanning the
sacrum and hip bone are two additional ligaments. The sacrospinous
ligament runs from the sacrum to the ischial spine, and the sacrotuberous
ligament runs from the sacrum to the ischial tuberosity. These ligaments
help to support and immobilize the sacrum as it carries the weight of the
body.
Ligaments of the Pelvis
Sacrum
Posterior
superior ;
iliac spine Posterior
sacroiliac
" ligament
llium 9
Greater
sciatic
foramen
pe rOspInous Sacrospinous
igament ligament
Ischial spine
Pubis Lesser sciatic
; foramen
Ischium
Ischial Sacrotuberous
tuberosity ligament
Subpubic
angle
Obturator
foramen
Ischiopubic ramus
The posterior sacroiliac ligament supports the
sacroiliac joint. The sacrospinous ligament spans the
sacrum to the ischial spine, and the sacrotuberous
ligament spans the sacrum to the ischial tuberosity.
The sacrospinous and sacrotuberous ligaments
contribute to the formation of the greater and lesser
sciatic foramens.
Note:
fepeeeety
a epenstex COLLEGE
Watch this video for a 3-D view of the pelvis and its associated ligaments.
What is the large opening in the bony pelvis, located between the ischium
and pubic regions, and what two parts of the pubis contribute to the
formation of this opening?
The sacrospinous and sacrotuberous ligaments also help to define two
openings on the posterolateral sides of the pelvis through which muscles,
nerves, and blood vessels for the lower limb exit. The superior opening is
the greater sciatic foramen. This large opening is formed by the greater
sciatic notch of the hip bone, the sacrum, and the sacrospinous ligament.
The smaller, more inferior lesser sciatic foramen is formed by the lesser
sciatic notch of the hip bone, together with the sacrospinous and
sacrotuberous ligaments.
The space enclosed by the bony pelvis is divided into two regions ({link]).
The broad, superior region, defined laterally by the large, fan-like portion of
the upper hip bone, is called the greater pelvis (greater pelvic cavity; false
pelvis). This broad area is occupied by portions of the small and large
intestines, and because it is more closely associated with the abdominal
cavity, it is sometimes referred to as the false pelvis. More inferiorly, the
narrow, rounded space of the lesser pelvis (lesser pelvic cavity; true pelvis)
contains the bladder and other pelvic organs, and thus is also known as the
true pelvis. The pelvic brim (also known as the pelvic inlet) forms the
superior margin of the lesser pelvis, separating it from the greater pelvis.
The pelvic brim is defined by a line formed by the upper margin of the
pubic symphysis anteriorly, and the pectineal line of the pubis, the arcuate
line of the ilium, and the sacral promontory (the anterior margin of the
superior sacrum) posteriorly. The inferior limit of the lesser pelvic cavity is
called the pelvic outlet. This large opening is defined by the inferior margin
of the pubic symphysis anteriorly, and the ischiopubic ramus, the ischial
tuberosity, the sacrotuberous ligament, and the inferior tip of the coccyx
posteriorly. Because of the anterior tilt of the pelvis, the lesser pelvis is also
angled, giving it an anterosuperior (pelvic inlet) to posteroinferior (pelvic
outlet) orientation.
Male and Female Pelvis
Female
Subpubic angle
The female pelvis is adapted for childbirth and is
broader, with a larger subpubic angle, a rounder pelvic
brim, and a wider and more shallow lesser pelvic
cavity than the male pelvis.
Comparison of the Female and Male Pelvis
The differences between the adult female and male pelvis relate to function
and body size. In general, the bones of the male pelvis are thicker and
heavier, adapted for support of the male’s heavier physical build and
stronger muscles. The greater sciatic notch of the male hip bone is narrower
and deeper than the broader notch of females. Because the female pelvis is
adapted for childbirth, it is wider than the male pelvis, as evidenced by the
distance between the anterior superior iliac spines (see [link]). The ischial
tuberosities of females are also farther apart, which increases the size of the
pelvic outlet. Because of this increased pelvic width, the subpubic angle is
larger in females (greater than 80 degrees) than it is in males (less than 70
degrees). The female sacrum is wider, shorter, and less curved, and the
sacral promontory projects less into the pelvic cavity, thus giving the female
pelvic inlet (pelvic brim) a more rounded or oval shape compared to males.
The lesser pelvic cavity of females is also wider and more shallow than the
narrower, deeper, and tapering lesser pelvis of males. Because of the
obvious differences between female and male hip bones, this is the one
bone of the body that allows for the most accurate sex determination. [link]
provides an overview of the general differences between the female and
male pelvis.
Overview of Differences between the Female and Male Pelvis
Pelvic
weight
Pelvic inlet
shape
Lesser pelvic
cavity shape
Subpubic
angle
Pelvic outlet
shape
Female pelvis
Bones of the pelvis
are lighter and thinner
Pelvic inlet has a
round or oval shape
Lesser pelvic cavity is
shorter and wider
Subpubic angle is
greater than 80
degrees
Pelvic outlet is
rounded and larger
Male pelvis
Bones of the pelvis are
thicker and heavier
Pelvic inlet is heart-
shaped
Lesser pelvic cavity is
longer and narrower
Subpubic angle is less
than 70 degrees
Pelvic outlet is smaller
Note:
Career Connection
Forensic Pathology and Forensic Anthropology
A forensic pathologist (also known as a medical examiner) is a medically
trained physician who has been specifically trained in pathology to
examine the bodies of the deceased to determine the cause of death. A
forensic pathologist applies his or her understanding of disease as well as
toxins, blood and DNA analysis, firearms and ballistics, and other factors
to assess the cause and manner of death. At times, a forensic pathologist
will be called to testify under oath in situations that involve a possible
crime. Forensic pathology is a field that has received much media attention
on television shows or following a high-profile death.
While forensic pathologists are responsible for determining whether the
cause of someone’s death was natural, a suicide, accidental, or a homicide,
there are times when uncovering the cause of death is more complex, and
other skills are needed. Forensic anthropology brings the tools and
knowledge of physical anthropology and human osteology (the study of the
skeleton) to the task of investigating a death. A forensic anthropologist
assists medical and legal professionals in identifying human remains. The
science behind forensic anthropology involves the study of archaeological
excavation; the examination of hair; an understanding of plants, insects,
and footprints; the ability to determine how much time has elapsed since
the person died; the analysis of past medical history and toxicology; the
ability to determine whether there are any postmortem injuries or
alterations of the skeleton; and the identification of the decedent (deceased
person) using skeletal and dental evidence.
Due to the extensive knowledge and understanding of excavation
techniques, a forensic anthropologist is an integral and invaluable team
member to have on-site when investigating a crime scene, especially when
the recovery of human skeletal remains is involved. When remains are
bought to a forensic anthropologist for examination, he or she must first
determine whether the remains are in fact human. Once the remains have
been identified as belonging to a person and not to an animal, the next step
is to approximate the individual’s age, sex, race, and height. The forensic
anthropologist does not determine the cause of death, but rather provides
information to the forensic pathologist, who will use all of the data
collected to make a final determination regarding the cause of death.
Chapter Review
The pelvic girdle, consisting of a hip bone, serves to attach a lower limb to
the axial skeleton. The hip bone articulates posteriorly at the sacroiliac joint
with the sacrum, which is part of the axial skeleton. The right and left hip
bones converge anteriorly and articulate with each other at the pubic
symphysis. The combination of the hip bone, the sacrum, and the coccyx
forms the pelvis. The pelvis has a pronounced anterior tilt. The primary
function of the pelvis is to support the upper body and transfer body weight
to the lower limbs. It also serves as the site of attachment for multiple
muscles.
The hip bone consists of three regions: the ilium, ischium, and pubis. The
ilium forms the large, fan-like region of the hip bone. The superior margin
of this area is the iliac crest. Located at either end of the iliac crest are the
anterior superior and posterior superior iliac spines. Inferior to these are the
anterior inferior and posterior inferior iliac spines. The auricular surface of
the ilium articulates with the sacrum to form the sacroiliac joint. The medial
surface of the upper ilium forms the iliac fossa, with the arcuate line
marking the inferior limit of this area. The posterior margin of the ilium has
the large greater sciatic notch.
The posterolateral portion of the hip bone is the ischium. It has the
expanded ischial tuberosity, which supports body weight when sitting. The
ischial ramus projects anteriorly and superiorly. The posterior margin of the
ischium has the shallow lesser sciatic notch and the ischial spine, which
separates the greater and lesser sciatic notches.
The pubis forms the anterior portion of the hip bone. The body of the pubis
articulates with the pubis of the opposite hip bone at the pubic symphysis.
The superior margin of the pubic body has the pubic tubercle. The pubis is
joined to the ilium by the superior pubic ramus, the superior surface of
which forms the pectineal line. The inferior pubic ramus projects inferiorly
and laterally. The pubic arch is formed by the pubic symphysis, the bodies
of the adjacent pubic bones, and the two inferior pubic rami. The inferior
pubic ramus joins the ischial ramus to form the ischiopubic ramus. The
subpubic angle is formed by the medial convergence of the right and left
ischiopubic rami.
The lateral side of the hip bone has the cup-like acetabulum, which is part
of the hip joint. The large anterior opening is the obturator foramen. The
sacroiliac joint is supported by the anterior and posterior sacroiliac
ligaments. The sacrum is also joined to the hip bone by the sacrospinous
ligament, which attaches to the ischial spine, and the sacrotuberous
ligament, which attaches to the ischial tuberosity. The sacrospinous and
sacrotuberous ligaments contribute to the formation of the greater and lesser
sciatic foramina.
The broad space of the upper pelvis is the greater pelvis, and the narrow,
inferior space is the lesser pelvis. These areas are separated by the pelvic
brim (pelvic inlet). The inferior opening of the pelvis is the pelvic outlet.
Compared to the male, the female pelvis is wider to accommodate
childbirth, has a larger subpubic angle, and a broader greater sciatic notch.
Interactive Link Questions
Exercise:
Problem:
Watch this video for a 3-D view of the pelvis and its associated
ligaments. What is the large opening in the bony pelvis, located
between the ischium and pubic regions, and what two parts of the
pubis contribute to the formation of this opening?
Solution:
The obturator foramen is located between the ischium and the pubis.
The superior and inferior pubic rami contribute to the boundaries of
the obturator foramen.
Review Questions
Exercise:
Problem:How many bones fuse in adulthood to form the hip bone?
Boop
ul B WN
Solution:
B
Exercise:
Problem: Which component forms the superior part of the hip bone?
a. ilium
b. pubis
c. ischium
d. sacrum
Solution:
A
Exercise:
Problem: Which of the following supports body weight when sitting?
a. iliac crest
b. ischial tuberosity
c. ischiopubic ramus
d. pubic body
Solution:
B
Exercise:
Problem:
The ischial spine is found between which of the following structures?
a. inferior pubic ramus and ischial ramus
b. pectineal line and arcuate line
c. lesser sciatic notch and greater sciatic notch
d. anterior superior iliac spine and posterior superior iliac spine
Solution:
C
Exercise:
Problem:The pelvis
a. has a subpubic angle that is larger in females
b. consists of the two hip bones, but does not include the sacrum or
coccyx
c. has an obturator foramen, an opening that is defined in part by the
Sacrospinous and sacrotuberous ligaments
d. has a space located inferior to the pelvic brim called the greater
pelvis
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Describe the articulations and ligaments that unite the four bones of
the pelvis to each other.
Solution:
The pelvis is formed by the combination of the right and left hip
bones, the sacrum, and the coccyx. The auricular surfaces of each hip
bone articulate with the auricular surface of the sacrum to form the
sacroiliac joint. This joint is supported on either side by the strong
anterior and posterior sacroiliac ligaments. The right and left hip bones
converge anteriorly, where the pubic bodies articulate with each other
to form the pubic symphysis joint. The sacrum is also attached to the
hip bone by the sacrospinous ligament, which spans the sacrum to the
ischial spine, and the sacrotuberous ligament, which runs from the
sacrum to the ischial tuberosity. The coccyx is attached to the inferior
end of the sacrum.
Exercise:
Problem:
Discuss the ways in which the female pelvis is adapted for childbirth.
Solution:
Compared to the male, the female pelvis is wider to accommodate
childbirth. Thus, the female pelvis has greater distances between the
anterior superior iliac spines and between the ischial tuberosities. The
greater width of the female pelvis results in a larger subpubic angle.
This angle, formed by the anterior convergence of the right and left
ischiopubic rami, is larger in females (greater than 80 degrees) than in
males (less than 70 degrees). The female sacral promontory does not
project anteriorly as far as it does in males, which gives the pelvic
brim (pelvic inlet) of the female a rounded or oval shape. The lesser
pelvic cavity is wider and more shallow in females, and the pelvic
outlet is larger than in males. Thus, the greater width of the female
pelvis, with its larger pelvic inlet, lesser pelvis, and pelvic outlet, are
important for childbirth because the baby must pass through the pelvis
during delivery.
Glossary
acetabulum
large, cup-shaped cavity located on the lateral side of the hip bone;
formed by the junction of the ilium, pubis, and ischium portions of the
hip bone
anterior inferior iliac spine
small, bony projection located on the anterior margin of the ilium,
below the anterior superior iliac spine
anterior sacroiliac ligament
strong ligament between the sacrum and the ilium portions of the hip
bone that supports the anterior side of the sacroiliac joint
anterior superior iliac spine
rounded, anterior end of the iliac crest
arcuate line of the ilium
smooth ridge located at the inferior margin of the iliac fossa; forms the
lateral portion of the pelvic brim
auricular surface of the ilium
roughened area located on the posterior, medial side of the ilium of the
hip bone; articulates with the auricular surface of the sacrum to form
the sacroiliac joint
coxal bone
hip bone
greater pelvis
(also, greater pelvic cavity or false pelvis) broad space above the
pelvic brim defined laterally by the fan-like portion of the upper ilium
greater sciatic foramen
pelvic opening formed by the greater sciatic notch of the hip bone, the
sacrum, and the sacrospinous ligament
greater Sciatic notch
large, U-shaped indentation located on the posterior margin of the
ilium, superior to the ischial spine
hip bone
coxal bone; single bone that forms the pelvic girdle; consists of three
areas, the ilium, ischium, and pubis
iliac crest
curved, superior margin of the ilium
iliac fossa
shallow depression found on the anterior and medial surfaces of the
upper ilium
ilium
superior portion of the hip bone
inferior pubic ramus
narrow segment of bone that passes inferiorly and laterally from the
pubic body; joins with the ischial ramus to form the ischiopubic ramus
ischial ramus
bony extension projecting anteriorly and superiorly from the ischial
tuberosity; joins with the inferior pubic ramus to form the ischiopubic
ramus
ischial spine
pointed, bony projection from the posterior margin of the ischium that
separates the greater sciatic notch and lesser sciatic notch
ischial tuberosity
large, roughened protuberance that forms the posteroinferior portion of
the hip bone; weight-bearing region of the pelvis when sitting
ischiopubic ramus
narrow extension of bone that connects the ischial tuberosity to the
pubic body; formed by the junction of the ischial ramus and inferior
pubic ramus
ischium
posteroinferior portion of the hip bone
lesser pelvis
(also, lesser pelvic cavity or true pelvis) narrow space located within
the pelvis, defined superiorly by the pelvic brim (pelvic inlet) and
inferiorly by the pelvic outlet
lesser sciatic foramen
pelvic opening formed by the lesser sciatic notch of the hip bone, the
Sacrospinous ligament, and the sacrotuberous ligament
lesser sciatic notch
shallow indentation along the posterior margin of the ischium, inferior
to the ischial spine
obturator foramen
large opening located in the anterior hip bone, between the pubis and
ischium regions
pectineal line
narrow ridge located on the superior surface of the superior pubic
ramus
pelvic brim
pelvic inlet; the dividing line between the greater and lesser pelvic
regions; formed by the superior margin of the pubic symphysis, the
pectineal lines of each pubis, the arcuate lines of each ilium, and the
sacral promontory
pelvic girdle
hip girdle; consists of a single hip bone, which attaches a lower limb to
the sacrum of the axial skeleton
pelvic inlet
pelvic brim
pelvic outlet
inferior opening of the lesser pelvis; formed by the inferior margin of
the pubic symphysis, right and left ischiopubic rami and sacrotuberous
ligaments, and the tip of the coccyx
pelvis
ring of bone consisting of the right and left hip bones, the sacrum, and
the coccyx
posterior inferior iliac spine
small, bony projection located at the inferior margin of the auricular
surface on the posterior ilium
posterior sacroiliac ligament
strong ligament spanning the sacrum and ilium of the hip bone that
supports the posterior side of the sacroiliac joint
posterior superior iliac spine
rounded, posterior end of the iliac crest
pubic arch
bony structure formed by the pubic symphysis, and the bodies and
inferior pubic rami of the right and left pubic bones
pubic body
enlarged, medial portion of the pubis region of the hip bone
pubic symphysis
joint formed by the articulation between the pubic bodies of the right
and left hip bones
pubic tubercle
small bump located on the superior aspect of the pubic body
pubis
anterior portion of the hip bone
sacroiliac joint
joint formed by the articulation between the auricular surfaces of the
sacrum and ilium
sacrospinous ligament
ligament that spans the sacrum to the ischial spine of the hip bone
sacrotuberous ligament
ligament that spans the sacrum to the ischial tuberosity of the hip bone
subpubic angle
inverted V-shape formed by the convergence of the right and left
ischiopubic rami; this angle is greater than 80 degrees in females and
less than 70 degrees in males
superior pubic ramus
narrow segment of bone that passes laterally from the pubic body to
join the ilium
Bones of the Lower Limb
By the end of this section, you will be able to:
e Identify the divisions of the lower limb and describe the bones of each
region
e Describe the bones and bony landmarks that articulate at each joint of
the lower limb
Like the upper limb, the lower limb is divided into three regions. The thigh
is that portion of the lower limb located between the hip joint and knee
joint. The leg is specifically the region between the knee joint and the ankle
joint. Distal to the ankle is the foot. The lower limb contains 30 bones.
These are the femur, patella, tibia, fibula, tarsal bones, metatarsal bones,
and phalanges (see [link]). The femur is the single bone of the thigh. The
patella is the kneecap and articulates with the distal femur. The tibia is the
larger, weight-bearing bone located on the medial side of the leg, and the
fibula is the thin bone of the lateral leg. The bones of the foot are divided
into three groups. The posterior portion of the foot is formed by a group of
seven bones, each of which is known as a tarsal bone, whereas the mid-
foot contains five elongated bones, each of which is a metatarsal bone.
The toes contain 14 small bones, each of which is a phalanx bone of the
foot.
Femur
The femur, or thigh bone, is the single bone of the thigh region ((link]). It is
the longest and strongest bone of the body, and accounts for approximately
one-quarter of a person’s total height. The rounded, proximal end is the
head of the femur, which articulates with the acetabulum of the hip bone
to form the hip joint. The fovea capitis is a minor indentation on the
medial side of the femoral head that serves as the site of attachment for the
ligament of the head of the femur. This ligament spans the femur and
acetabulum, but is weak and provides little support for the hip joint. It does,
however, carry an important artery that supplies the head of the femur.
Femur and Patella
Greater
trochanter
Greater
trochanter
Gluteal
tuberosity
Intertrochanteric line
Intertrochanteric
crest
Lesser trochanter
Linea
aspera
Adductor
tubercle Lateral
Lateral epicondyle
epicondyle Medial
oe Intercondylar
Patella fossa
Medial condyle Lateral
i condyle
Fibula —
Fibula
Anterior view Posterior view
The femur is the single bone of the thigh
region. It articulates superiorly with the
hip bone at the hip joint, and inferiorly
with the tibia at the knee joint. The patella
only articulates with the distal end of the
femur.
The narrowed region below the head is the neck of the femur. This is a
common area for fractures of the femur. The greater trochanter is the
large, upward, bony projection located above the base of the neck. Multiple
muscles that act across the hip joint attach to the greater trochanter, which,
because of its projection from the femur, gives additional leverage to these
muscles. The greater trochanter can be felt just under the skin on the lateral
side of your upper thigh. The lesser trochanter is a small, bony
prominence that lies on the medial aspect of the femur, just below the neck.
A single, powerful muscle attaches to the lesser trochanter. Running
between the greater and lesser trochanters on the anterior side of the femur
is the roughened intertrochanteric line. The trochanters are also connected
on the posterior side of the femur by the larger intertrochanteric crest.
The elongated shaft of the femur has a slight anterior bowing or curvature.
At its proximal end, the posterior shaft has the gluteal tuberosity, a
roughened area extending inferiorly from the greater trochanter. More
inferiorly, the gluteal tuberosity becomes continuous with the linea aspera
(“rough line”). This is the roughened ridge that passes distally along the
posterior side of the mid-femur. Multiple muscles of the hip and thigh
regions make long, thin attachments to the femur along the linea aspera.
The distal end of the femur has medial and lateral bony expansions. On the
lateral side, the smooth portion that covers the distal and posterior aspects
of the lateral expansion is the lateral condyle of the femur. The roughened
area on the outer, lateral side of the condyle is the lateral epicondyle of the
femur. Similarly, the smooth region of the distal and posterior medial
femur is the medial condyle of the femur, and the irregular outer, medial
side of this is the medial epicondyle of the femur. The lateral and medial
condyles articulate with the tibia to form the knee joint. The epicondyles
provide attachment for muscles and supporting ligaments of the knee. The
adductor tubercle is a small bump located at the superior margin of the
medial epicondyle. Posteriorly, the medial and lateral condyles are
separated by a deep depression called the intercondylar fossa. Anteriorly,
the smooth surfaces of the condyles join together to form a wide groove
called the patellar surface, which provides for articulation with the patella
bone. The combination of the medial and lateral condyles with the patellar
surface gives the distal end of the femur a horseshoe (U) shape.
Watch this video to view how a fracture of the mid-femur is surgically
repaired. How are the two portions of the broken femur stabilized during
surgical repair of a fractured femur?
Patella
The patella (kneecap) is largest sesamoid bone of the body (see [link]). A
sesamoid bone is a bone that is incorporated into the tendon of a muscle
where that tendon crosses a joint. The sesamoid bone articulates with the
underlying bones to prevent damage to the muscle tendon due to rubbing
against the bones during movements of the joint. The patella is found in the
tendon of the quadriceps femoris muscle, the large muscle of the anterior
thigh that passes across the anterior knee to attach to the tibia. The patella
articulates with the patellar surface of the femur and thus prevents rubbing
of the muscle tendon against the distal femur. The patella also lifts the
tendon away from the knee joint, which increases the leverage power of the
quadriceps femoris muscle as it acts across the knee. The patella does not
articulate with the tibia.
Note:
openstax COLLEGE
. a
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Pit | |
Visit this site to perform a virtual knee replacement surgery. The prosthetic
knee components must be properly aligned to function properly. How is
this alignment ensured?
Note:
Homeostatic Imbalances
Runner’s Knee
Runner’s knee, also known as patellofemoral syndrome, is the most
common overuse injury among runners. It is most frequent in adolescents
and young adults, and is more common in females. It often results from
excessive running, particularly downhill, but may also occur in athletes
who do a lot of knee bending, such as jumpers, skiers, cyclists, weight
lifters, and soccer players. It is felt as a dull, aching pain around the front
of the knee and deep to the patella. The pain may be felt when walking or
running, going up or down stairs, kneeling or squatting, or after sitting with
the knee bent for an extended period.
Patellofemoral syndrome may be initiated by a variety of causes, including
individual variations in the shape and movement of the patella, a direct
blow to the patella, or flat feet or improper shoes that cause excessive
turning in or out of the feet or leg. These factors may cause in an
imbalance in the muscle pull that acts on the patella, resulting in an
abnormal tracking of the patella that allows it to deviate too far toward the
lateral side of the patellar surface on the distal femur.
Because the hips are wider than the knee region, the femur has a diagonal
orientation within the thigh, in contrast to the vertically oriented tibia of
the leg ({link]). The Q-angle is a measure of how far the femur is angled
laterally away from vertical. The Q-angle is normally 10—15 degrees, with
females typically having a larger Q-angle due to their wider pelvis. During
extension of the knee, the quadriceps femoris muscle pulls the patella both
superiorly and laterally, with the lateral pull greater in women due to their
large Q-angle. This makes women more vulnerable to developing
patellofemoral syndrome than men. Normally, the large lip on the lateral
side of the patellar surface of the femur compensates for the lateral pull on
the patella, and thus helps to maintain its proper tracking.
However, if the pull produced by the medial and lateral sides of the
quadriceps femoris muscle is not properly balanced, abnormal tracking of
the patella toward the lateral side may occur. With continued use, this
produces pain and could result in damage to the articulating surfaces of the
patella and femur, and the possible future development of arthritis.
Treatment generally involves stopping the activity that produces knee pain
for a period of time, followed by a gradual resumption of activity. Proper
strengthening of the quadriceps femoris muscle to correct for imbalances is
also important to help prevent reoccurrence.
The Q-Angle
Q-angle
Anterior view
The Q-angle is a
measure of the amount
of lateral deviation of
the femur from the
vertical line of the
tibia. Adult females
have a larger Q-angle
due to their wider
pelvis than adult
males.
Tibia
The tibia (shin bone) is the medial bone of the leg and is larger than the
fibula, with which it is paired ({link]). The tibia is the main weight-bearing
bone of the lower leg and the second longest bone of the body, after the
femur. The medial side of the tibia is located immediately under the skin,
allowing it to be easily palpated down the entire length of the medial leg.
Tibia and Fibula
fa a
Lateral condyle :
Articular surface
of lateral condyle
Articular surface
of medial condyle
Medial Head of fibula
aa ia Anterior border
Interosseous
membrane
Tibial tuberosity
Soleal line
Fibula Fibula
Medial malleolus
Lateral malleolus
Lateral malleolus
Articular surface
Anterior view Posterior view
The tibia is the larger, weight-bearing bone
located on the medial side of the leg. The
fibula is the slender bone of the lateral side
of the leg and does not bear weight.
The proximal end of the tibia is greatly expanded. The two sides of this
expansion form the medial condyle of the tibia and the lateral condyle of
the tibia. The tibia does not have epicondyles. The top surface of each
condyle is smooth and flattened. These areas articulate with the medial and
lateral condyles of the femur to form the knee joint. Between the
articulating surfaces of the tibial condyles is the intercondylar eminence,
an irregular, elevated area that serves as the inferior attachment point for
two supporting ligaments of the knee.
The tibial tuberosity is an elevated area on the anterior side of the tibia,
near its proximal end. It is the final site of attachment for the muscle tendon
associated with the patella. More inferiorly, the shaft of the tibia becomes
triangular in shape. The anterior apex of
MH this triangle forms the anterior border of the tibia, which begins at
the tibial tuberosity and runs inferiorly along the length of the tibia. Both
the anterior border and the medial side of the triangular shaft are located
immediately under the skin and can be easily palpated along the entire
length of the tibia. A small ridge running down the lateral side of the tibial
shaft is the interosseous border of the tibia. This is for the attachment of
the interosseous membrane of the leg, the sheet of dense connective tissue
that unites the tibia and fibula bones. Located on the posterior side of the
tibia is the soleal line, a diagonally running, roughened ridge that begins
below the base of the lateral condyle, and runs down and medially across
the proximal third of the posterior tibia. Muscles of the posterior leg attach
to this line.
The large expansion found on the medial side of the distal tibia is the
medial malleolus (“little hammer”). This forms the large bony bump found
on the medial side of the ankle region. Both the smooth surface on the
inside of the medial malleolus and the smooth area at the distal end of the
tibia articulate with the talus bone of the foot as part of the ankle joint. On
the lateral side of the distal tibia is a wide groove called the fibular notch.
This area articulates with the distal end of the fibula, forming the distal
tibiofibular joint.
Fibula
The fibula is the slender bone located on the lateral side of the leg (see
[link]). The fibula does not bear weight. It serves primarily for muscle
attachments and thus is largely surrounded by muscles. Only the proximal
and distal ends of the fibula can be palpated.
The head of the fibula is the small, knob-like, proximal end of the fibula. It
articulates with the inferior aspect of the lateral tibial condyle, forming the
proximal tibiofibular joint. The thin shaft of the fibula has the
interosseous border of the fibula, a narrow ridge running down its medial
side for the attachment of the interosseous membrane that spans the fibula
and tibia. The distal end of the fibula forms the lateral malleolus, which
forms the easily palpated bony bump on the lateral side of the ankle. The
deep (medial) side of the lateral malleolus articulates with the talus bone of
the foot as part of the ankle joint. The distal fibula also articulates with the
fibular notch of the tibia.
Tarsal Bones
The posterior half of the foot is formed by seven tarsal bones ([link]). The
most superior bone is the talus. This has a relatively square-shaped, upper
surface that articulates with the tibia and fibula to form the ankle joint.
Three areas of articulation form the ankle joint: The superomedial surface
of the talus bone articulates with the medial malleolus of the tibia, the top of
the talus articulates with the distal end of the tibia, and the lateral side of the
talus articulates with the lateral malleolus of the fibula. Inferiorly, the talus
articulates with the calcaneus (heel bone), the largest bone of the foot,
which forms the heel. Body weight is transferred from the tibia to the talus
to the calcaneus, which rests on the ground. The medial calcaneus has a
prominent bony extension called the sustentaculum tali (“support for the
talus”) that supports the medial side of the talus bone.
Bones of the Foot
| \ t | Navicular Talus
\ Intermediate
Tarsals cuneiform
Metatarsals
Phalanges First metatarsal
Facet for medial
malleolus
Sustentaculum
tali (talar shelf)
Distal ————___{ :
¢€ Medial Calcaneus
Proximal Y Distal cuneiform =i ee
halanges phalange alcaneal tuberosi
i 9 : / Medial view
Middle
phalange
Medial Proximal
cuneiform phalange Facet for
lateral malleolus Navicular Intermediate cuneiform
Intermediate
cuneiform
Lateral
cuneiform Lateral cuneiform
Navicular Cuboid
Talus
Trochlea
of talus
Calcaneus
Calcaneus Cuboid Fifth metatarsal
Superior view Lateral view
The bones of the foot are divided into three groups. The posterior
foot is formed by the seven tarsal bones. The mid-foot has the five
metatarsal bones. The toes contain the phalanges.
The cuboid bone articulates with the anterior end of the calcaneus bone.
The cuboid has a deep groove running across its inferior surface, which
provides passage for a muscle tendon. The talus bone articulates anteriorly
with the navicular bone, which in turn articulates anteriorly with the three
cuneiform (“wedge-shaped”) bones. These bones are the medial
cuneiform, the intermediate cuneiform, and the lateral cuneiform. Each
of these bones has a broad superior surface and a narrow inferior surface,
which together produce the transverse (medial-lateral) curvature of the foot.
The navicular and lateral cuneiform bones also articulate with the medial
side of the cuboid bone.
Note:
— openstax COLLEGE
1
Oe
Use this tutorial to review the bones of the foot. Which tarsal bones are in
the proximal, intermediate, and distal groups?
Metatarsal Bones
The anterior half of the foot is formed by the five metatarsal bones, which
are located between the tarsal bones of the posterior foot and the phalanges
of the toes (see [link]). These elongated bones are numbered 1-5, starting
with the medial side of the foot. The first metatarsal bone is shorter and
thicker than the others. The second metatarsal is the longest. The base of
the metatarsal bone is the proximal end of each metatarsal bone. These
articulate with the cuboid or cuneiform bones. The base of the fifth
metatarsal has a large, lateral expansion that provides for muscle
attachments. This expanded base of the fifth metatarsal can be felt as a bony
bump at the midpoint along the lateral border of the foot. The expanded
distal end of each metatarsal is the head of the metatarsal bone. Each
metatarsal bone articulates with the proximal phalanx of a toe to forma
metatarsophalangeal joint. The heads of the metatarsal bones also rest on
the ground and form the ball (anterior end) of the foot.
Phalanges
The toes contain a total of 14 phalanx bones (phalanges), arranged in a
similar manner as the phalanges of the fingers (see [link]). The toes are
numbered 1-5, starting with the big toe (hallux). The big toe has two
phalanx bones, the proximal and distal phalanges. The remaining toes all
have proximal, middle, and distal phalanges. A joint between adjacent
phalanx bones is called an interphalangeal joint.
Note:
eS
wees Openstax COLLEGE
aha
View this link to learn about a bunion, a localized swelling on the medial
side of the foot, next to the first metatarsophalangeal joint, at the base of
the big toe. What is a bunion and what type of shoe is most likely to cause
this to develop?
Arches of the Foot
When the foot comes into contact with the ground during walking, running,
or jumping activities, the impact of the body weight puts a tremendous
amount of pressure and force on the foot. During running, the force applied
to each foot as it contacts the ground can be up to 2.5 times your body
weight. The bones, joints, ligaments, and muscles of the foot absorb this
force, thus greatly reducing the amount of shock that is passed superiorly
into the lower limb and body. The arches of the foot play an important role
in this shock-absorbing ability. When weight is applied to the foot, these
arches will flatten somewhat, thus absorbing energy. When the weight is
removed, the arch rebounds, giving “spring” to the step. The arches also
serve to distribute body weight side to side and to either end of the foot.
The foot has a transverse arch, a medial longitudinal arch, and a lateral
longitudinal arch (see [link]). The transverse arch forms the medial-lateral
curvature of the mid-foot. It is formed by the wedge shapes of the
cuneiform bones and bases (proximal ends) of the first to fourth metatarsal
bones. This arch helps to distribute body weight from side to side within the
foot, thus allowing the foot to accommodate uneven terrain.
The longitudinal arches run down the length of the foot. The lateral
longitudinal arch is relatively flat, whereas the medial longitudinal arch is
larger (taller). The longitudinal arches are formed by the tarsal bones
posteriorly and the metatarsal bones anteriorly. These arches are supported
at either end, where they contact the ground. Posteriorly, this support is
provided by the calcaneus bone and anteriorly by the heads (distal ends) of
the metatarsal bones. The talus bone, which receives the weight of the body,
is located at the top of the longitudinal arches. Body weight is then
conveyed from the talus to the ground by the anterior and posterior ends of
these arches. Strong ligaments unite the adjacent foot bones to prevent
disruption of the arches during weight bearing. On the bottom of the foot,
additional ligaments tie together the anterior and posterior ends of the
arches. These ligaments have elasticity, which allows them to stretch
somewhat during weight bearing, thus allowing the longitudinal arches to
spread. The stretching of these ligaments stores energy within the foot,
rather than passing these forces into the leg. Contraction of the foot muscles
also plays an important role in this energy absorption. When the weight is
removed, the elastic ligaments recoil and pull the ends of the arches closer
together. This recovery of the arches releases the stored energy and
improves the energy efficiency of walking.
Stretching of the ligaments that support the longitudinal arches can lead to
pain. This can occur in overweight individuals, with people who have jobs
that involve standing for long periods of time (such as a waitress), or
walking or running long distances. If stretching of the ligaments is
prolonged, excessive, or repeated, it can result in a gradual lengthening of
the supporting ligaments, with subsequent depression or collapse of the
longitudinal arches, particularly on the medial side of the foot. This
condition is called pes planus (“flat foot” or “fallen arches”).
Chapter Review
The lower limb is divided into three regions. These are the thigh, located
between the hip and knee joints; the leg, located between the knee and
ankle joints; and distal to the ankle, the foot. There are 30 bones in each
lower limb. These are the femur, patella, tibia, fibula, seven tarsal bones,
five metatarsal bones, and 14 phalanges.
The femur is the single bone of the thigh. Its rounded head articulates with
the acetabulum of the hip bone to form the hip joint. The head has the fovea
capitis for attachment of the ligament of the head of the femur. The narrow
neck joins inferiorly with the greater and lesser trochanters. Passing
between these bony expansions are the intertrochanteric line on the anterior
femur and the larger intertrochanteric crest on the posterior femur. On the
posterior shaft of the femur is the gluteal tuberosity proximally and the
linea aspera in the mid-shaft region. The expanded distal end consists of
three articulating surfaces: the medial and lateral condyles, and the patellar
surface. The outside margins of the condyles are the medial and lateral
epicondyles. The adductor tubercle is on the superior aspect of the medial
epicondyle.
The patella is a sesamoid bone located within a muscle tendon. It articulates
with the patellar surface on the anterior side of the distal femur, thereby
protecting the muscle tendon from rubbing against the femur.
The leg contains the large tibia on the medial side and the slender fibula on
the lateral side. The tibia bears the weight of the body, whereas the fibula
does not bear weight. The interosseous border of each bone is the
attachment site for the interosseous membrane of the leg, the connective
tissue sheet that unites the tibia and fibula.
The proximal tibia consists of the expanded medial and lateral condyles,
which articulate with the medial and lateral condyles of the femur to form
the knee joint. Between the tibial condyles is the intercondylar eminence.
On the anterior side of the proximal tibia is the tibial tuberosity, which is
continuous inferiorly with the anterior border of the tibia. On the posterior
side, the proximal tibia has the curved soleal line. The bony expansion on
the medial side of the distal tibia is the medial malleolus. The groove on the
lateral side of the distal tibia is the fibular notch.
The head of the fibula forms the proximal end and articulates with the
underside of the lateral condyle of the tibia. The distal fibula articulates
with the fibular notch of the tibia. The expanded distal end of the fibula is
the lateral malleolus.
The posterior foot is formed by the seven tarsal bones. The talus articulates
superiorly with the distal tibia, the medial malleolus of the tibia, and the
lateral malleolus of the fibula to form the ankle joint. The talus articulates
inferiorly with the calcaneus bone. The sustentaculum tali of the calcaneus
helps to support the talus. Anterior to the talus is the navicular bone, and
anterior to this are the medial, intermediate, and lateral cuneiform bones.
The cuboid bone is anterior to the calcaneus.
The five metatarsal bones form the anterior foot. The base of these bones
articulate with the cuboid or cuneiform bones. The metatarsal heads, at their
distal ends, articulate with the proximal phalanges of the toes. The big toe
(toe number 1) has proximal and distal phalanx bones. The remaining toes
have proximal, middle, and distal phalanges.
Interactive Link Questions
Exercise:
Problem:
Watch this video to view how a fracture of the mid-femur is surgically
repaired. How are the two portions of the broken femur stabilized
during surgical repair of a fractured femur?
Solution:
A hole is drilled into the greater trochanter, the bone marrow
(medullary) space inside the femur is enlarged, and finally an
intramedullary rod is inserted into the femur. This rod is then anchored
to the bone with screws.
Exercise:
Problem:
Visit this site to perform a virtual knee replacement surgery. The
prosthetic knee components must be properly aligned to function
properly. How is this alignment ensured?
Solution:
Metal cutting jigs are attached to the bones to ensure that the bones are
cut properly prior to the attachment of prosthetic components.
Exercise:
Problem:
Use this tutorial to review the bones of the foot. Which tarsal bones are
in the proximal, intermediate, and distal groups?
Solution:
The proximal group of tarsal bones includes the calcaneus and talus
bones, the navicular bone is intermediate, and the distal group consists
of the cuboid bone plus the medial, intermediate, and lateral cuneiform
bones.
Exercise:
Problem:
View this link to learn about a bunion, a localized swelling on the
medial side of the foot, next to the first metatarsophalangeal joint, at
the base of the big toe. What is a bunion and what type of shoe is most
likely to cause this to develop?
Solution:
A bunion results from the deviation of the big toe toward the second
toe, which causes the distal end of the first metatarsal bone to stick out.
A bunion may also be caused by prolonged pressure on the foot from
pointed shoes with a narrow toe box that compresses the big toe and
pushes it toward the second toe.
Review Questions
Exercise:
Problem:
Which bony landmark of the femur serves as a site for muscle
attachments?
a. fovea capitis
b. lesser trochanter
c. head
d. medial condyle
Solution:
B
Exercise:
Problem: What structure contributes to the knee joint?
a. lateral malleolus of the fibula
b. tibial tuberosity
c. medial condyle of the tibia
d. lateral epicondyle of the femur
Solution:
C
Exercise:
Problem: Which tarsal bone articulates with the tibia and fibula?
a. calcaneus
b. cuboid
c. navicular
d. talus
Solution:
D
Exercise:
Problem: What is the total number of bones found in the foot and toes?
a. 7
a
aN
BS
Solution:
C
Exercise:
Problem: The tibia
a. has an expanded distal end called the lateral malleolus
b. is not a weight-bearing bone
c. is firmly anchored to the fibula by an interosseous membrane
d. can be palpated (felt) under the skin only at its proximal and
distal ends
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Define the regions of the lower limb, name the bones found in each
region, and describe the bony landmarks that articulate together to
form the hip, knee, and ankle joints.
Solution:
The lower limb is divided into three regions. The thigh is the region
located between the hip and knee joints. It contains the femur and the
patella. The hip joint is formed by the articulation between the
acetabulum of the hip bone and the head of the femur. The leg is the
region between the knee and ankle joints, and contains the tibia
(medially) and the fibula (laterally). The knee joint is formed by the
articulations between the medial and lateral condyles of the femur, and
the medial and lateral condyles of the tibia. Also associated with the
knee is the patella, which articulates with the patellar surface of the
distal femur. The foot is found distal to the ankle and contains 26
bones. The ankle joint is formed by the articulations between the talus
bone of the foot and the distal end of the tibia, the medial malleolus of
the tibia, and the lateral malleolus of the fibula. The posterior foot
contains the seven tarsal bones, which are the talus, calcaneus,
navicular, cuboid, and the medial, intermediate, and lateral cuneiform
bones. The anterior foot consists of the five metatarsal bones, which
are numbered 1-5 starting on the medial side of the foot. The toes
contain 14 phalanx bones, with the big toe (toe number 1) having a
proximal and a distal phalanx, and the other toes having proximal,
middle, and distal phalanges.
Exercise:
Problem:
The talus bone of the foot receives the weight of the body from the
tibia. The talus bone then distributes this weight toward the ground in
two directions: one-half of the body weight is passed in a posterior
direction and one-half of the weight is passed in an anterior direction.
Describe the arrangement of the tarsal and metatarsal bones that are
involved in both the posterior and anterior distribution of body weight.
Solution:
The talus bone articulates superiorly with the tibia and fibula at the
ankle joint, with body weight passed from the tibia to the talus. Body
weight from the talus is transmitted to the ground by both ends of the
medial and lateral longitudinal foot arches. Weight is passed
posteriorly through both arches to the calcaneus bone, which forms the
heel of the foot and is in contact with the ground. On the medial side
of the foot, body weight is passed anteriorly from the talus bone to the
navicular bone, and then to the medial, intermediate, and lateral
cuneiform bones. The cuneiform bones pass the weight anteriorly to
the first, second, and third metatarsal bones, whose heads (distal ends)
are in contact with the ground. On the lateral side, body weight is
passed anteriorly from the talus through the calcaneus, cuboid, and
fourth and fifth metatarsal bones. The talus bone thus transmits body
weight posteriorly to the calcaneus and anteriorly through the
navicular, cuneiform, and cuboid bones, and metatarsals one through
five.
Glossary
adductor tubercle
small, bony bump located on the superior aspect of the medial
epicondyle of the femur
ankle joint
joint that separates the leg and foot portions of the lower limb; formed
by the articulations between the talus bone of the foot inferiorly, and
the distal end of the tibia, medial malleolus of the tibia, and lateral
malleolus of the fibula superiorly
anterior border of the tibia
narrow, anterior margin of the tibia that extends inferiorly from the
tibial tuberosity
base of the metatarsal bone
expanded, proximal end of each metatarsal bone
calcaneus
heel bone; posterior, inferior tarsal bone that forms the heel of the foot
cuboid
tarsal bone that articulates posteriorly with the calcaneus bone,
medially with the lateral cuneiform bone, and anteriorly with the
fourth and fifth metatarsal bones
distal tibiofibular joint
articulation between the distal fibula and the fibular notch of the tibia
femur
thigh bone; the single bone of the thigh
fibula
thin, non-weight-bearing bone found on the lateral side of the leg
fibular notch
wide groove on the lateral side of the distal tibia for articulation with
the fibula at the distal tibiofibular joint
foot
portion of the lower limb located distal to the ankle joint
fovea capitis
minor indentation on the head of the femur that serves as the site of
attachment for the ligament to the head of the femur
gluteal tuberosity
roughened area on the posterior side of the proximal femur, extending
inferiorly from the base of the greater trochanter
greater trochanter
large, bony expansion of the femur that projects superiorly from the
base of the femoral neck
hallux
big toe; digit 1 of the foot
head of the femur
rounded, proximal end of the femur that articulates with the
acetabulum of the hip bone to form the hip joint
head of the fibula
small, knob-like, proximal end of the fibula; articulates with the
inferior aspect of the lateral condyle of the tibia
head of the metatarsal bone
expanded, distal end of each metatarsal bone
hip joint
joint located at the proximal end of the lower limb; formed by the
articulation between the acetabulum of the hip bone and the head of
the femur
intercondylar eminence
irregular elevation on the superior end of the tibia, between the
articulating surfaces of the medial and lateral condyles
intercondylar fossa
deep depression on the posterior side of the distal femur that separates
the medial and lateral condyles
intermediate cuneiform
middle of the three cuneiform tarsal bones; articulates posteriorly with
the navicular bone, medially with the medial cuneiform bone, laterally
with the lateral cuneiform bone, and anteriorly with the second
metatarsal bone
interosseous border of the fibula
small ridge running down the medial side of the fibular shaft; for
attachment of the interosseous membrane between the fibula and tibia
interosseous border of the tibia
small ridge running down the lateral side of the tibial shaft; for
attachment of the interosseous membrane between the tibia and fibula
interosseous membrane of the leg
sheet of dense connective tissue that unites the shafts of the tibia and
fibula bones
intertrochanteric crest
short, prominent ridge running between the greater and lesser
trochanters on the posterior side of the proximal femur
intertrochanteric line
small ridge running between the greater and lesser trochanters on the
anterior side of the proximal femur
knee joint
joint that separates the thigh and leg portions of the lower limb;
formed by the articulations between the medial and lateral condyles of
the femur, and the medial and lateral condyles of the tibia
lateral condyle of the femur
smooth, articulating surface that forms the distal and posterior sides of
the lateral expansion of the distal femur
lateral condyle of the tibia
lateral, expanded region of the proximal tibia that includes the smooth
surface that articulates with the lateral condyle of the femur as part of
the knee joint
lateral cuneiform
most lateral of the three cuneiform tarsal bones; articulates posteriorly
with the navicular bone, medially with the intermediate cuneiform
bone, laterally with the cuboid bone, and anteriorly with the third
metatarsal bone
lateral epicondyle of the femur
roughened area of the femur located on the lateral side of the lateral
condyle
lateral malleolus
expanded distal end of the fibula
leg
portion of the lower limb located between the knee and ankle joints
lesser trochanter
small, bony projection on the medial side of the proximal femur, at the
base of the femoral neck
ligament of the head of the femur
ligament that spans the acetabulum of the hip bone and the fovea
capitis of the femoral head
linea aspera
longitudinally running bony ridge located in the middle third of the
posterior femur
medial condyle of the femur
smooth, articulating surface that forms the distal and posterior sides of
the medial expansion of the distal femur
medial condyle of the tibia
medial, expanded region of the proximal tibia that includes the smooth
surface that articulates with the medial condyle of the femur as part of
the knee joint
medial cuneiform
most medial of the three cuneiform tarsal bones; articulates posteriorly
with the navicular bone, laterally with the intermediate cuneiform
bone, and anteriorly with the first and second metatarsal bones
medial epicondyle of the femur
roughened area of the distal femur located on the medial side of the
medial condyle
medial malleolus
bony expansion located on the medial side of the distal tibia
metatarsal bone
one of the five elongated bones that forms the anterior half of the foot;
numbered 1-5, starting on the medial side of the foot
metatarsophalangeal joint
articulation between a metatarsal bone of the foot and the proximal
phalanx bone of a toe
navicular
tarsal bone that articulates posteriorly with the talus bone, laterally
with the cuboid bone, and anteriorly with the medial, intermediate, and
lateral cuneiform bones
neck of the femur
narrowed region located inferior to the head of the femur
patella
kneecap; the largest sesamoid bone of the body; articulates with the
distal femur
patellar surface
smooth groove located on the anterior side of the distal femur, between
the medial and lateral condyles; site of articulation for the patella
phalanx bone of the foot
(plural = phalanges) one of the 14 bones that form the toes; these
include the proximal and distal phalanges of the big toe, and the
proximal, middle, and distal phalanx bones of toes two through five
proximal tibiofibular joint
articulation between the head of the fibula and the inferior aspect of
the lateral condyle of the tibia
shaft of the femur
cylindrically shaped region that forms the central portion of the femur
shaft of the fibula
elongated, slender portion located between the expanded ends of the
fibula
shaft of the tibia
triangular-shaped, central portion of the tibia
soleal line
small, diagonally running ridge located on the posterior side of the
proximal tibia
sustentaculum tali
bony ledge extending from the medial side of the calcaneus bone
talus
tarsal bone that articulates superiorly with the tibia and fibula at the
ankle joint; also articulates inferiorly with the calcaneus bone and
anteriorly with the navicular bone
tarsal bone
one of the seven bones that make up the posterior foot; includes the
calcaneus, talus, navicular, cuboid, medial cuneiform, intermediate
cuneiform, and lateral cuneiform bones
thigh
portion of the lower limb located between the hip and knee joints
tibia
shin bone; the large, weight-bearing bone located on the medial side of
the leg
tibial tuberosity
elevated area on the anterior surface of the proximal tibia
Classification of Joints
By the end of this section, you will be able to:
e Distinguish between the functional and structural classifications for
joints
¢ Describe the three functional types of joints and give an example of
each
e List the three types of diarthrodial joints
A joint, also called an articulation, is any place where adjacent bones or
bone and cartilage come together (articulate with each other) to form a
connection. Joints are classified both structurally and functionally.
Structural classifications of joints take into account whether the adjacent
bones are strongly anchored to each other by fibrous connective tissue or
cartilage, or whether the adjacent bones articulate with each other within a
fluid-filled space called a joint cavity. Functional classifications describe
the degree of movement available between the bones, ranging from
immobile, to slightly mobile, to freely moveable joints. The amount of
movement available at a particular joint of the body is related to the
functional requirements for that joint. Thus immobile or slightly moveable
joints serve to protect internal organs, give stability to the body, and allow
for limited body movement. In contrast, freely moveable joints allow for
much more extensive movements of the body and limbs.
Structural Classification of Joints
The structural classification of joints is based on whether the articulating
surfaces of the adjacent bones are directly connected by fibrous connective
tissue or cartilage, or whether the articulating surfaces contact each other
within a fluid-filled joint cavity. These differences serve to divide the joints
of the body into three structural classifications. A fibrous joint is where the
adjacent bones are united by fibrous connective tissue. At a cartilaginous
joint, the bones are joined by hyaline cartilage or fibrocartilage. At a
synovial joint, the articulating surfaces of the bones are not directly
connected, but instead come into contact with each other within a joint
cavity that is filled with a lubricating fluid. Synovial joints allow for free
movement between the bones and are the most common joints of the body.
Functional Classification of Joints
The functional classification of joints is determined by the amount of
mobility found between the adjacent bones. Joints are thus functionally
classified as a synarthrosis or immobile joint, an amphiarthrosis or slightly
moveable joint, or as a diarthrosis, which is a freely moveable joint
(arthroun = “to fasten by a joint”). Depending on their location, fibrous
joints may be functionally classified as a synarthrosis (immobile joint) or an
amphiarthrosis (slightly mobile joint). Cartilaginous joints are also
functionally classified as either a synarthrosis or an amphiarthrosis joint.
All synovial joints are functionally classified as a diarthrosis joint.
Synarthrosis
An immobile or nearly immobile joint is called a synarthrosis. The
immobile nature of these joints provide for a strong union between the
articulating bones. This is important at locations where the bones provide
protection for internal organs. Examples include sutures, the fibrous joints
between the bones of the skull that surround and protect the brain ([link]),
and the manubriosternal joint, the cartilaginous joint that unites the
manubrium and body of the sternum for protection of the heart.
Suture Joints of Skull
Coronal
suture
Lambdoid Squamous
suture suture
The suture joints of the skull are an
example of a synarthrosis, an
immobile or essentially immobile
joint.
Amphiarthrosis
An amphiarthrosis is a joint that has limited mobility. An example of this
type of joint is the cartilaginous joint that unites the bodies of adjacent
vertebrae. Filling the gap between the vertebrae is a thick pad of
fibrocartilage called an intervertebral disc ({link]). Each intervertebral disc
strongly unites the vertebrae but still allows for a limited amount of
movement between them. However, the small movements available
between adjacent vertebrae can sum together along the length of the
vertebral column to provide for large ranges of body movements.
Another example of an amphiarthrosis is the pubic symphysis of the pelvis.
This is a cartilaginous joint in which the pubic regions of the right and left
hip bones are strongly anchored to each other by fibrocartilage. This joint
normally has very little mobility. The strength of the pubic symphysis is
important in conferring weight-bearing stability to the pelvis.
Intervertebral Disc
i
Vertebral body
Intervertebral disc
Lateral view
An intervertebral disc unites the
bodies of adjacent vertebrae within
the vertebral column. Each disc
allows for limited movement
between the vertebrae and thus
functionally forms an
amphiarthrosis type of joint.
Intervertebral discs are made of
fibrocartilage and thereby
structurally form a symphysis type
of cartilaginous joint.
Diarthrosis
A freely mobile joint is classified as a diarthrosis. These types of joints
include all synovial joints of the body, which provide the majority of body
movements. Most diarthrotic joints are found in the appendicular skeleton
and thus give the limbs a wide range of motion. These joints are divided
into three categories, based on the number of axes of motion provided by
each. An axis in anatomy is described as the movements in reference to the
three anatomical planes: transverse, frontal, and sagittal. Thus, diarthroses
are Classified as uniaxial (for movement in one plane), biaxial (for
movement in two planes), or multiaxial joints (for movement in all three
anatomical planes).
A uniaxial joint only allows for a motion in a single plane (around a single
axis). The elbow joint, which only allows for bending or straightening, is an
example of a uniaxial joint. A biaxial joint allows for motions within two
planes. An example of a biaxial joint is a metacarpophalangeal joint
(knuckle joint) of the hand. The joint allows for movement along one axis
to produce bending or straightening of the finger, and movement along a
second axis, which allows for spreading of the fingers away from each other
and bringing them together. A joint that allows for the several directions of
movement is called a multiaxial joint (polyaxial or triaxial joint). This type
of diarthrotic joint allows for movement along three axes ([link]). The
shoulder and hip joints are multiaxial joints. They allow the upper or lower
limb to move in an anterior-posterior direction and a medial-lateral
direction. In addition, the limb can also be rotated around its long axis. This
third movement results in rotation of the limb so that its anterior surface is
moved either toward or away from the midline of the body.
Multiaxial Joint
Acetabulum of
hip bone
Head of femur
A multiaxial joint, such as the hip joint, allows for
three types of movement: anterior-posterior,
medial-lateral, and rotational.
Chapter Review
Structural classifications of the body joints are based on how the bones are
held together and articulate with each other. At fibrous joints, the adjacent
bones are directly united to each other by fibrous connective tissue.
Similarly, at a cartilaginous joint, the adjacent bones are united by cartilage.
In contrast, at a synovial joint, the articulating bone surfaces are not directly
united to each other, but come together within a fluid-filled joint cavity.
The functional classification of body joints is based on the degree of
movement found at each joint. A synarthrosis is a joint that is essentially
immobile. This type of joint provides for a strong connection between the
adjacent bones, which serves to protect internal structures such as the brain
or heart. Examples include the fibrous joints of the skull sutures and the
cartilaginous manubriosternal joint. A joint that allows for limited
movement is an amphiarthrosis. An example is the pubic symphysis of the
pelvis, the cartilaginous joint that strongly unites the right and left hip bones
of the pelvis. The cartilaginous joints in which vertebrae are united by
intervertebral discs provide for small movements between the adjacent
vertebrae and are also an amphiarthrosis type of joint. Thus, based on their
movement ability, both fibrous and cartilaginous joints are functionally
classified as a synarthrosis or amphiarthrosis.
The most common type of joint is the diarthrosis, which is a freely
moveable joint. All synovial joints are functionally classified as diarthroses.
A uniaxial diarthrosis, such as the elbow, is a joint that only allows for
movement within a single anatomical plane. Joints that allow for
movements in two planes are biaxial joints, such as the
metacarpophalangeal joints of the fingers. A multiaxial joint, such as the
shoulder or hip joint, allows for three planes of motions.
Review Questions
Exercise:
Problem:
The joint between adjacent vertebrae that includes an invertebral disc
is classified as which type of joint?
a. diarthrosis
b. multiaxial
c. amphiarthrosis
d. synarthrosis
Solution:
C
Exercise:
Problem: Which of these joints is classified as a synarthrosis?
a. the pubic symphysis
b. the manubriosternal joint
c. an invertebral disc
d. the shoulder joint
Solution:
B
Exercise:
Problem: Which of these joints is classified as a biaxial diarthrosis?
a. the metacarpophalangeal joint
b. the hip joint
c. the elbow joint
d. the pubic symphysis
Solution:
A
Exercise:
Problem:Synovial joints
a. may be functionally classified as a synarthrosis
b. are joints where the bones are connected to each other by hyaline
cartilage
c. may be functionally classified as a amphiarthrosis
d. are joints where the bones articulate with each other within a
fluid-filled joint cavity
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Define how joints are classified based on function. Describe and give
an example for each functional type of joint.
Solution:
Functional classification of joints is based on the degree of mobility
exhibited by the joint. A synarthrosis is an immobile or nearly
immobile joint. An example is the manubriosternal joint or the joints
between the skull bones surrounding the brain. An amphiarthrosis is a
slightly moveable joint, such as the pubic symphysis or an
intervertebral cartilaginous joint. A diarthrosis is a freely moveable
joint. These are subdivided into three categories. A uniaxial diarthrosis
allows movement within a single anatomical plane or axis of motion.
The elbow joint is an example. A biaxial diarthrosis, such as the
metacarpophalangeal joint, allows for movement along two planes or
axes. The hip and shoulder joints are examples of a multiaxial
diarthrosis. These allow movements along three planes or axes.
Exercise:
Problem:
Explain the reasons for why joints differ in their degree of mobility.
Solution:
The functional needs of joints vary and thus joints differ in their degree
of mobility. A synarthrosis, which is an immobile joint, serves to
strongly connect bones thus protecting internal organs such as the heart
or brain. A slightly moveable amphiarthrosis provides for small
movements, which in the vertebral column can add together to yield a
much larger overall movement. The freedom of movement provided by
a diarthrosis can allow for large movements, such as is seen with most
joints of the limbs.
Glossary
amphiarthrosis
slightly mobile joint
articulation
joint of the body
biaxial joint
type of diarthrosis; a joint that allows for movements within two
planes (two axes)
cartilaginous joint
joint at which the bones are united by hyaline cartilage
(synchondrosis) or fibrocartilage (symphysis)
diarthrosis
freely mobile joint
fibrous joint
joint where the articulating areas of the adjacent bones are connected
by fibrous connective tissue
joint
site at which two or more bones or bone and cartilage come together
(articulate)
joint cavity
space enclosed by the articular capsule of a synovial joint that is filled
with synovial fluid and contains the articulating surfaces of the
adjacent bones
multiaxial joint
type of diarthrosis; a joint that allows for movements within three
planes (three axes)
synarthrosis
immobile or nearly immobile joint
synovial joint
joint at which the articulating surfaces of the bones are located within
a joint cavity formed by an articular capsule
uniaxial joint
type of diarthrosis; joint that allows for motion within only one plane
(one axis)
Fibrous Joints
By the end of this section, you will be able to:
e Describe the structural features of fibrous joints
e Distinguish between a suture, syndesmosis, and gomphosis
e Give an example of each type of fibrous joint
At a fibrous joint, the adjacent bones are directly connected to each other by
fibrous connective tissue, and thus the bones do not have a joint cavity
between them ({link]). The gap between the bones may be narrow or wide.
There are three types of fibrous joints. A suture is the narrow fibrous joint
found between most bones of the skull. At a syndesmosis joint, the bones
are more widely separated but are held together by a narrow band of fibrous
connective tissue called a ligament or a wide sheet of connective tissue
called an interosseous membrane. This type of fibrous joint is found
between the shaft regions of the long bones in the forearm and in the leg.
Lastly, a gomphosis is the narrow fibrous joint between the roots of a tooth
and the bony socket in the jaw into which the tooth fits.
Fibrous Joints
. 7 Suture line :
Vv n e ~
ea . ~
\ ‘en, ) =D.
| __ Suture = a |
a Ulna ——— | | ; |
= Ly Bhs Radius
_ fx
AS Syndesmosis
= af v/\ Dense HT | Socket Root of
i fibrous AniabRIGHER 7 Hie
% connective /*ntebracnial | Genpheus
~~ tissue Meena | \
j membrane ON
ZN
Periodontal
ligament
(a) (b) (c)
Fibrous joints form strong connections between bones. (a)
Sutures join most bones of the skull. (b) An interosseous
membrane forms a syndesmosis between the radius and
ulna bones of the forearm. (c) A gomphosis is a specialized
fibrous joint that anchors a tooth to its socket in the jaw.
Suture
All the bones of the skull, except for the mandible, are joined to each other
by a fibrous joint called a suture. The fibrous connective tissue found at a
suture (“to bind or sew”) strongly unites the adjacent skull bones and thus
helps to protect the brain and form the face. In adults, the skull bones are
closely opposed and fibrous connective tissue fills the narrow gap between
the bones. The suture is frequently convoluted, forming a tight union that
prevents most movement between the bones. (See [link]a.) Thus, skull
sutures are functionally classified as a synarthrosis, although some sutures
may allow for slight movements between the cranial bones.
In newborns and infants, the areas of connective tissue between the bones
are much wider, especially in those areas on the top and sides of the skull
that will become the sagittal, coronal, squamous, and lambdoid sutures.
These broad areas of connective tissue are called fontanelles ((link]).
During birth, the fontanelles provide flexibility to the skull, allowing the
bones to push closer together or to overlap slightly, thus aiding movement
of the infant’s head through the birth canal. After birth, these expanded
regions of connective tissue allow for rapid growth of the skull and
enlargement of the brain. The fontanelles greatly decrease in width during
the first year after birth as the skull bones enlarge. When the connective
tissue between the adjacent bones is reduced to a narrow layer, these fibrous
joints are now called sutures. At some sutures, the connective tissue will
ossify and be converted into bone, causing the adjacent bones to fuse to
each other. This fusion between bones is called a synostosis (“joined by
bone”). Examples of synostosis fusions between cranial bones are found
both early and late in life. At the time of birth, the frontal and maxillary
bones consist of right and left halves joined together by sutures, which
disappear by the eighth year as the halves fuse together to form a single
bone. Late in life, the sagittal, coronal, and lambdoid sutures of the skull
will begin to ossify and fuse, causing the suture line to gradually disappear.
The Newborn Skull
Anterior fontanelle
Parietal bone Frontal bone
Ossification
center
Posterior
fontanelle Sphenoidal
fontanelle
Mastoid
fontanelle
Occipital bone Temporal bone (squamous portion)
Lateral view
The fontanelles of a newborn’s skull are
broad areas of fibrous connective tissue
that form fibrous joints between the bones
of the skull.
Syndesmosis
A syndesmosis (“fastened with a band”) is a type of fibrous joint in which
two parallel bones are united to each other by fibrous connective tissue. The
gap between the bones may be narrow, with the bones joined by ligaments,
or the gap may be wide and filled in by a broad sheet of connective tissue
called an interosseous membrane.
In the forearm, the wide gap between the shaft portions of the radius and
ulna bones are strongly united by an interosseous membrane (see [link]b).
Similarly, in the leg, the shafts of the tibia and fibula are also united by an
interosseous membrane. In addition, at the distal tibiofibular joint, the
articulating surfaces of the bones lack cartilage and the narrow gap between
the bones is anchored by fibrous connective tissue and ligaments on both
the anterior and posterior aspects of the joint. Together, the interosseous
membrane and these ligaments form the tibiofibular syndesmosis.
The syndesmoses found in the forearm and leg serve to unite parallel bones
and prevent their separation. However, a syndesmosis does not prevent all
movement between the bones, and thus this type of fibrous joint is
functionally classified as an amphiarthrosis. In the leg, the syndesmosis
between the tibia and fibula strongly unites the bones, allows for little
movement, and firmly locks the talus bone in place between the tibia and
fibula at the ankle joint. This provides strength and stability to the leg and
ankle, which are important during weight bearing. In the forearm, the
interosseous membrane is flexible enough to allow for rotation of the radius
bone during forearm movements. Thus in contrast to the stability provided
by the tibiofibular syndesmosis, the flexibility of the antebrachial
interosseous membrane allows for the much greater mobility of the forearm.
The interosseous membranes of the leg and forearm also provide areas for
muscle attachment. Damage to a syndesmotic joint, which usually results
from a fracture of the bone with an accompanying tear of the interosseous
membrane, will produce pain, loss of stability of the bones, and may
damage the muscles attached to the interosseous membrane. If the fracture
site is not properly immobilized with a cast or splint, contractile activity by
these muscles can cause improper alignment of the broken bones during
healing.
Gomphosis
A gomphosis (“fastened with bolts”) is the specialized fibrous joint that
anchors the root of a tooth into its bony socket within the maxillary bone
(upper jaw) or mandible bone (lower jaw) of the skull. A gomphosis is also
known as a peg-and-socket joint. Spanning between the bony walls of the
socket and the root of the tooth are numerous short bands of dense
connective tissue, each of which is called a periodontal ligament (see
[link]c). Due to the immobility of a gomphosis, this type of joint is
functionally classified as a synarthrosis.
Chapter Review
Fibrous joints are where adjacent bones are strongly united by fibrous
connective tissue. The gap filled by connective tissue may be narrow or
wide. The three types of fibrous joints are sutures, gomphoses, and
syndesmoses. A suture is the narrow fibrous joint that unites most bones of
the skull. At a gomphosis, the root of a tooth is anchored across a narrow
gap by periodontal ligaments to the walls of its socket in the bony jaw. A
syndesmosis is the type of fibrous joint found between parallel bones. The
gap between the bones may be wide and filled with a fibrous interosseous
membrane, or it may narrow with ligaments spanning between the bones.
Syndesmoses are found between the bones of the forearm (radius and ulna)
and the leg (tibia and fibula). Fibrous joints strongly unite adjacent bones
and thus serve to provide protection for internal organs, strength to body
regions, or weight-bearing stability.
Review Questions
Exercise:
Problem: Which type of fibrous joint connects the tibia and fibula?
a. syndesmosis
b. symphysis
c. suture
d. gomphosis
Solution:
A
Exercise:
Problem: An example of a wide fibrous joint is
a. the interosseous membrane of the forearm
b. a gomphosis
c. a suture joint
d. a synostosis
Solution:
A
Exercise:
Problem:A gomphosis
a. is formed by an interosseous membrane
b. connects the tibia and fibula bones of the leg
c. contains a joint cavity
d. anchors a tooth to the jaw
Solution:
D
Exercise:
Problem: A syndesmosis is
a. a Narrow fibrous joint
b. the type of joint that unites bones of the skull
c. a fibrous joint that unites parallel bones
d. the type of joint that anchors the teeth in the jaws
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Distinguish between a narrow and wide fibrous joint and give an
example of each.
Solution:
Narrow fibrous joints are found at a suture, gomphosis, or
syndesmosis. A suture is the fibrous joint that joins the bones of the
skull to each other (except the mandible). A gomphosis is the fibrous
joint that anchors each tooth to its bony socket within the upper or
lower jaw. The tooth is connected to the bony jaw by periodontal
ligaments. A narrow syndesmosis is found at the distal tibiofibular
joint where the bones are united by fibrous connective tissue and
ligaments. A syndesmosis can also form a wide fibrous joint where the
shafts of two parallel bones are connected by a broad interosseous
membrane. The radius and ulna bones of the forearm and the tibia and
fibula bones of the leg are united by interosseous membranes.
Exercise:
Problem:
The periodontal ligaments are made of collagen fibers and are
responsible for connecting the roots of the teeth to the jaws. Describe
how scurvy, a disease that inhibits collagen production, can affect the
teeth.
Solution:
The teeth are anchored into their sockets within the bony jaws by the
periodontal ligaments. This is a gomphosis type of fibrous joint. In
scurvy, collagen production is inhibited and the periodontal ligaments
become weak. This will cause the teeth to become loose or even to fall
out.
Glossary
fontanelles
expanded areas of fibrous connective tissue that separate the braincase
bones of the skull prior to birth and during the first year after birth
gomphosis
type of fibrous joint in which the root of a tooth is anchored into its
bony jaw socket by strong periodontal ligaments
interosseous membrane
wide sheet of fibrous connective tissue that fills the gap between two
parallel bones, forming a syndesmosis; found between the radius and
ulna of the forearm and between the tibia and fibula of the leg
ligament
strong band of dense connective tissue spanning between bones
periodontal ligament
band of dense connective tissue that anchors the root of a tooth into the
bony jaw socket
suture
fibrous joint that connects the bones of the skull (except the mandible);
an immobile joint (synarthrosis)
syndesmosis
type of fibrous joint in which two separated, parallel bones are
connected by an interosseous membrane
synostosis
site at which adjacent bones or bony components have fused together
Cartilaginous Joints
By the end of this section, you will be able to:
e Describe the structural features of cartilaginous joints
e Distinguish between a synchondrosis and symphysis
e Give an example of each type of cartilaginous joint
As the name indicates, at a cartilaginous joint, the adjacent bones are united
by cartilage, a tough but flexible type of connective tissue. These types of
joints lack a joint cavity and involve bones that are joined together by either
hyaline cartilage or fibrocartilage ({link]). There are two types of
cartilaginous joints. A synchondrosis is a cartilaginous joint where the
bones are joined by hyaline cartilage. Also classified as a synchondrosis are
places where bone is united to a cartilage structure, such as between the
anterior end of a rib and the costal cartilage of the thoracic cage. The
second type of cartilaginous joint is a symphysis, where the bones are
joined by fibrocartilage.
Cartiliginous Joints
A
(temporary
hyaline
cartilage joint)
At cartilaginous joints, bones are united by hyaline
cartilage to form a synchondrosis or by fibrocartilage to
form a symphysis. (a) The hyaline cartilage of the
epiphyseal plate (growth plate) forms a synchondrosis that
unites the shaft (diaphysis) and end (epiphysis) of a long
bone and allows the bone to grow in length. (b) The pubic
portions of the right and left hip bones of the pelvis are
joined together by fibrocartilage, forming the pubic
symphysis.
Synchondrosis
A synchondrosis (“joined by cartilage’) is a cartilaginous joint where
bones are joined together by hyaline cartilage, or where bone is united to
hyaline cartilage. A synchondrosis may be temporary or permanent. A
temporary synchondrosis is the epiphyseal plate (growth plate) of a growing
long bone. The epiphyseal plate is the region of growing hyaline cartilage
that unites the diaphysis (shaft) of the bone to the epiphysis (end of the
bone). Bone lengthening involves growth of the epiphyseal plate cartilage
and its replacement by bone, which adds to the diaphysis. For many years
during childhood growth, the rates of cartilage growth and bone formation
are equal and thus the epiphyseal plate does not change in overall thickness
as the bone lengthens. During the late teens and early 20s, growth of the
cartilage slows and eventually stops. The epiphyseal plate is then
completely replaced by bone, and the diaphysis and epiphysis portions of
the bone fuse together to form a single adult bone. This fusion of the
diaphysis and epiphysis is a synostosis. Once this occurs, bone lengthening
ceases. For this reason, the epiphyseal plate is considered to be a temporary
synchondrosis. Because cartilage is softer than bone tissue, injury to a
growing long bone can damage the epiphyseal plate cartilage, thus stopping
bone growth and preventing additional bone lengthening.
Growing layers of cartilage also form synchondroses that join together the
ilium, ischium, and pubic portions of the hip bone during childhood and
adolescence. When body growth stops, the cartilage disappears and is
replaced by bone, forming synostoses and fusing the bony components
together into the single hip bone of the adult. Similarly, synostoses unite the
sacral vertebrae that fuse together to form the adult sacrum.
Note:
r
Lares
7 .
openstax COLLEGE
.
Visit this website to view a radiograph (X-ray image) of a child’s hand and
wrist. The growing bones of child have an epiphyseal plate that forms a
synchondrosis between the shaft and end of a long bone. Being less dense
than bone, the area of epiphyseal cartilage is seen on this radiograph as the
dark epiphyseal gaps located near the ends of the long bones, including the
radius, ulna, metacarpal, and phalanx bones. Which of the bones in this
image do not show an epiphyseal plate (epiphyseal gap)?
Examples of permanent synchondroses are found in the thoracic cage. One
example is the first sternocostal joint, where the first rib is anchored to the
manubrium by its costal cartilage. (The articulations of the remaining costal
cartilages to the sternum are all synovial joints.) Additional synchondroses
are formed where the anterior end of the other 11 ribs is joined to its costal
cartilage. Unlike the temporary synchondroses of the epiphyseal plate, these
permanent synchondroses retain their hyaline cartilage and thus do not
ossify with age. Due to the lack of movement between the bone and
cartilage, both temporary and permanent synchondroses are functionally
classified as a synarthrosis.
Symphysis
A cartilaginous joint where the bones are joined by fibrocartilage is called a
symphysis (“growing together”). Fibrocartilage is very strong because it
contains numerous bundles of thick collagen fibers, thus giving it a much
greater ability to resist pulling and bending forces when compared with
hyaline cartilage. This gives symphyses the ability to strongly unite the
adjacent bones, but can still allow for limited movement to occur. Thus, a
symphysis is functionally classified as an amphiarthrosis.
The gap separating the bones at a symphysis may be narrow or wide.
Examples in which the gap between the bones is narrow include the pubic
symphysis and the manubriosternal joint. At the pubic symphysis, the pubic
portions of the right and left hip bones of the pelvis are joined together by
fibrocartilage across a narrow gap. Similarly, at the manubriosternal joint,
fibrocartilage unites the manubrium and body portions of the sternum.
The intervertebral symphysis is a wide symphysis located between the
bodies of adjacent vertebrae of the vertebral column. Here a thick pad of
fibrocartilage called an intervertebral disc strongly unites the adjacent
vertebrae by filling the gap between them. The width of the intervertebral
symphysis is important because it allows for small movements between the
adjacent vertebrae. In addition, the thick intervertebral disc provides
cushioning between the vertebrae, which is important when carrying heavy
objects or during high-impact activities such as running or jumping.
Chapter Review
There are two types of cartilaginous joints. A synchondrosis is formed
when the adjacent bones are united by hyaline cartilage. A temporary
synchondrosis is formed by the epiphyseal plate of a growing long bone,
which is lost when the epiphyseal plate ossifies as the bone reaches
maturity. The synchondrosis is thus replaced by a synostosis. Permanent
synchondroses that do not ossify are found at the first sternocostal joint and
between the anterior ends of the bony ribs and the junction with their costal
cartilage. A symphysis is where the bones are joined by fibrocartilage and
the gap between the bones may be narrow or wide. A narrow symphysis is
found at the manubriosternal joint and at the pubic symphysis. A wide
symphysis is the intervertebral symphysis in which the bodies of adjacent
vertebrae are united by an intervertebral disc.
Interactive Link Questions
Exercise:
Problem:
Go to this website to view a radiograph (X-ray image) of a child’s
hand and wrist. The growing bones of child have an epiphyseal plate
that forms a synchondrosis between the shaft and end of a long bone.
Being less dense than bone, the area of epiphyseal cartilage is seen on
this radiograph as the dark epiphyseal gaps located near the ends of the
long bones, including the radius, ulna, metacarpal, and phalanx bones.
Which of the bones in this image do not show an epiphyseal plate
(epiphyseal gap)?
Solution:
Although they are still growing, the carpal bones of the wrist area do
not show an epiphyseal plate. Instead of elongating, these bones grow
in diameter by adding new bone to their surfaces.
Review Questions
Exercise:
Problem:A cartilaginous joint
a. has a joint cavity
b. is called a symphysis when the bones are united by fibrocartilage
c. anchors the teeth to the jaws
d. is formed by a wide sheet of fibrous connective tissue
Solution:
B
Exercise:
Problem: A synchondrosis is
a. found at the pubic symphysis
b. where bones are connected together with fibrocartilage
c. a type of fibrous joint
d. found at the first sternocostal joint of the thoracic cage
Solution:
D
Exercise:
Problem: Which of the following are joined by a symphysis?
a. adjacent vertebrae
b. the first rib and the sternum
c. the end and shaft of a long bone
d. the radius and ulna bones
Solution:
A
Exercise:
Problem:
The epiphyseal plate of a growing long bone in a child is classified as
a
a. synchondrosis
b. synostosis
c. symphysis
d. syndesmosis
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Describe the two types of cartilaginous joints and give examples of
each.
Solution:
Cartilaginous joints are where the adjacent bones are joined by
cartilage. At a synchondrosis, the bones are united by hyaline cartilage.
The epiphyseal plate of growing long bones and the first sternocostal
joint that unites the first rib to the sternum are examples of
synchondroses. At a symphysis, the bones are joined by fibrocartilage,
which is strong and flexible. Symphysis joints include the
intervertebral symphysis between adjacent vertebrae and the pubic
symphysis that joins the pubic portions of the right and left hip bones.
Exercise:
Problem:
Both functional and structural classifications can be used to describe
an individual joint. Define the first sternocostal joint and the pubic
symphysis using both functional and structural characteristics.
Solution:
The first sternocostal joint is a synchondrosis type of cartilaginous
joint in which hyaline cartilage unites the first rib to the manubrium of
the sternum. This forms an immobile (synarthrosis) type of joint. The
pubic symphysis is a slightly mobile (amphiarthrosis) cartilaginous
joint, where the pubic portions of the right and left hip bones are
united by fibrocartilage, thus forming a symphysis.
Glossary
symphysis
type of cartilaginous joint where the bones are joined by fibrocartilage
synchondrosis
type of cartilaginous joint where the bones are joined by hyaline
cartilage
Synovial Joints
By the end of this section, you will be able to:
¢ Describe the structural features of a synovial joint
e Discuss the function of additional structures associated with synovial
joints
e List the six types of synovial joints and give an example of each
Synovial joints are the most common type of joint in the body ((link]). A
key structural characteristic for a synovial joint that is not seen at fibrous or
cartilaginous joints is the presence of a joint cavity. This fluid-filled space is
the site at which the articulating surfaces of the bones contact each other.
Also unlike fibrous or cartilaginous joints, the articulating bone surfaces at
a synovial joint are not directly connected to each other with fibrous
connective tissue or cartilage. This gives the bones of a synovial joint the
ability to move smoothly against each other, allowing for increased joint
mobility.
Synovial Joints
Bone
Synovial
membrane
Articular
capsule
Articular
cartilage
Joint cavity
containing
Bone synovial fluid
Synovial joints allow for smooth
movements between the adjacent bones.
The joint is surrounded by an articular
capsule that defines a joint cavity filled
with synovial fluid. The articulating
surfaces of the bones are covered by a
thin layer of articular cartilage. Ligaments
support the joint by holding the bones
together and resisting excess or abnormal
joint motions.
Structural Features of Synovial Joints
Synovial joints are characterized by the presence of a joint cavity. The walls
of this space are formed by the articular capsule, a fibrous connective
tissue structure that is attached to each bone just outside the area of the
bone’s articulating surface. The bones of the joint articulate with each other
within the joint cavity.
Friction between the bones at a synovial joint is prevented by the presence
of the articular cartilage, a thin layer of hyaline cartilage that covers the
entire articulating surface of each bone. However, unlike at a cartilaginous
joint, the articular cartilages of each bone are not continuous with each
other. Instead, the articular cartilage acts like a Teflon® coating over the
bone surface, allowing the articulating bones to move smoothly against
each other without damaging the underlying bone tissue. Lining the inner
surface of the articular capsule is a thin synovial membrane. The cells of
this membrane secrete synovial fluid (synovia = “a thick fluid”), a thick,
slimy fluid that provides lubrication to further reduce friction between the
bones of the joint. This fluid also provides nourishment to the articular
cartilage, which does not contain blood vessels. The ability of the bones to
move smoothly against each other within the joint cavity, and the freedom
of joint movement this provides, means that each synovial joint is
functionally classified as a diarthrosis.
Outside of their articulating surfaces, the bones are connected together by
ligaments, which are strong bands of fibrous connective tissue. These
strengthen and support the joint by anchoring the bones together and
preventing their separation. Ligaments allow for normal movements at a
joint, but limit the range of these motions, thus preventing excessive or
abnormal joint movements. Ligaments are classified based on their
relationship to the fibrous articular capsule. An extrinsic ligament is
located outside of the articular capsule, an intrinsic ligament is fused to or
incorporated into the wall of the articular capsule, and an intracapsular
ligament is located inside of the articular capsule.
At many synovial joints, additional support is provided by the muscles and
their tendons that act across the joint. A tendon is the dense connective
tissue structure that attaches a muscle to bone. As forces acting on a joint
increase, the body will automatically increase the overall strength of
contraction of the muscles crossing that joint, thus allowing the muscle and
its tendon to serve as a “dynamic ligament” to resist forces and support the
joint. This type of indirect support by muscles is very important at the
shoulder joint, for example, where the ligaments are relatively weak.
Additional Structures Associated with Synovial Joints
A few synovial joints of the body have a fibrocartilage structure located
between the articulating bones. This is called an articular disc, which is
generally small and oval-shaped, or a meniscus, which is larger and C-
shaped. These structures can serve several functions, depending on the
specific joint. In some places, an articular disc may act to strongly unite the
bones of the joint to each other. Examples of this include the articular discs
found at the sternoclavicular joint or between the distal ends of the radius
and ulna bones. At other synovial joints, the disc can provide shock
absorption and cushioning between the bones, which is the function of each
meniscus within the knee joint. Finally, an articular disc can serve to
smooth the movements between the articulating bones, as seen at the
temporomandibular joint. Some synovial joints also have a fat pad, which
can serve as a cushion between the bones.
Additional structures located outside of a synovial joint serve to prevent
friction between the bones of the joint and the overlying muscle tendons or
skin. A bursa (plural = bursae) is a thin connective tissue sac filled with
lubricating liquid. They are located in regions where skin, ligaments,
muscles, or muscle tendons can rub against each other, usually near a body
joint ((link]). Bursae reduce friction by separating the adjacent structures,
preventing them from rubbing directly against each other. Bursae are
classified by their location. A subcutaneous bursa is located between the
skin and an underlying bone. It allows skin to move smoothly over the
bone. Examples include the prepatellar bursa located over the kneecap and
the olecranon bursa at the tip of the elbow. A submuscular bursa is found
between a muscle and an underlying bone, or between adjacent muscles.
These prevent rubbing of the muscle during movements. A large
submuscular bursa, the trochanteric bursa, is found at the lateral hip,
between the greater trochanter of the femur and the overlying gluteus
maximus muscle. A subtendinous bursa is found between a tendon and a
bone. Examples include the subacromial bursa that protects the tendon of
shoulder muscle as it passes under the acromion of the scapula, and the
suprapatellar bursa that separates the tendon of the large anterior thigh
muscle from the distal femur just above the knee.
Bursae
_\ |
Posterior
cruciate
ligament
Tendon of
quadriceps
femoris
Suprapatellar
bursa
Patella
Prepatellar
bursa
Anterior Synovial cavity
reas Infrapatellar
ligament
2 fat pad
Infrapatellar
Tibia bursa
Patellar
ligament
Bursae are fluid-filled sacs that serve to
prevent friction between skin, muscle, or
tendon and an underlying bone. Three
major bursae and a fat pad are part of the
complex joint that unites the femur and
tibia of the leg.
A tendon sheath is similar in structure to a bursa, but smaller. It is a
connective tissue sac that surrounds a muscle tendon at places where the
tendon crosses a joint. It contains a lubricating fluid that allows for smooth
motions of the tendon during muscle contraction and joint movements.
Note:
Homeostatic Imbalances
Bursitis
Bursitis is the inflammation of a bursa near a joint. This will cause pain,
swelling, or tenderness of the bursa and surrounding area, and may also
result in joint stiffness. Bursitis is most commonly associated with the
bursae found at or near the shoulder, hip, knee, or elbow joints. At the
shoulder, subacromial bursitis may occur in the bursa that separates the
acromion of the scapula from the tendon of a shoulder muscle as it passes
deep to the acromion. In the hip region, trochanteric bursitis can occur in
the bursa that overlies the greater trochanter of the femur, just below the
lateral side of the hip. Ischial bursitis occurs in the bursa that separates the
skin from the ischial tuberosity of the pelvis, the bony structure that is
weight bearing when sitting. At the knee, inflammation and swelling of the
bursa located between the skin and patella bone is prepatellar bursitis
(“housemaid’s knee”), a condition more commonly seen today in roofers or
floor and carpet installers who do not use knee pads. At the elbow,
olecranon bursitis is inflammation of the bursa between the skin and
olecranon process of the ulna. The olecranon forms the bony tip of the
elbow, and bursitis here is also known as “student’s elbow.”
Bursitis can be either acute (lasting only a few days) or chronic. It can arise
from muscle overuse, trauma, excessive or prolonged pressure on the skin,
rheumatoid arthritis, gout, or infection of the joint. Repeated acute
episodes of bursitis can result in a chronic condition. Treatments for the
disorder include antibiotics if the bursitis is caused by an infection, or anti-
inflammatory agents, such as nonsteroidal anti-inflammatory drugs
(NSAIDs) or corticosteroids if the bursitis is due to trauma or overuse.
Chronic bursitis may require that fluid be drained, but additional surgery is
usually not required.
Types of Synovial Joints
Synovial joints are subdivided based on the shapes of the articulating
surfaces of the bones that form each joint. The six types of synovial joints
are pivot, hinge, condyloid, saddle, plane, and ball-and socket-joints
({link]).
Types of Synovial Joints
(f) Ball-and-socket joint
(hip joint)
(a) Pivot joint
(between C1 and
C2 vertebrae)
=a
T
(b) Hinge joint
(elbow)
(e) Condyloid joint
(between radius and
carpal bones of wrist)
(d) Plane joint
(between tarsal bones)
(c) Saddle joint
(between trapezium
carpal bone and 1st
metacarpal bone)
The six types of synovial joints allow the body to move in a variety
of ways. (a) Pivot joints allow for rotation around an axis, such as
between the first and second cervical vertebrae, which allows for
side-to-side rotation of the head. (b) The hinge joint of the elbow
works like a door hinge. (c) The articulation between the trapezium
carpal bone and the first metacarpal bone at the base of the thumb is a
saddle joint. (d) Plane joints, such as those between the tarsal bones
of the foot, allow for limited gliding movements between bones. (e)
The radiocarpal joint of the wrist is a condyloid joint. (f) The hip and
shoulder joints are the only ball-and-socket joints of the body.
Pivot Joint
At a pivot joint, a rounded portion of a bone is enclosed within a ring
formed partially by the articulation with another bone and partially by a
ligament (see [link]a). The bone rotates within this ring. Since the rotation
is around a single axis, pivot joints are functionally classified as a uniaxial
diarthrosis type of joint. An example of a pivot joint is the atlantoaxial joint,
found between the C1 (atlas) and C2 (axis) vertebrae. Here, the upward
projecting dens of the axis articulates with the inner aspect of the atlas,
where it is held in place by a ligament. Rotation at this joint allows you to
turn your head from side to side. A second pivot joint is found at the
proximal radioulnar joint. Here, the head of the radius is largely encircled
by a ligament that holds it in place as it articulates with the radial notch of
the ulna. Rotation of the radius allows for forearm movements.
Hinge Joint
In a hinge joint, the convex end of one bone articulates with the concave
end of the adjoining bone (see [link]b). This type of joint allows only for
bending and straightening motions along a single axis, and thus hinge joints
are functionally classified as uniaxial joints. A good example is the elbow
joint, with the articulation between the trochlea of the humerus and the
trochlear notch of the ulna. Other hinge joints of the body include the knee,
ankle, and interphalangeal joints between the phalanx bones of the fingers
and toes.
Condyloid Joint
At a condyloid joint (ellipsoid joint), the shallow depression at the end of
one bone articulates with a rounded structure from an adjacent bone or
bones (see [link]e). The knuckle (metacarpophalangeal) joints of the hand
between the distal end of a metacarpal bone and the proximal phalanx bone
are condyloid joints. Another example is the radiocarpal joint of the wrist,
between the shallow depression at the distal end of the radius bone and the
rounded scaphoid, lunate, and triquetrum carpal bones. In this case, the
articulation area has a more oval (elliptical) shape. Functionally, condyloid
joints are biaxial joints that allow for two planes of movement. One
movement involves the bending and straightening of the fingers or the
anterior-posterior movements of the hand. The second movement is a side-
to-side movement, which allows you to spread your fingers apart and bring
them together, or to move your hand in a medial-going or lateral-going
direction.
Saddle Joint
At a saddle joint, both of the articulating surfaces for the bones have a
saddle shape, which is concave in one direction and convex in the other (see
[link]c). This allows the two bones to fit together like a rider sitting on a
saddle. Saddle joints are functionally classified as biaxial joints. The
primary example is the first carpometacarpal joint, between the trapezium
(a carpal bone) and the first metacarpal bone at the base of the thumb. This
joint provides the thumb the ability to move away from the palm of the
hand along two planes. Thus, the thumb can move within the same plane as
the palm of the hand, or it can jut out anteriorly, perpendicular to the palm.
This movement of the first carpometacarpal joint is what gives humans their
distinctive “opposable” thumbs. The sternoclavicular joint is also classified
as a Saddle joint.
Plane Joint
Ata plane joint (gliding joint), the articulating surfaces of the bones are
flat or slightly curved and of approximately the same size, which allows the
bones to slide against each other (see [link]d). The motion at this type of
joint is usually small and tightly constrained by surrounding ligaments.
Based only on their shape, plane joints can allow multiple movements,
including rotation. Thus plane joints can be functionally classified as a
multiaxial joint. However, not all of these movements are available to every
plane joint due to limitations placed on it by ligaments or neighboring
bones. Thus, depending upon the specific joint of the body, a plane joint
may exhibit only a single type of movement or several movements. Plane
joints are found between the carpal bones (intercarpal joints) of the wrist or
tarsal bones (intertarsal joints) of the foot, between the clavicle and
acromion of the scapula (acromioclavicular joint), and between the superior
and inferior articular processes of adjacent vertebrae (zygapophysial joints).
Ball-and-Socket Joint
The joint with the greatest range of motion is the ball-and-socket joint. At
these joints, the rounded head of one bone (the ball) fits into the concave
articulation (the socket) of the adjacent bone (see [link ]f). The hip joint and
the glenohumeral (shoulder) joint are the only ball-and-socket joints of the
body. At the hip joint, the head of the femur articulates with the acetabulum
of the hip bone, and at the shoulder joint, the head of the humerus
articulates with the glenoid cavity of the scapula.
Ball-and-socket joints are classified functionally as multiaxial joints. The
femur and the humerus are able to move in both anterior-posterior and
medial-lateral directions and they can also rotate around their long axis. The
shallow socket formed by the glenoid cavity allows the shoulder joint an
extensive range of motion. In contrast, the deep socket of the acetabulum
and the strong supporting ligaments of the hip joint serve to constrain
movements of the femur, reflecting the need for stability and weight-
bearing ability at the hip.
Note:
Watch this video to see an animation of synovial joints in action. Synovial
joints are places where bones articulate with each other inside of a joint
cavity. The different types of synovial joints are the ball-and-socket joint
(shoulder joint), hinge joint (knee), pivot joint (atlantoaxial joint, between
C1 and C2 vertebrae of the neck), condyloid joint (radiocarpal joint of the
wrist), saddle joint (first carpometacarpal joint, between the trapezium
carpal bone and the first metacarpal bone, at the base of the thumb), and
plane joint (facet joints of vertebral column, between superior and inferior
articular processes). Which type of synovial joint allows for the widest
range of motion?
Note:
Aging and the...
Joints
Arthritis is a common disorder of synovial joints that involves
inflammation of the joint. This often results in significant joint pain, along
with swelling, stiffness, and reduced joint mobility. There are more than
100 different forms of arthritis. Arthritis may arise from aging, damage to
the articular cartilage, autoimmune diseases, bacterial or viral infections, or
unknown (probably genetic) causes.
The most common type of arthritis is osteoarthritis, which is associated
with aging and “wear and tear” of the articular cartilage ((link]). Risk
factors that may lead to osteoarthritis later in life include injury to a joint;
jobs that involve physical labor; sports with running, twisting, or throwing
actions; and being overweight. These factors put stress on the articular
cartilage that covers the surfaces of bones at synovial joints, causing the
cartilage to gradually become thinner. As the articular cartilage layer wears
down, more pressure is placed on the bones. The joint responds by
increasing production of the lubricating synovial fluid, but this can lead to
swelling of the joint cavity, causing pain and joint stiffness as the articular
capsule is stretched. The bone tissue underlying the damaged articular
cartilage also responds by thickening, producing irregularities and causing
the articulating surface of the bone to become rough or bumpy. Joint
movement then results in pain and inflammation. In its early stages,
symptoms of osteoarthritis may be reduced by mild activity that “warms
up” the joint, but the symptoms may worsen following exercise. In
individuals with more advanced osteoarthritis, the affected joints can
become more painful and therefore are difficult to use effectively, resulting
in increased immobility. There is no cure for osteoarthritis, but several
treatments can help alleviate the pain. Treatments may include lifestyle
changes, such as weight loss and low-impact exercise, and over-the-
counter or prescription medications that help to alleviate the pain and
inflammation. For severe cases, joint replacement surgery (arthroplasty)
may be required.
Joint replacement is a very invasive procedure, so other treatments are
always tried before surgery. However arthroplasty can provide relief from
chronic pain and can enhance mobility within a few months following the
surgery. This type of surgery involves replacing the articular surfaces of the
bones with prosthesis (artificial components). For example, in hip
arthroplasty, the worn or damaged parts of the hip joint, including the head
and neck of the femur and the acetabulum of the pelvis, are removed and
replaced with artificial joint components. The replacement head for the
femur consists of a rounded ball attached to the end of a shaft that is
inserted inside the diaphysis of the femur. The acetabulum of the pelvis is
reshaped and a replacement socket is fitted into its place. The parts, which
are always built in advance of the surgery, are sometimes custom made to
produce the best possible fit for a patient.
Gout is a form of arthritis that results from the deposition of uric acid
crystals within a body joint. Usually only one or a few joints are affected,
such as the big toe, knee, or ankle. The attack may only last a few days, but
may return to the same or another joint. Gout occurs when the body makes
too much uric acid or the kidneys do not properly excrete it. A diet with
excessive fructose has been implicated in raising the chances of a
susceptible individual developing gout.
Other forms of arthritis are associated with various autoimmune diseases,
bacterial infections of the joint, or unknown genetic causes. Autoimmune
diseases, including rheumatoid arthritis, scleroderma, or systemic lupus
erythematosus, produce arthritis because the immune system of the body
attacks the body joints. In rheumatoid arthritis, the joint capsule and
synovial membrane become inflamed. As the disease progresses, the
articular cartilage is severely damaged or destroyed, resulting in joint
deformation, loss of movement, and severe disability. The most commonly
involved joints are the hands, feet, and cervical spine, with corresponding
joints on both sides of the body usually affected, though not always to the
same extent. Rheumatoid arthritis is also associated with lung fibrosis,
vasculitis (inflammation of blood vessels), coronary heart disease, and
premature mortality. With no known cure, treatments are aimed at
alleviating symptoms. Exercise, anti-inflammatory and pain medications,
various specific disease-modifying anti-rheumatic drugs, or surgery are
used to treat rheumatoid arthritis.
Osteoarthritis
WL
Decreased
Normal hip joint joint space
Exposed bone
Worn
cartilage
Osteoarthritis of a synovial joint results from
aging or prolonged joint wear and tear. These
cause erosion and loss of the articular cartilage
covering the surfaces of the bones, resulting in
inflammation that causes joint stiffness and
pain.
Visit this website to learn about a patient who arrives at the hospital with
joint pain and weakness in his legs. What caused this patient’s weakness?
Rtg
Watch this animation to observe hip replacement surgery (total hip
arthroplasty), which can be used to alleviate the pain and loss of joint
mobility associated with osteoarthritis of the hip joint. What is the most
common cause of hip disability?
Watch this video to learn about the symptoms and treatments for
rheumatoid arthritis. Which system of the body malfunctions in rheumatoid
arthritis and what does this cause?
Chapter Review
Synovial joints are the most common type of joints in the body. They are
characterized by the presence of a joint cavity, inside of which the bones of
the joint articulate with each other. The articulating surfaces of the bones at
a synovial joint are not directly connected to each other by connective
tissue or cartilage, which allows the bones to move freely against each
other. The walls of the joint cavity are formed by the articular capsule.
Friction between the bones is reduced by a thin layer of articular cartilage
covering the surfaces of the bones, and by a lubricating synovial fluid,
which is secreted by the synovial membrane.
Synovial joints are strengthened by the presence of ligaments, which hold
the bones together and resist excessive or abnormal movements of the joint.
Ligaments are classified as extrinsic ligaments if they are located outside of
the articular capsule, intrinsic ligaments if they are fused to the wall of the
articular capsule, or intracapsular ligaments if they are located inside the
articular capsule. Some synovial joints also have an articular disc
(meniscus), which can provide padding between the bones, smooth their
movements, or strongly join the bones together to strengthen the joint.
Muscles and their tendons acting across a joint can also increase their
contractile strength when needed, thus providing indirect support for the
joint.
Bursae contain a lubricating fluid that serves to reduce friction between
structures. Subcutaneous bursae prevent friction between the skin and an
underlying bone, submuscular bursae protect muscles from rubbing against
a bone or another muscle, and a subtendinous bursa prevents friction
between bone and a muscle tendon. Tendon sheaths contain a lubricating
fluid and surround tendons to allow for smooth movement of the tendon as
it crosses a joint.
Based on the shape of the articulating bone surfaces and the types of
movement allowed, synovial joints are classified into six types. At a pivot
joint, one bone is held within a ring by a ligament and its articulation with a
second bone. Pivot joints only allow for rotation around a single axis. These
are found at the articulation between the C1 (atlas) and the dens of the C2
(axis) vertebrae, which provides the side-to-side rotation of the head, or at
the proximal radioulnar joint between the head of the radius and the radial
notch of the ulna, which allows for rotation of the radius during forearm
movements. Hinge joints, such as at the elbow, knee, ankle, or
interphalangeal joints between phalanx bones of the fingers and toes, allow
only for bending and straightening of the joint. Pivot and hinge joints are
functionally classified as uniaxial joints.
Condyloid joints are found where the shallow depression of one bone
receives a rounded bony area formed by one or two bones. Condyloid joints
are found at the base of the fingers (metacarpophalangeal joints) and at the
wrist (radiocarpal joint). At a saddle joint, the articulating bones fit together
like a rider and a saddle. An example is the first carpometacarpal joint
located at the base of the thumb. Both condyloid and saddle joints are
functionally classified as biaxial joints.
Plane joints are formed between the small, flattened surfaces of adjacent
bones. These joints allow the bones to slide or rotate against each other, but
the range of motion is usually slight and tightly limited by ligaments or
surrounding bones. This type of joint is found between the articular
processes of adjacent vertebrae, at the acromioclavicular joint, or at the
intercarpal joints of the hand and intertarsal joints of the foot. Ball-and-
socket joints, in which the rounded head of a bone fits into a large
depression or socket, are found at the shoulder and hip joints. Both plane
and ball-and-sockets joints are classified functionally as multiaxial joints.
However, ball-and-socket joints allow for large movements, while the
motions between bones at a plane joint are small.
Interactive Link Questions
Exercise:
Problem:
Watch this video to see an animation of synovial joints in action.
Synovial joints are places where bones articulate with each other
inside of a joint cavity. The different types of synovial joints are the
ball-and-socket joint (shoulder joint), hinge joint (knee), pivot joint
(atlantoaxial joint, between C1 and C2 vertebrae of the neck),
condyloid joint (radiocarpal joint of the wrist), saddle joint (first
carpometacarpal joint, between the trapezium carpal bone and the first
metacarpal bone, at the base of the thumb), and plane joint (facet joints
of vertebral column, between superior and inferior articular processes).
Which type of synovial joint allows for the widest ranges of motion?
Solution:
Ball-and-socket joint.
Exercise:
Problem:
Visit this website to read about a patient who arrives at the hospital
with joint pain and weakness in his legs. What caused this patient’s
weakness?
Solution:
Gout is due to the accumulation of uric acid crystals in the body.
Usually these accumulate within joints, causing joint pain. This patient
also had crystals that accumulated in the space next to his spinal cord,
thus compressing the spinal cord and causing muscle weakness.
Exercise:
Problem:
Watch this animation to observe hip replacement surgery (total hip
arthroplasty), which can be used to alleviate the pain and loss of joint
mobility associated with osteoarthritis of the hip joint. What is the
most common cause of hip disability?
Solution:
The most common cause of hip disability is osteoarthritis, a chronic
disease in which the articular cartilage of the joint wears away,
resulting in severe hip pain and stiffness.
Exercise:
Problem:
Watch this video to learn about the symptoms and treatments for
rheumatoid arthritis. Which system of the body malfunctions in
rheumatoid arthritis and what does this cause?
Solution:
The immune system malfunctions and attacks healthy cells in the
lining of your joints. This causes inflammation and pain in the joints
and surrounding tissues.
Review Questions
Exercise:
Problem: Which type of joint provides the greatest range of motion?
a. ball-and-socket
b. hinge
c. condyloid
d. plane
Solution:
A
Exercise:
Problem: Which type of joint allows for only uniaxial movement?
a. saddle joint
b. hinge joint
c. condyloid joint
d. ball-and-socket joint
Solution:
B
Exercise:
Problem: Which of the following is a type of synovial joint?
a. a synostosis
b. a suture
c. a plane joint
d. a synchondrosis
Solution:
C
Exercise:
Problem:A bursa
a. surrounds a tendon at the point where the tendon crosses a joint
b. secretes the lubricating fluid for a synovial joint
c. prevents friction between skin and bone, or a muscle tendon and
bone
d. is the strong band of connective tissue that holds bones together at
a synovial joint
Solution:
@
Exercise:
Problem: At synovial joints,
a. the articulating ends of the bones are directly connected by
fibrous connective tissue
b. the ends of the bones are enclosed within a space called a
subcutaneous bursa
c. intrinsic ligaments are located entirely inside of the articular
capsule
d. the joint cavity is filled with a thick, lubricating fluid
Solution:
a)
Exercise:
Problem: At a synovial joint, the synovial membrane
a. forms the fibrous connective walls of the joint cavity
b. is the layer of cartilage that covers the articulating surfaces of the
bones
c. forms the intracapsular ligaments
d. secretes the lubricating synovial fluid
Solution:
D
Exercise:
Problem: Condyloid joints
a. are a type of ball-and-socket joint
b. include the radiocarpal joint
c. are a uniaxial diarthrosis joint
d. are found at the proximal radioulnar joint
Solution:
B
Exercise:
Problem:A meniscus is
a. a fibrocartilage pad that provides padding between bones
b. a fluid-filled space that prevents friction between a muscle tendon
and underlying bone
c. the articular cartilage that covers the ends of a bone at a synovial
joint
d. the lubricating fluid within a synovial joint
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Describe the characteristic structures found at all synovial joints.
Solution:
All synovial joints have a joint cavity filled with synovial fluid that is
the site at which the bones of the joint articulate with each other. The
articulating surfaces of the bones are covered by articular cartilage, a
thin layer of hyaline cartilage. The walls of the joint cavity are formed
by the connective tissue of the articular capsule. The synovial
membrane lines the interior surface of the joint cavity and secretes the
synovial fluid. Synovial joints are directly supported by ligaments,
which span between the bones of the joint. These may be located
outside of the articular capsule (extrinsic ligaments), incorporated or
fused to the wall of the articular capsule (intrinsic ligaments), or found
inside of the articular capsule (intracapsular ligaments). Ligaments
hold the bones together and also serve to resist or prevent excessive or
abnormal movements of the joint.
Exercise:
Problem:
Describe the structures that provide direct and indirect support for a
synovial joint.
Solution:
Direct support for a synovial joint is provided by ligaments that
strongly unite the bones of the joint and serve to resist excessive or
abnormal movements. Some joints, such as the sternoclavicular joint,
have an articular disc that is attached to both bones, where it provides
direct support by holding the bones together. Indirect joint support is
provided by the muscles and their tendons that act across a joint.
Muscles will increase their contractile force to help support the joint
by resisting forces acting on it.
Glossary
articular capsule
connective tissue structure that encloses the joint cavity of a synovial
joint
articular cartilage
thin layer of hyaline cartilage that covers the articulating surfaces of
bones at a synovial joint
articular disc
meniscus; a fibrocartilage structure found between the bones of some
synovial joints; provides padding or smooths movements between the
bones; strongly unites the bones together
ball-and-socket joint
synovial joint formed between the spherical end of one bone (the ball)
that fits into the depression of a second bone (the socket); found at the
hip and shoulder joints; functionally classified as a multiaxial joint
bursa
connective tissue sac containing lubricating fluid that prevents friction
between adjacent structures, such as skin and bone, tendons and bone,
or between muscles
condyloid joint
synovial joint in which the shallow depression at the end of one bone
receives a rounded end from a second bone or a rounded structure
formed by two bones; found at the metacarpophalangeal joints of the
fingers or the radiocarpal joint of the wrist; functionally classified as a
biaxial joint
extrinsic ligament
ligament located outside of the articular capsule of a synovial joint
hinge joint
synovial joint at which the convex surface of one bone articulates with
the concave surface of a second bone; includes the elbow, knee, ankle,
and interphalangeal joints; functionally classified as a uniaxial joint
intracapsular ligament
ligament that is located within the articular capsule of a synovial joint
intrinsic ligament
ligament that is fused to or incorporated into the wall of the articular
capsule of a synovial joint
meniscus
articular disc
pivot joint
synovial joint at which the rounded portion of a bone rotates within a
ring formed by a ligament and an articulating bone; functionally
classified as uniaxial joint
plane joint
synovial joint formed between the flattened articulating surfaces of
adjacent bones; functionally classified as a multiaxial joint
proximal radioulnar joint
articulation between head of radius and radial notch of ulna; uniaxial
pivot joint that allows for rotation of radius during
pronation/supination of forearm
saddle joint
synovial joint in which the articulating ends of both bones are convex
and concave in shape, such as at the first carpometacarpal joint at the
base of the thumb; functionally classified as a biaxial joint
subcutaneous bursa
bursa that prevents friction between skin and an underlying bone
submuscular bursa
bursa that prevents friction between bone and a muscle or between
adjacent muscles
subtendinous bursa
bursa that prevents friction between bone and a muscle tendon
synovial fluid
thick, lubricating fluid that fills the interior of a synovial joint
synovial membrane
thin layer that lines the inner surface of the joint cavity at a synovial
joint; produces the synovial fluid
tendon
dense connective tissue structure that anchors a muscle to bone
tendon sheath
connective tissue that surrounds a tendon at places where the tendon
crosses a joint; contains a lubricating fluid to prevent friction and
allow smooth movements of the tendon
Types of Body Movements
By the end of this section, you will be able to:
¢ Define the different types of body movements
¢ Identify the joints that allow for these motions
Synovial joints allow the body a tremendous range of movements. Each
movement at a synovial joint results from the contraction or relaxation of
the muscles that are attached to the bones on either side of the articulation.
The type of movement that can be produced at a synovial joint is
determined by its structural type. While the ball-and-socket joint gives the
greatest range of movement at an individual joint, in other regions of the
body, several joints may work together to produce a particular movement.
Overall, each type of synovial joint is necessary to provide the body with its
great flexibility and mobility. There are many types of movement that can
occur at synovial joints ({link]). Movement types are generally paired, with
one being the opposite of the other. Body movements are always described
in relation to the anatomical position of the body: upright stance, with upper
limbs to the side of body and palms facing forward. Refer to [link] as you
go through this section.
Note:
meee OPENStAX COLLEGE
—
Watch this video to learn about anatomical motions. What motions involve
increasing or decreasing the angle of the foot at the ankle?
Movements of the Body, Part 1
Extension
Flexion Extension y =
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f }
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——s#
Extension
(a) and (b) Angular movements: flexion and extension at the shoulder and knees (c) Angular movements: flexion and extension
of the neck
Extension
S y
WY 4) pf
AS
ie
\s
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iat | : yi
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f A
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Extension \ \ Flexion a4}
— \\ ly f/ Lateral
{ a, Un! | rotation
: Adduction
Medial
rotation
(d) Angular movements: flexion (e) Angular movements: abduction, adduction, (f) Rotation of the head, neck, and lower limb
and extension of the vertebral and circumduction of the upper limb at the
column shoulder
Synovial joints give the body many ways in which to move.
(a)—(b) Flexion and extension motions are in the sagittal
(anterior—posterior) plane of motion. These movements take
place at the shoulder, hip, elbow, knee, wrist,
metacarpophalangeal, metatarsophalangeal, and
interphalangeal joints. (c)-(d) Anterior bending of the head or
vertebral column is flexion, while any posterior-going
movement is extension. (e) Abduction and adduction are
motions of the limbs, hand, fingers, or toes in the coronal
(medial—lateral) plane of movement. Moving the limb or hand
laterally away from the body, or spreading the fingers or toes,
is abduction. Adduction brings the limb or hand toward or
across the midline of the body, or brings the fingers or toes
together. Circumduction is the movement of the limb, hand, or
fingers in a circular pattern, using the sequential combination
of flexion, adduction, extension, and abduction motions.
Adduction/abduction and circumduction take place at the
shoulder, hip, wrist, metacarpophalangeal, and
metatarsophalangeal joints. (f) Turning of the head side to side
or twisting of the body is rotation. Medial and lateral rotation
of the upper limb at the shoulder or lower limb at the hip
involves turning the anterior surface of the limb toward the
midline of the body (medial or internal rotation) or away from
the midline (lateral or external rotation).
Movements of the Body, Part 2
| \
| \ ee
Pronation | \ Supination
(Radius / | (radius and
rotates / ulna are
over ulna) parallel)
Dorsiflexion
\
= ani —\} Plantar flexion / ,
LU JM \ <a D>" Eversion
(g) Pronation (P) and supination (S) (h) Dorsiflexion and plantar flexion (i) Inversion and eversion
A
Retraction Protraction
Elevation of
of mandible of mandible
( | eae
fs : Depression
‘4 of mandible
(j) Protraction and retraction (k) Elevation and depression (I) Opposition
(g) Supination of the forearm turns the hand to the palm
forward position in which the radius and ulna are parallel,
while forearm pronation turns the hand to the palm
backward position in which the radius crosses over the ulna
to form an "X." (h) Dorsiflexion of the foot at the ankle
joint moves the top of the foot toward the leg, while plantar
flexion lifts the heel and points the toes. (i) Eversion of the
foot moves the bottom (sole) of the foot away from the
midline of the body, while foot inversion faces the sole
toward the midline. (j) Protraction of the mandible pushes
the chin forward, and retraction pulls the chin back. (k)
Depression of the mandible opens the mouth, while
elevation closes it. (1) Opposition of the thumb brings the
tip of the thumb into contact with the tip of the fingers of
the same hand and reposition brings the thumb back next to
the index finger.
Flexion and Extension
Flexion and extension are movements that take place within the sagittal
plane and involve anterior or posterior movements of the body or limbs. For
the vertebral column, flexion (anterior flexion) is an anterior (forward)
bending of the neck or body, while extension involves a posterior-directed
motion, such as straightening from a flexed position or bending backward.
Lateral flexion is the bending of the neck or body toward the right or left
side. These movements of the vertebral column involve both the symphysis
joint formed by each intervertebral disc, as well as the plane type of
synovial joint formed between the inferior articular processes of one
vertebra and the superior articular processes of the next lower vertebra.
In the limbs, flexion decreases the angle between the bones (bending of the
joint), while extension increases the angle and straightens the joint. For the
upper limb, all anterior-going motions are flexion and all posterior-going
motions are extension. These include anterior-posterior movements of the
arm at the shoulder, the forearm at the elbow, the hand at the wrist, and the
fingers at the metacarpophalangeal and interphalangeal joints. For the
thumb, extension moves the thumb away from the palm of the hand, within
the same plane as the palm, while flexion brings the thumb back against the
index finger or into the palm. These motions take place at the first
carpometacarpal joint. In the lower limb, bringing the thigh forward and
upward is flexion at the hip joint, while any posterior-going motion of the
thigh is extension. Note that extension of the thigh beyond the anatomical
(standing) position is greatly limited by the ligaments that support the hip
joint. Knee flexion is the bending of the knee to bring the foot toward the
posterior thigh, and extension is the straightening of the knee. Flexion and
extension movements are seen at the hinge, condyloid, saddle, and ball-and-
socket joints of the limbs (see [link ]a-d).
Hyperextension is the abnormal or excessive extension of a joint beyond
its normal range of motion, thus resulting in injury. Similarly, hyperflexion
is excessive flexion at a joint. Hyperextension injuries are common at hinge
joints such as the knee or elbow. In cases of “whiplash” in which the head is
suddenly moved backward and then forward, a patient may experience both
hyperextension and hyperflexion of the cervical region.
Abduction and Adduction
Abduction and adduction motions occur within the coronal plane and
involve medial-lateral motions of the limbs, fingers, toes, or thumb.
Abduction moves the limb laterally away from the midline of the body,
while adduction is the opposing movement that brings the limb toward the
body or across the midline. For example, abduction is raising the arm at the
shoulder joint, moving it laterally away from the body, while adduction
brings the arm down to the side of the body. Similarly, abduction and
adduction at the wrist moves the hand away from or toward the midline of
the body. Spreading the fingers or toes apart is also abduction, while
bringing the fingers or toes together is adduction. For the thumb, abduction
is the anterior movement that brings the thumb to a 90° perpendicular
position, pointing straight out from the palm. Adduction moves the thumb
back to the anatomical position, next to the index finger. Abduction and
adduction movements are seen at condyloid, saddle, and ball-and-socket
joints (see [link]e).
Circumduction
Circumduction is the movement of a body region in a circular manner, in
which one end of the body region being moved stays relatively stationary
while the other end describes a circle. It involves the sequential
combination of flexion, adduction, extension, and abduction at a joint. This
type of motion is found at biaxial condyloid and saddle joints, and at
multiaxial ball-and-sockets joints (see [link]e).
Rotation
Rotation can occur within the vertebral column, at a pivot joint, or at a
ball-and-socket joint. Rotation of the neck or body is the twisting
movement produced by the summation of the small rotational movements
available between adjacent vertebrae. At a pivot joint, one bone rotates in
relation to another bone. This is a uniaxial joint, and thus rotation is the
only motion allowed at a pivot joint. For example, at the atlantoaxial joint,
the first cervical (C1) vertebra (atlas) rotates around the dens, the upward
projection from the second cervical (C2) vertebra (axis). This allows the
head to rotate from side to side as when shaking the head “no.” The
proximal radioulnar joint is a pivot joint formed by the head of the radius
and its articulation with the ulna. This joint allows for the radius to rotate
along its length during pronation and supination movements of the forearm.
Rotation can also occur at the ball-and-socket joints of the shoulder and hip.
Here, the humerus and femur rotate around their long axis, which moves the
anterior surface of the arm or thigh either toward or away from the midline
of the body. Movement that brings the anterior surface of the limb toward
the midline of the body is called medial (internal) rotation. Conversely,
rotation of the limb so that the anterior surface moves away from the
midline is lateral (external) rotation (see [link |f). Be sure to distinguish
medial and lateral rotation, which can only occur at the multiaxial shoulder
and hip joints, from circumduction, which can occur at either biaxial or
multiaxial joints.
Supination and Pronation
Supination and pronation are movements of the forearm. In the anatomical
position, the upper limb is held next to the body with the palm facing
forward. This is the supinated position of the forearm. In this position, the
radius and ulna are parallel to each other. When the palm of the hand faces
backward, the forearm is in the pronated position, and the radius and ulna
form an X-shape.
Supination and pronation are the movements of the forearm that go between
these two positions. Pronation is the motion that moves the forearm from
the supinated (anatomical) position to the pronated (palm backward)
position. This motion is produced by rotation of the radius at the proximal
radioulnar joint, accompanied by movement of the radius at the distal
radioulnar joint. The proximal radioulnar joint is a pivot joint that allows
for rotation of the head of the radius. Because of the slight curvature of the
shaft of the radius, this rotation causes the distal end of the radius to cross
over the distal ulna at the distal radioulnar joint. This crossing over brings
the radius and ulna into an X-shape position. Supination is the opposite
motion, in which rotation of the radius returns the bones to their parallel
positions and moves the palm to the anterior facing (supinated) position. It
helps to remember that supination is the motion you use when scooping up
soup with a spoon (see [link]g).
Dorsiflexion and Plantar Flexion
Dorsiflexion and plantar flexion are movements at the ankle joint, which
is a hinge joint. Lifting the front of the foot, so that the top of the foot
moves toward the anterior leg is dorsiflexion, while lifting the heel of the
foot from the ground or pointing the toes downward is plantar flexion.
These are the only movements available at the ankle joint (see [link]h).
Inversion and Eversion
Inversion and eversion are complex movements that involve the multiple
plane joints among the tarsal bones of the posterior foot (intertarsal joints)
and thus are not motions that take place at the ankle joint. Inversion is the
turning of the foot to angle the bottom of the foot toward the midline, while
eversion turns the bottom of the foot away from the midline. The foot has a
greater range of inversion than eversion motion. These are important
motions that help to stabilize the foot when walking or running on an
uneven surface and aid in the quick side-to-side changes in direction used
during active sports such as basketball, racquetball, or soccer (see [link ]i).
Protraction and Retraction
Protraction and retraction are anterior-posterior movements of the scapula
or mandible. Protraction of the scapula occurs when the shoulder is moved
forward, as when pushing against something or throwing a ball. Retraction
is the opposite motion, with the scapula being pulled posteriorly and
medially, toward the vertebral column. For the mandible, protraction occurs
when the lower jaw is pushed forward, to stick out the chin, while retraction
pulls the lower jaw backward. (See [link]j.)
Depression and Elevation
Depression and elevation are downward and upward movements of the
scapula or mandible. The upward movement of the scapula and shoulder is
elevation, while a downward movement is depression. These movements
are used to shrug your shoulders. Similarly, elevation of the mandible is the
upward movement of the lower jaw used to close the mouth or bite on
something, and depression is the downward movement that produces
opening of the mouth (see [link ]k).
Excursion
Excursion is the side to side movement of the mandible. Lateral excursion
moves the mandible away from the midline, toward either the right or left
side. Medial excursion returns the mandible to its resting position at the
midline.
Superior Rotation and Inferior Rotation
Superior and inferior rotation are movements of the scapula and are defined
by the direction of movement of the glenoid cavity. These motions involve
rotation of the scapula around a point inferior to the scapular spine and are
produced by combinations of muscles acting on the scapula. During
superior rotation, the glenoid cavity moves upward as the medial end of
the scapular spine moves downward. This is a very important motion that
contributes to upper limb abduction. Without superior rotation of the
scapula, the greater tubercle of the humerus would hit the acromion of the
scapula, thus preventing any abduction of the arm above shoulder height.
Superior rotation of the scapula is thus required for full abduction of the
upper limb. Superior rotation is also used without arm abduction when
carrying a heavy load with your hand or on your shoulder. You can feel this
rotation when you pick up a load, such as a heavy book bag and carry it on
only one shoulder. To increase its weight-bearing support for the bag, the
shoulder lifts as the scapula superiorly rotates. Inferior rotation occurs
during limb adduction and involves the downward motion of the glenoid
cavity with upward movement of the medial end of the scapular spine.
Opposition and Reposition
Opposition is the thumb movement that brings the tip of the thumb in
contact with the tip of a finger. This movement is produced at the first
carpometacarpal joint, which is a saddle joint formed between the
trapezium carpal bone and the first metacarpal bone. Thumb opposition is
produced by a combination of flexion and abduction of the thumb at this
joint. Returning the thumb to its anatomical position next to the index finger
is called reposition (see [link]l).
Movements of the Joints
Type of
Joint Movement Example
Atlantoaxial joint (C1-
Pivot Uniaxial joint; allows C2 vertebrae
rotational movement articulation); proximal
radioulnar joint
Uniaxial joint; allows Knee; elbow; ankle;
Hinge flexion/extension interphalangeal joints
movements of fingers and toes
Movements of the Joints
Type of
Joint Movement
Biaxial joint; allows
flexion/extension,
Condyloid abduction/adduction, and
circumduction
movements
Biaxial joint; allows
flexion/extension,
Saddle abduction/adduction, and
circumduction
movements
Multiaxial joint; allows
inversion and eversion of
foot, or flexion,
extension, and lateral
flexion of the vertebral
column
Plane
Multiaxial joint; allows
flexion/extension,
Ball-and- abduction/adduction,
socket circumduction, and
medial/lateral rotation
movements
Chapter Review
The variety of movements provided by the different types of synovial joints
Example
Metacarpophalangeal
(knuckle) joints of
fingers; radiocarpal
joint of wrist;
metatarsophalangeal
joints for toes
First carpometacarpal
joint of the thumb;
stemoclavicular joint
Intertarsal joints of
foot; superior-inferior
articular process
articulations between
vertebrae
Shoulder and hip joints
allows for a large range of body motions and gives you tremendous
mobility. These movements allow you to flex or extend your body or limbs,
medially rotate and adduct your arms and flex your elbows to hold a heavy
object against your chest, raise your arms above your head, rotate or shake
your head, and bend to touch the toes (with or without bending your knees).
Each of the different structural types of synovial joints also allow for
specific motions. The atlantoaxial pivot joint provides side-to-side rotation
of the head, while the proximal radioulnar articulation allows for rotation of
the radius during pronation and supination of the forearm. Hinge joints,
such as at the knee and elbow, allow only for flexion and extension.
Similarly, the hinge joint of the ankle only allows for dorsiflexion and
plantar flexion of the foot.
Condyloid and saddle joints are biaxial. These allow for flexion and
extension, and abduction and adduction. The sequential combination of
flexion, adduction, extension, and abduction produces circumduction.
Multiaxial plane joints provide for only small motions, but these can add
together over several adjacent joints to produce body movement, such as
inversion and eversion of the foot. Similarly, plane joints allow for flexion,
extension, and lateral flexion movements of the vertebral column. The
multiaxial ball and socket joints allow for flexion-extension, abduction-
adduction, and circumduction. In addition, these also allow for medial
(internal) and lateral (external) rotation. Ball-and-socket joints have the
greatest range of motion of all synovial joints.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn about anatomical motions. What motions
involve increasing or decreasing the angle of the foot at the ankle?
Solution:
Dorsiflexion of the foot at the ankle decreases the angle of the ankle
joint, while plantar flexion increases the angle of the ankle joint.
Chapter Review
Exercise:
Problem:
The joints between the articular processes of adjacent vertebrae can
contribute to which movement?
a. lateral flexion
b. circumduction
c. dorsiflexion
d. abduction
Solution:
A
Exercise:
Problem:
Which motion moves the bottom of the foot away from the midline of
the body?
a. elevation
b. dorsiflexion
c. eversion
d. plantar flexion
Solution:
GC
Exercise:
Problem:
Movement of a body region in a circular movement at a condyloid
joint is what type of motion?
a. rotation
b. elevation
c. abduction
d. circumduction
Solution:
D
Exercise:
Problem: Supination is the motion that moves the
a. hand from the palm backward position to the palm forward
position
b. foot so that the bottom of the foot faces the midline of the body
c. hand from the palm forward position to the palm backward
position
d. scapula in an upward direction
Solution:
A
Exercise:
Problem:
Movement at the shoulder joint that moves the upper limb laterally
away from the body is called
a. elevation
b. eversion
c. abduction
d. lateral rotation
Solution:
‘Ss
Critical Thinking Questions
Exercise:
Problem:
Briefly define the types of joint movements available at a ball-and-
socket joint.
Solution:
Ball-and-socket joints are multiaxial joints that allow for flexion and
extension, abduction and adduction, circumduction, and medial and
lateral rotation.
Exercise:
Problem:
Discuss the joints involved and movements required for you to cross
your arms together in front of your chest.
Solution:
To cross your arms, you need to use both your shoulder and elbow
joints. At the shoulder, the arm would need to flex and medially rotate.
At the elbow, the forearm would need to be flexed.
Glossary
abduction
movement in the coronal plane that moves a limb laterally away from
the body; spreading of the fingers
adduction
movement in the coronal plane that moves a limb medially toward or
across the midline of the body; bringing fingers together
circumduction
circular motion of the arm, thigh, hand, thumb, or finger that is
produced by the sequential combination of flexion, abduction,
extension, and adduction
depression
downward (inferior) motion of the scapula or mandible
dorsiflexion
movement at the ankle that brings the top of the foot toward the
anterior leg
elevation
upward (superior) motion of the scapula or mandible
eversion
foot movement involving the intertarsal joints of the foot in which the
bottom of the foot is turned laterally, away from the midline
extension
movement in the sagittal plane that increases the angle of a joint
(straightens the joint); motion involving posterior bending of the
vertebral column or returning to the upright position from a flexed
position
flexion
movement in the sagittal plane that decreases the angle of a joint
(bends the joint); motion involving anterior bending of the vertebral
column
hyperextension
excessive extension of joint, beyond the normal range of movement
hyperflexion
excessive flexion of joint, beyond the normal range of movement
inferior rotation
movement of the scapula during upper limb adduction in which the
glenoid cavity of the scapula moves in a downward direction as the
medial end of the scapular spine moves in an upward direction
inversion
foot movement involving the intertarsal joints of the foot in which the
bottom of the foot is turned toward the midline
lateral excursion
side-to-side movement of the mandible away from the midline, toward
either the right or left side
lateral flexion
bending of the neck or body toward the right or left side
lateral (external) rotation
movement of the arm at the shoulder joint or the thigh at the hip joint
that moves the anterior surface of the limb away from the midline of
the body
medial excursion
side-to-side movement that returns the mandible to the midline
medial (internal) rotation
movement of the arm at the shoulder joint or the thigh at the hip joint
that brings the anterior surface of the limb toward the midline of the
body
opposition
thumb movement that brings the tip of the thumb in contact with the
tip of a finger
plantar flexion
foot movement at the ankle in which the heel is lifted off of the ground
pronated position
forearm position in which the palm faces backward
pronation
forearm motion that moves the palm of the hand from the palm
forward to the palm backward position
protraction
anterior motion of the scapula or mandible
reposition
movement of the thumb from opposition back to the anatomical
position (next to index finger)
retraction
posterior motion of the scapula or mandible
rotation
movement of a bone around a central axis (atlantoaxial joint) or around
its long axis (proximal radioulnar joint; shoulder or hip joint); twisting
of the vertebral column resulting from the summation of small motions
between adjacent vertebrae
superior rotation
movement of the scapula during upper limb abduction in which the
glenoid cavity of the scapula moves in an upward direction as the
medial end of the scapular spine moves in a downward direction
supinated position
forearm position in which the palm faces anteriorly (anatomical
position)
supination
forearm motion that moves the palm of the hand from the palm
backward to the palm forward position
Anatomy of Selected Synovial Joints
By the end of this section, you will be able to:
e Describe the bones that articulate together to form selected synovial
joints
e Discuss the movements available at each joint
e Describe the structures that support and prevent excess movements at
each joint
Each synovial joint of the body is specialized to perform certain
movements. The movements that are allowed are determined by the
structural classification for each joint. For example, a multiaxial ball-and-
socket joint has much more mobility than a uniaxial hinge joint. However,
the ligaments and muscles that support a joint may place restrictions on the
total range of motion available. Thus, the ball-and-socket joint of the
shoulder has little in the way of ligament support, which gives the shoulder
a very large range of motion. In contrast, movements at the hip joint are
restricted by strong ligaments, which reduce its range of motion but confer
stability during standing and weight bearing.
This section will examine the anatomy of selected synovial joints of the
body. Anatomical names for most joints are derived from the names of the
bones that articulate at that joint, although some joints, such as the elbow,
hip, and knee joints are exceptions to this general naming scheme.
Articulations of the Vertebral Column
In addition to being held together by the intervertebral discs, adjacent
vertebrae also articulate with each other at synovial joints formed between
the superior and inferior articular processes called zygapophysial joints
(facet joints) (see [link]). These are plane joints that provide for only
limited motions between the vertebrae. The orientation of the articular
processes at these joints varies in different regions of the vertebral column
and serves to determine the types of motions available in each vertebral
region. The cervical and lumbar regions have the greatest ranges of
motions.
In the neck, the articular processes of cervical vertebrae are flattened and
generally face upward or downward. This orientation provides the cervical
vertebral column with extensive ranges of motion for flexion, extension,
lateral flexion, and rotation. In the thoracic region, the downward projecting
and overlapping spinous processes, along with the attached thoracic cage,
greatly limit flexion, extension, and lateral flexion. However, the flattened
and vertically positioned thoracic articular processes allow for the greatest
range of rotation within the vertebral column. The lumbar region allows for
considerable extension, flexion, and lateral flexion, but the orientation of
the articular processes largely prohibits rotation.
The articulations formed between the skull, the atlas (C1 vertebra), and the
axis (C2 vertebra) differ from the articulations in other vertebral areas and
play important roles in movement of the head. The atlanto-occipital joint
is formed by the articulations between the superior articular processes of the
atlas and the occipital condyles on the base of the skull. This articulation
has a pronounced U-shaped curvature, oriented along the anterior-posterior
axis. This allows the skull to rock forward and backward, producing flexion
and extension of the head. This moves the head up and down, as when
shaking your head “yes.”
The atlantoaxial joint, between the atlas and axis, consists of three
articulations. The paired superior articular processes of the axis articulate
with the inferior articular processes of the atlas. These articulating surfaces
are relatively flat and oriented horizontally. The third articulation is the
pivot joint formed between the dens, which projects upward from the body
of the axis, and the inner aspect of the anterior arch of the atlas ({link]). A
strong ligament passes posterior to the dens to hold it in position against the
anterior arch. These articulations allow the atlas to rotate on top of the axis,
moving the head toward the right or left, as when shaking your head “no.”
Atlantoaxial Joint
Dens of
C2 (axis)
Anterior arch
Superior articular of C1 (atlas)
facet
Ligament
Superior view of atlas
The atlantoaxial joint is a pivot type
of joint between the dens portion of
the axis (C2 vertebra) and the anterior
arch of the atlas (C1 vertebra), with
the dens held in place by a ligament.
Temporomandibular Joint
The temporomandibular joint (TMJ) is the joint that allows for opening
(mandibular depression) and closing (mandibular elevation) of the mouth,
as well as side-to-side and protraction/retraction motions of the lower jaw.
This joint involves the articulation between the mandibular fossa and
articular tubercle of the temporal bone, with the condyle (head) of the
mandible. Located between these bony structures, filling the gap between
the skull and mandible, is a flexible articular disc ([link]). This disc serves
to smooth the movements between the temporal bone and mandibular
condyle.
Movement at the TMJ during opening and closing of the mouth involves
both gliding and hinge motions of the mandible. With the mouth closed, the
mandibular condyle and articular disc are located within the mandibular
fossa of the temporal bone. During opening of the mouth, the mandible
hinges downward and at the same time is pulled anteriorly, causing both the
condyle and the articular disc to glide forward from the mandibular fossa
onto the downward projecting articular tubercle. The net result is a forward
and downward motion of the condyle and mandibular depression. The
temporomandibular joint is supported by an extrinsic ligament that anchors
the mandible to the skull. This ligament spans the distance between the base
of the skull and the lingula on the medial side of the mandibular ramus.
Dislocation of the TMJ may occur when opening the mouth too wide (such
as when taking a large bite) or following a blow to the jaw, resulting in the
mandibular condyle moving beyond (anterior to) the articular tubercle. In
this case, the individual would not be able to close his or her mouth.
Temporomandibular joint disorder is a painful condition that may arise due
to arthritis, wearing of the articular cartilage covering the bony surfaces of
the joint, muscle fatigue from overuse or grinding of the teeth, damage to
the articular disc within the joint, or jaw injury. Temporomandibular joint
disorders can also cause headache, difficulty chewing, or even the inability
to move the jaw (lock jaw). Pharmacologic agents for pain or other
therapies, including bite guards, are used as treatments.
Temporomandibular Joint
Articular disc
Mandibular
fossa
uperior SOT aay rticular
joint cay tubercle
a
Interior ——_$ ——————
joint cavity ; Articular
capsule
Mandibular
condyle
Ramus of
mandible
The temporomandibular joint is the
articulation between the temporal bone of
the skull and the condyle of the mandible,
with an articular disc located between
these bones. During depression of the
mandible (opening of the mouth), the
mandibular condyle moves both forward
and hinges downward as it travels from
the mandibular fossa onto the articular
tubercle.
a
gC
Watch this video to learn about TMJ. Opening of the mouth requires the
combination of two motions at the temporomandibular joint, an anterior
gliding motion of the articular disc and mandible and the downward
hinging of the mandible. What is the initial movement of the mandible
during opening and how much mouth opening does this produce?
Shoulder Joint
The shoulder joint is called the glenohumeral joint. This is a ball-and-
socket joint formed by the articulation between the head of the humerus and
the glenoid cavity of the scapula ([link]). This joint has the largest range of
motion of any joint in the body. However, this freedom of movement is due
to the lack of structural support and thus the enhanced mobility is offset by
a loss of stability.
Glenohumeral Joint
Clavicle Acromioclavicular ligament
Tendon of
supraspinatus
muscle ——
Glenoid labrum
Acromion of scapula
= Coracoacromial
FBT ligament
; eel Subacromial bursa
Glenoid cavity SSRs
Scapula ———__ “s= 5
~~
Articular capsule
Tendon sheath
Tendon of biceps
brachii muscles
Articular cartilage Q\ .
7 (long heed)
Articular capsule:
Synovial membrane
Fibrous membrane
Head of humerus
ml Humerus
The glenohumeral (shoulder) joint is a ball-and-
socket joint that provides the widest range of
motions. It has a loose articular capsule and is
supported by ligaments and the rotator cuff
muscles.
The large range of motions at the shoulder joint is provided by the
articulation of the large, rounded humeral head with the small and shallow
glenoid cavity, which is only about one third of the size of the humeral
head. The socket formed by the glenoid cavity is deepened slightly by a
small lip of fibrocartilage called the glenoid labrum, which extends around
the outer margin of the cavity. The articular capsule that surrounds the
glenohumeral joint is relatively thin and loose to allow for large motions of
the upper limb. Some structural support for the joint is provided by
thickenings of the articular capsule wall that form weak intrinsic ligaments.
These include the coracohumeral ligament, running from the coracoid
process of the scapula to the anterior humerus, and three ligaments, each
called a glenohumeral ligament, located on the anterior side of the
articular capsule. These ligaments help to strengthen the superior and
anterior capsule walls.
However, the primary support for the shoulder joint is provided by muscles
crossing the joint, particularly the four rotator cuff muscles. These muscles
(supraspinatus, infraspinatus, teres minor, and subscapularis) arise from the
scapula and attach to the greater or lesser tubercles of the humerus. As these
muscles cross the shoulder joint, their tendons encircle the head of the
humerus and become fused to the anterior, superior, and posterior walls of
the articular capsule. The thickening of the capsule formed by the fusion of
these four muscle tendons is called the rotator cuff. Two bursae, the
subacromial bursa and the subscapular bursa, help to prevent friction
between the rotator cuff muscle tendons and the scapula as these tendons
cross the glenohumeral joint. In addition to their individual actions of
moving the upper limb, the rotator cuff muscles also serve to hold the head
of the humerus in position within the glenoid cavity. By constantly
adjusting their strength of contraction to resist forces acting on the shoulder,
these muscles serve as “dynamic ligaments” and thus provide the primary
structural support for the glenohumeral joint.
Injuries to the shoulder joint are common. Repetitive use of the upper limb,
particularly in abduction such as during throwing, swimming, or racquet
sports, may lead to acute or chronic inflammation of the bursa or muscle
tendons, a tear of the glenoid labrum, or degeneration or tears of the rotator
cuff. Because the humeral head is strongly supported by muscles and
ligaments around its anterior, superior, and posterior aspects, most
dislocations of the humerus occur in an inferior direction. This can occur
when force is applied to the humerus when the upper limb is fully abducted,
as when diving to catch a baseball and landing on your hand or elbow.
Inflammatory responses to any shoulder injury can lead to the formation of
scar tissue between the articular capsule and surrounding structures, thus
reducing shoulder mobility, a condition called adhesive capsulitis (“frozen
shoulder”).
Note:
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Watch this video for a tutorial on the anatomy of the shoulder joint. What
movements are available at the shoulder joint?
Note:
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Watch this video to learn more about the anatomy of the shoulder joint,
including bones, joints, muscles, nerves, and blood vessels. What is the
shape of the glenoid labrum in cross-section, and what is the importance of
this shape?
Elbow Joint
The elbow joint is a uniaxial hinge joint formed by the humeroulnar joint,
the articulation between the trochlea of the humerus and the trochlear notch
of the ulna. Also associated with the elbow are the humeroradial joint and
the proximal radioulnar joint. All three of these joints are enclosed within a
single articular capsule ([link]).
The articular capsule of the elbow is thin on its anterior and posterior
aspects, but is thickened along its outside margins by strong intrinsic
ligaments. These ligaments prevent side-to-side movements and
hyperextension. On the medial side is the triangular ulnar collateral
ligament. This arises from the medial epicondyle of the humerus and
attaches to the medial side of the proximal ulna. The strongest part of this
ligament is the anterior portion, which resists hyperextension of the elbow.
The ulnar collateral ligament may be injured by frequent, forceful
extensions of the forearm, as is seen in baseball pitchers. Reconstructive
surgical repair of this ligament is referred to as Tommy John surgery, named
for the former major league pitcher who was the first person to have this
treatment.
The lateral side of the elbow is supported by the radial collateral ligament.
This arises from the lateral epicondyle of the humerus and then blends into
the lateral side of the annular ligament. The annular ligament encircles the
head of the radius. This ligament supports the head of the radius as it
articulates with the radial notch of the ulna at the proximal radioulnar joint.
This is a pivot joint that allows for rotation of the radius during supination
and pronation of the forearm.
Elbow Joint
Humerus Articular
capsule
Fat pad
Synovial
membrane
Tendon ; c Synovial
of triceps 7 - - cavity
muscle \F) Articular
i. cartilage
Bursa | \\ of trochlea
i. \ } ) fj = Tendon of
SY . +
Trochlea wi branchialis
By — . muscle
Articular }
cartilage of the
trochlear notch
Olecranon bursa
|
| eve ' . NX Una
al Coronoid
= process
(a) Medial sagittal section through right elbow (lateral view)
Articular capsule
oF Anular ligament
\ : a ——
ee Radius
»
Ulnar \ = = aa
collateral —Lil ; D, ~—
ligament AW ) “aa
k TU) \\\ __ Una
, Cael
a
Coronoid process
(c) Medial view of right elbow joint
Humerus j |
ie
\ Lateral epicondyle
\ a ae Ulna
Articular | =a jn ular
capsule Min : ade
p / tT Radial
collateral
\
Radius
\ ligament
—_ aw
Olecranon ye =
process
(b) Lateral view of right elbow joint
(a) The elbow is a hinge joint that allows only for flexion and
extension of the forearm. (b) It is supported by the ulnar and radial
collateral ligaments. (c) The annular ligament supports the head of the
radius at the proximal radioulnar joint, the pivot joint that allows for
rotation of the radius.
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Watch this animation to learn more about the anatomy of the elbow joint.
Which structures provide the main stability for the elbow?
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Watch this video to learn more about the anatomy of the elbow joint,
including bones, joints, muscles, nerves, and blood vessels. What are the
functions of the articular cartilage?
Hip Joint
The hip joint is a multiaxial ball-and-socket joint between the head of the
femur and the acetabulum of the hip bone ({link]). The hip carries the
weight of the body and thus requires strength and stability during standing
and walking. For these reasons, its range of motion is more limited than at
the shoulder joint.
The acetabulum is the socket portion of the hip joint. This space is deep and
has a large articulation area for the femoral head, thus giving stability and
weight bearing ability to the joint. The acetabulum is further deepened by
the acetabular labrum, a fibrocartilage lip attached to the outer margin of
the acetabulum. The surrounding articular capsule is strong, with several
thickened areas forming intrinsic ligaments. These ligaments arise from the
hip bone, at the margins of the acetabulum, and attach to the femur at the
base of the neck. The ligaments are the iliofemoral ligament, pubofemoral
ligament, and ischiofemoral ligament, all of which spiral around the head
and neck of the femur. The ligaments are tightened by extension at the hip,
thus pulling the head of the femur tightly into the acetabulum when in the
upright, standing position. Very little additional extension of the thigh is
permitted beyond this vertical position. These ligaments thus stabilize the
hip joint and allow you to maintain an upright standing position with only
minimal muscle contraction. Inside of the articular capsule, the ligament of
the head of the femur (ligamentum teres) spans between the acetabulum
and femoral head. This intracapsular ligament is normally slack and does
not provide any significant joint support, but it does provide a pathway for
an important artery that supplies the head of the femur.
The hip is prone to osteoarthritis, and thus was the first joint for which a
replacement prosthesis was developed. A common injury in elderly
individuals, particularly those with weakened bones due to osteoporosis, is
a “broken hip,” which is actually a fracture of the femoral neck. This may
result from a fall, or it may cause the fall. This can happen as one lower
limb is taking a step and all of the body weight is placed on the other limb,
causing the femoral neck to break and producing a fall. Any accompanying
disruption of the blood supply to the femoral neck or head can lead to
necrosis of these areas, resulting in bone and cartilage death. Femoral
fractures usually require surgical treatment, after which the patient will
need mobility assistance for a prolonged period, either from family
members or in a long-term care facility. Consequentially, the associated
health care costs of “broken hips” are substantial. In addition, hip fractures
are associated with increased rates of morbidity (incidences of disease) and
mortality (death). Surgery for a hip fracture followed by prolonged bed rest
may lead to life-threatening complications, including pneumonia, infection
of pressure ulcers (bedsores), and thrombophlebitis (deep vein thrombosis;
blood clot formation) that can result in a pulmonary embolism (blood clot
within the lung).
Hip Joint
Articular cartilage Coxal (hip) bone
Acetabular labrum
Ligament of the
head of the femur
Synovial cavity
Articular capsule
(a) Frontal section through the right hip joint
Anterior inferior
A ) lliofemoral
iliac spine ligament
Greater
tronchanter » “~% = \Os
Pubofemoral .
ligament
(b) Anterior view of right hip joint, capsule in place
Ischium
v lliofemoral
/ ean ligament
_ Greater
trochanter
of femur
Ischiofemoral
ligament
(c) Posterior view of right hip joint, capsule in place
(a) The ball-and-socket joint of the
hip is a multiaxial joint that provides
both stability and a wide range of
motion. (b—c) When standing, the
supporting ligaments are tight,
pulling the head of the femur into the
acetabulum.
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Watch this video for a tutorial on the anatomy of the hip joint. What is a
possible consequence following a fracture of the femoral neck within the
capsule of the hip joint?
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1
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Watch this video to learn more about the anatomy of the hip joint,
including bones, joints, muscles, nerves, and blood vessels. Where is the
articular cartilage thickest within the hip joint?
Knee Joint
The knee joint is the largest joint of the body ([link]). It actually consists of
three articulations. The femoropatellar joint is found between the patella
and the distal femur. The medial tibiofemoral joint and lateral
tibiofemoral joint are located between the medial and lateral condyles of
the femur and the medial and lateral condyles of the tibia. All of these
articulations are enclosed within a single articular capsule. The knee
functions as a hinge joint, allowing flexion and extension of the leg. This
action is generated by both rolling and gliding motions of the femur on the
tibia. In addition, some rotation of the leg is available when the knee is
flexed, but not when extended. The knee is well constructed for weight
bearing in its extended position, but is vulnerable to injuries associated with
hyperextension, twisting, or blows to the medial or lateral side of the joint,
particularly while weight bearing.
At the femoropatellar joint, the patella slides vertically within a groove on
the distal femur. The patella is a sesamoid bone incorporated into the tendon
of the quadriceps femoris muscle, the large muscle of the anterior thigh.
The patella serves to protect the quadriceps tendon from friction against the
distal femur. Continuing from the patella to the anterior tibia just below the
knee is the patellar ligament. Acting via the patella and patellar ligament,
the quadriceps femoris is a powerful muscle that acts to extend the leg at
the knee. It also serves as a “dynamic ligament” to provide very important
support and stabilization for the knee joint.
The medial and lateral tibiofemoral joints are the articulations between the
rounded condyles of the femur and the relatively flat condyles of the tibia.
During flexion and extension motions, the condyles of the femur both roll
and glide over the surfaces of the tibia. The rolling action produces flexion
or extension, while the gliding action serves to maintain the femoral
condyles centered over the tibial condyles, thus ensuring maximal bony,
weight-bearing support for the femur in all knee positions. As the knee
comes into full extension, the femur undergoes a slight medial rotation in
relation to tibia. The rotation results because the lateral condyle of the
femur is slightly smaller than the medial condyle. Thus, the lateral condyle
finishes its rolling motion first, followed by the medial condyle. The
resulting small medial rotation of the femur serves to “lock” the knee into
its fully extended and most stable position. Flexion of the knee is initiated
by a slight lateral rotation of the femur on the tibia, which “unlocks” the
knee. This lateral rotation motion is produced by the popliteus muscle of the
posterior leg.
Located between the articulating surfaces of the femur and tibia are two
articular discs, the medial meniscus and lateral meniscus (see [link]b).
Each is a C-shaped fibrocartilage structure that is thin along its inside
margin and thick along the outer margin. They are attached to their tibial
condyles, but do not attach to the femur. While both menisci are free to
move during knee motions, the medial meniscus shows less movement
because it is anchored at its outer margin to the articular capsule and tibial
collateral ligament. The menisci provide padding between the bones and
help to fill the gap between the round femoral condyles and flattened tibial
condyles. Some areas of each meniscus lack an arterial blood supply and
thus these areas heal poorly if damaged.
The knee joint has multiple ligaments that provide support, particularly in
the extended position (see [link]c). Outside of the articular capsule, located
at the sides of the knee, are two extrinsic ligaments. The fibular collateral
ligament (lateral collateral ligament) is on the lateral side and spans from
the lateral epicondyle of the femur to the head of the fibula. The tibial
collateral ligament (medial collateral ligament) of the medial knee runs
from the medial epicondyle of the femur to the medial tibia. As it crosses
the knee, the tibial collateral ligament is firmly attached on its deep side to
the articular capsule and to the medial meniscus, an important factor when
considering knee injuries. In the fully extended knee position, both
collateral ligaments are taut (tight), thus serving to stabilize and support the
extended knee and preventing side-to-side or rotational motions between
the femur and tibia.
The articular capsule of the posterior knee is thickened by intrinsic
ligaments that help to resist knee hyperextension. Inside the knee are two
intracapsular ligaments, the anterior cruciate ligament and posterior
cruciate ligament. These ligaments are anchored inferiorly to the tibia at
the intercondylar eminence, the roughened area between the tibial condyles.
The cruciate ligaments are named for whether they are attached anteriorly
or posteriorly to this tibial region. Each ligament runs diagonally upward to
attach to the inner aspect of a femoral condyle. The cruciate ligaments are
named for the X-shape formed as they pass each other (cruciate means
“cross”). The posterior cruciate ligament is the stronger ligament. It serves
to support the knee when it is flexed and weight bearing, as when walking
downhill. In this position, the posterior cruciate ligament prevents the femur
from sliding anteriorly off the top of the tibia. The anterior cruciate
ligament becomes tight when the knee is extended, and thus resists
hyperextension.
Knee Joint
Femur Tendon of
quadriceps femoris
Articular
Suprapatellar bursa
capsule
Patella
Prepatellar bursa
Posterior
cruciate
ligament Synovial cavity
; Lateral meniscus
meniscus
Infrapatellar
Anterior fat pad
cruciate
ligament
Tibia
Infrapatellar
bursa
Patellar ligament
(a) Sagittal section through the right knee joint
Quadriceps
femoris muscle
Tendon of —————————_
quadriceps
femoris muscle
a
Medial patellar
Lateral rosie |e Ay LX retinaculum
retinaculum
Z Tibial collateral
Fibular collateral ligament
ligament
W! Patellar ligament
Fibula = Tibia
(c) Anterior view of right knee
Anterior cruciate :
ligament prienoe
—_—— Articular
cartilage on
lateral tibial
condyle
Articular cartilage yo
on medial F
tibial condyle c
Medial meniscus <<) Sf
— Lateral
meniscus
\ <a
Posterior cruciate
ligament
(b) Superior view of the right tibia in the knee joint, showing
the menisci and cruciate ligaments
(a) The knee joint is the largest joint of the body. (b)-(c) It is supported
by the tibial and fibular collateral ligaments located on the sides of the
knee outside of the articular capsule, and the anterior and posterior
cruciate ligaments found inside the capsule. The medial and lateral
menisci provide padding and support between the femoral condyles
and tibial condyles.
Note:
Dees 0
Per
—
mess" Openstax COLLEGE
Watch this video to learn more about the flexion and extension of the knee,
as the femur both rolls and glides on the tibia to maintain stable contact
between the bones in all knee positions. The patella glides along a groove
on the anterior side of the distal femur. The collateral ligaments on the
sides of the knee become tight in the fully extended position to help
stabilize the knee. The posterior cruciate ligament supports the knee when
flexed and the anterior cruciate ligament becomes tight when the knee
comes into full extension to resist hyperextension. What are the ligaments
that support the knee joint?
Note:
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Watch this video to learn more about the anatomy of the knee joint,
including bones, joints, muscles, nerves, and blood vessels. Which
ligament of the knee keeps the tibia from sliding too far forward in relation
to the femur and which ligament keeps the tibia from sliding too far
backward?
Note:
Disorders of the...
Joints
Injuries to the knee are common. Since this joint is primarily supported by
muscles and ligaments, injuries to any of these structures will result in pain
or knee instability. Injury to the posterior cruciate ligament occurs when
the knee is flexed and the tibia is driven posteriorly, such as falling and
landing on the tibial tuberosity or hitting the tibia on the dashboard when
not wearing a seatbelt during an automobile accident. More commonly,
injuries occur when forces are applied to the extended knee, particularly
when the foot is planted and unable to move. Anterior cruciate ligament
injuries can result with a forceful blow to the anterior knee, producing
hyperextension, or when a runner makes a quick change of direction that
produces both twisting and hyperextension of the knee.
A worse combination of injuries can occur with a hit to the lateral side of
the extended knee ([link]). A moderate blow to the lateral knee will cause
the medial side of the joint to open, resulting in stretching or damage to the
tibial collateral ligament. Because the medial meniscus is attached to the
tibial collateral ligament, a stronger blow can tear the ligament and also
damage the medial meniscus. This is one reason that the medial meniscus
is 20 times more likely to be injured than the lateral meniscus. A powerful
blow to the lateral knee produces a “terrible triad” injury, in which there is
a sequential injury to the tibial collateral ligament, medial meniscus, and
anterior cruciate ligament.
Arthroscopic surgery has greatly improved the surgical treatment of knee
injuries and reduced subsequent recovery times. This procedure involves a
small incision and the insertion into the joint of an arthroscope, a pencil-
thin instrument that allows for visualization of the joint interior. Small
surgical instruments are also inserted via additional incisions. These tools
allow a surgeon to remove or repair a torn meniscus or to reconstruct a
ruptured cruciate ligament. The current method for anterior cruciate
ligament replacement involves using a portion of the patellar ligament.
Holes are drilled into the cruciate ligament attachment points on the tibia
and femur, and the patellar ligament graft, with small areas of attached
bone still intact at each end, is inserted into these holes. The bone-to-bone
sites at each end of the graft heal rapidly and strongly, thus enabling a rapid
recovery.
Knee Injury
Lateral Medial
Torn tibial (medial)
Direction of force collateral ligament
Torn anterior Medial meniscus
cruciate ligament
Anterior view
A strong blow to the lateral side of the extended
knee will cause three injuries, in sequence: tearing
of the tibial collateral ligament, damage to the
medial meniscus, and rupture of the anterior
cruciate ligament.
Note:
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Watch this video to learn more about different knee injuries and diagnostic
testing of the knee. What are the most common causes of anterior cruciate
ligament injury?
Ankle and Foot Joints
The ankle is formed by the talocrural joint ((link]). It consists of the
articulations between the talus bone of the foot and the distal ends of the
tibia and fibula of the leg (crural = “leg”). The superior aspect of the talus
bone is square-shaped and has three areas of articulation. The top of the
talus articulates with the inferior tibia. This is the portion of the ankle joint
that carries the body weight between the leg and foot. The sides of the talus
are firmly held in position by the articulations with the medial malleolus of
the tibia and the lateral malleolus of the fibula, which prevent any side-to-
side motion of the talus. The ankle is thus a uniaxial hinge joint that allows
only for dorsiflexion and plantar flexion of the foot.
Additional joints between the tarsal bones of the posterior foot allow for the
movements of foot inversion and eversion. Most important for these
movements is the subtalar joint, located between the talus and calcaneus
bones. The joints between the talus and navicular bones and the calcaneus
and cuboid bones are also important contributors to these movements. All
of the joints between tarsal bones are plane joints. Together, the small
motions that take place at these joints all contribute to the production of
inversion and eversion foot motions.
Like the hinge joints of the elbow and knee, the talocrural joint of the ankle
is supported by several strong ligaments located on the sides of the joint.
These ligaments extend from the medial malleolus of the tibia or lateral
malleolus of the fibula and anchor to the talus and calcaneus bones. Since
they are located on the sides of the ankle joint, they allow for dorsiflexion
and plantar flexion of the foot. They also prevent abnormal side-to-side and
twisting movements of the talus and calcaneus bones during eversion and
inversion of the foot. On the medial side is the broad deltoid ligament. The
deltoid ligament supports the ankle joint and also resists excessive eversion
of the foot. The lateral side of the ankle has several smaller ligaments.
These include the anterior talofibular ligament and the posterior
talofibular ligament, both of which span between the talus bone and the
lateral malleolus of the fibula, and the calcaneofibular ligament, located
between the calcaneus bone and fibula. These ligaments support the ankle
and also resist excess inversion of the foot.
Ankle Joint
Tibia
Medial malleolus
Deltoid ligament
Medial view
Fibula Tibia
Posterior and anterior inferior
tibiofibular ligaments
Interosseous
membrane Anterior talofibular ligament
Calcaneofibular ligament Subtalar joint
Lateral view
The talocrural (ankle) joint is a uniaxial
hinge joint that only allows for
dorsiflexion or plantar flexion of the foot.
Movements at the subtalar joint, between
the talus and calcaneus bones, combined
with motions at other intertarsal joints,
enables eversion/inversion movements of
the foot. Ligaments that unite the medial
or lateral malleolus with the talus and
calcaneus bones serve to support the
talocrural joint and to resist excess
eversion or inversion of the foot.
[=]: Le]
Watch this video for a tutorial on the anatomy of the ankle joint. What are
the three ligaments found on the lateral side of the ankle joint?
Note:
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Watch this video to learn more about the anatomy of the ankle joint,
including bones, joints, muscles, nerves, and blood vessels. Which type of
joint used in woodworking does the ankle joint resemble?
Note:
Disorders of the...
Joints
The ankle is the most frequently injured joint in the body, with the most
common injury being an inversion ankle sprain. A sprain is the stretching
or tearing of the supporting ligaments. Excess inversion causes the talus
bone to tilt laterally, thus damaging the ligaments on the lateral side of the
ankle. The anterior talofibular ligament is most commonly injured,
followed by the calcaneofibular ligament. In severe inversion injuries, the
forceful lateral movement of the talus not only ruptures the lateral ankle
ligaments, but also fractures the distal fibula.
Less common are eversion sprains of the ankle, which involve stretching of
the deltoid ligament on the medial side of the ankle. Forcible eversion of
the foot, for example, with an awkward landing from a jump or when a
football player has a foot planted and is hit on the lateral ankle, can result
in a Pott’s fracture and dislocation of the ankle joint. In this injury, the very
strong deltoid ligament does not tear, but instead shears off the medial
malleolus of the tibia. This frees the talus, which moves laterally and
fractures the distal fibula. In extreme cases, the posterior margin of the
tibia may also be sheared off.
Above the ankle, the distal ends of the tibia and fibula are united by a
strong syndesmosis formed by the interosseous membrane and ligaments at
the distal tibiofibular joint. These connections prevent separation between
the distal ends of the tibia and fibula and maintain the talus locked into
position between the medial malleolus and lateral malleolus. Injuries that
produce a lateral twisting of the leg on top of the planted foot can result in
stretching or tearing of the tibiofibular ligaments, producing a syndesmotic
ankle sprain or “high ankle sprain.”
Most ankle sprains can be treated using the RICE technique: Rest, Ice,
Compression, and Elevation. Reducing joint mobility using a brace or cast
may be required for a period of time. More severe injuries involving
ligament tears or bone fractures may require surgery.
Note:
—
mess" Openstax COLLEGE
Watch this video to learn more about the ligaments of the ankle joint, ankle
sprains, and treatment. During an inversion ankle sprain injury, all three
ligaments that resist excessive inversion of the foot may be injured. What
is the sequence in which these three ligaments are injured?
Chapter Review
Although synovial joints share many common features, each joint of the
body is specialized for certain movements and activities. The joints of the
upper limb provide for large ranges of motion, which give the upper limb
great mobility, thus enabling actions such as the throwing of a ball or typing
on a keyboard. The joints of the lower limb are more robust, giving them
greater strength and the stability needed to support the body weight during
running, jumping, or kicking activities.
The joints of the vertebral column include the symphysis joints formed by
each intervertebral disc and the plane synovial joints between the superior
and inferior articular processes of adjacent vertebrae. Each of these joints
provide for limited motions, but these sum together to produce flexion,
extension, lateral flexion, and rotation of the neck and body. The range of
motions available in each region of the vertebral column varies, with all of
these motions available in the cervical region. Only rotation is allowed in
the thoracic region, while the lumbar region has considerable extension,
flexion, and lateral flexion, but rotation is prevented. The atlanto-occipital
joint allows for flexion and extension of the head, while the atlantoaxial
joint is a pivot joint that provides for rotation of the head.
The temporomandibular joint is the articulation between the condyle of the
mandible and the mandibular fossa and articular tubercle of the skull
temporal bone. An articular disc is located between the bony components of
this joint. A combination of gliding and hinge motions of the mandibular
condyle allows for elevation/depression, protraction/retraction, and side-to-
side motions of the lower jaw.
The glenohumeral (shoulder) joint is a multiaxial ball-and-socket joint that
provides flexion/extension, abduction/adduction, circumduction, and
medial/lateral rotation of the humerus. The head of the humerus articulates
with the glenoid cavity of the scapula. The glenoid labrum extends around
the margin of the glenoid cavity. Intrinsic ligaments, including the
coracohumeral ligament and glenohumeral ligaments, provide some support
for the shoulder joint. However, the primary support comes from muscles
crossing the joint whose tendons form the rotator cuff. These muscle
tendons are protected from friction against the scapula by the subacromial
bursa and subscapular bursa.
The elbow is a uniaxial hinge joint that allows for flexion/extension of the
forearm. It includes the humeroulnar joint and the humeroradial joint. The
medial elbow is supported by the ulnar collateral ligament and the radial
collateral ligament supports the lateral side. These ligaments prevent side-
to-side movements and resist hyperextension of the elbow. The proximal
radioulnar joint is a pivot joint that allows for rotation of the radius during
pronation/supination of the forearm. The annular ligament surrounds the
head of the radius to hold it in place at this joint.
The hip joint is a ball-and-socket joint whose motions are more restricted
than at the shoulder to provide greater stability during weight bearing. The
hip joint is the articulation between the head of the femur and the
acetabulum of the hip bone. The acetabulum is deepened by the acetabular
labrum. The iliofemoral, pubofemoral, and ischiofemoral ligaments
strongly support the hip joint in the upright, standing position. The ligament
of the head of the femur provides little support but carries an important
artery that supplies the femur.
The knee includes three articulations. The femoropatellar joint is between
the patella and distal femur. The patella, a sesamoid bone incorporated into
the tendon of the quadriceps femoris muscle of the anterior thigh, serves to
protect this tendon from rubbing against the distal femur during knee
movements. The medial and lateral tibiofemoral joints, between the
condyles of the femur and condyles of the tibia, are modified hinge joints
that allow for knee extension and flexion. During these movements, the
condyles of the femur both roll and glide over the surface of the tibia. As
the knee comes into full extension, a slight medial rotation of the femur
serves to “lock” the knee into its most stable, weight-bearing position. The
reverse motion, a small lateral rotation of the femur, is required to initiate
knee flexion. When the knee is flexed, some rotation of the leg is available.
Two extrinsic ligaments, the tibial collateral ligament on the medial side
and the fibular collateral ligament on the lateral side, serve to resist
hyperextension or rotation of the extended knee joint. Two intracapsular
ligaments, the anterior cruciate ligament and posterior cruciate ligament,
span between the tibia and the inner aspects of the femoral condyles. The
anterior cruciate ligament resists hyperextension of the knee, while the
posterior cruciate ligament prevents anterior sliding of the femur, thus
supporting the knee when it is flexed and weight bearing. The medial and
lateral menisci, located between the femoral and tibial condyles, are
articular discs that provide padding and improve the fit between the bones.
The talocrural joint forms the ankle. It consists of the articulation between
the talus bone and the medial malleolus of the tibia, the distal end of the
tibia, and the lateral malleolus of the fibula. This is a uniaxial hinge joint
that allows only dorsiflexion and plantar flexion of the foot. Gliding
motions at the subtalar and intertarsal joints of the foot allow for
inversion/eversion of the foot. The ankle joint is supported on the medial
side by the deltoid ligament, which prevents side-to-side motions of the
talus at the talocrural joint and resists excessive eversion of the foot. The
lateral ankle is supported by the anterior and posterior talofibular ligaments
and the calcaneofibular ligament. These support the ankle joint and also
resist excess inversion of the foot. An inversion ankle sprain, a common
injury, will result in injury to one or more of these lateral ankle ligaments.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn about TMJ. Opening of the mouth requires
the combination of two motions at the temporomandibular joint, an
anterior gliding motion of the articular disc and mandible and the
downward hinging of the mandible. What is the initial movement of
the mandible during opening and how much mouth opening does this
produce?
Solution:
The first motion is rotation (hinging) of the mandible, but this only
produces about 20 mm (0.78 in) of mouth opening.
Exercise:
Problem:
Watch this video for a tutorial on the anatomy of the shoulder joint.
What movements are available at the shoulder joint?
Solution:
The shoulder joint is a ball-and-socket joint that allows for flexion-
extension, abduction-adduction, medial rotation, lateral rotation, and
circumduction of the humerus.
Exercise:
Problem:
Watch this video to learn about the anatomy of the shoulder joint,
including bones, joints, muscles, nerves, and blood vessels. What is the
shape of the glenoid labrum in cross-section, and what is the
importance of this shape?
Solution:
The glenoid labrum is wedge-shaped in cross-section. This is
important because it creates an elevated rim around the glenoid cavity,
which creates a deeper socket for the head of the humerus to fit into.
Exercise:
Problem:
Watch this animation to learn more about the anatomy of the elbow
joint. What structures provide the main stability for the elbow?
Solution:
The structures that stabilize the elbow include the coronoid process,
the radial (lateral) collateral ligament, and the anterior portion of the
ulnar (medial) collateral ligament.
Exercise:
Problem:
Watch this video to learn more about the anatomy of the elbow joint,
including bones, joints, muscles, nerves, and blood vessels. What are
the functions of the articular cartilage?
Solution:
The articular cartilage functions to absorb shock and to provide an
extremely smooth surface that makes movement between bones easy,
without damaging the bones.
Exercise:
Problem:
Watch this video for a tutorial on the anatomy of the hip joint. What is
a possible consequence following a fracture of the femoral neck within
the capsule of the hip joint?
Solution:
An intracapsular fracture of the neck of the femur can result in
disruption of the arterial blood supply to the head of the femur, which
may lead to avascular necrosis of the femoral head.
Exercise:
Problem:
Watch this video to learn more about the anatomy of the hip joint,
including bones, joints, muscles, nerves, and blood vessels. Where is
the articular cartilage thickest within the hip joint?
Solution:
The articular cartilage is thickest in the upper and back part of the
acetabulum, the socket portion of the hip joint. These regions receive
most of the force from the head of the femur during walking and
running.
Exercise:
Problem:
Watch this video to learn more about the flexion and extension of the
knee, as the femur both rolls and glides on the tibia to maintain stable
contact between the bones in all knee positions. The patella glides
along a groove on the anterior side of the distal femur. The collateral
ligaments on the sides of the knee become tight in the fully extended
position to help stabilize the knee. The posterior cruciate ligament
supports the knee when flexed and the anterior cruciate ligament
becomes tight when the knee comes into full extension to resist
hyperextension. What are the ligaments that support the knee joint?
Solution:
There are five ligaments associated with the knee joint. The tibial
collateral ligament is located on the medial side of the knee and the
fibular collateral ligament is located on the lateral side. The anterior
and posterior cruciate ligaments are located inside the knee joint.
Exercise:
Problem:
Watch this video to learn more about the anatomy of the knee joint,
including bones, joints, muscles, nerves, and blood vessels. Which
ligament of the knee keeps the tibia from sliding too far forward in
relation to the femur and which ligament keeps the tibia from sliding
too far backward?
Solution:
The anterior cruciate ligament prevents the tibia from sliding too far
forward in relation to the femur and the posterior cruciate ligament
keeps the tibia from sliding too far backward.
Exercise:
Problem:
Watch this video to learn more about different knee injuries and
diagnostic testing of the knee. What are the most causes of anterior
cruciate ligament injury?
Solution:
The anterior cruciate ligament (ACL) is most commonly injured when
traumatic force is applied to the knee during a twisting motion or when
side standing or landing from a jump.
Exercise:
Problem:
Watch this video for a tutorial on the anatomy of the ankle joint. What
are the three ligaments found on the lateral side of the ankle joint?
Solution:
The ligaments of the lateral ankle are the anterior and posterior
talofibular ligaments and the calcaneofibular ligament. These
ligaments support the ankle joint and resist excess inversion of the
foot.
Exercise:
Problem:
Watch this video to learn more about the anatomy of the ankle joint,
including bones, joints, muscles, nerves, and blood vessels. The ankle
joint resembles what type of joint used in woodworking?
Solution:
Because of the square shape of the ankle joint, it has been compared to
a mortise-and-tendon type of joint.
Exercise:
Problem:
Watch this video to learn about the ligaments of the ankle joint, ankle
sprains, and treatment. During an inversion ankle sprain injury, all
three ligaments that resist excessive inversion of the foot may be
injured. What is the sequence in which these three ligaments are
injured?
Solution:
An inversion ankle sprain may injure all three ligaments located on the
lateral side of the ankle. The sequence of injury would be the anterior
talofibular ligament first, followed by the calcaneofibular ligament
second, and finally, the posterior talofibular ligament third.
Review Questions
Exercise:
Problem:
The primary support for the glenohumeral joint is provided by the
a. coracohumeral ligament
b. glenoid labrum
c. rotator cuff muscles
d. subacromial bursa
Solution:
C
Exercise:
Problem: The proximal radioulnar joint
a. is supported by the annular ligament
b. contains an articular disc that strongly unites the bones
c. is supported by the ulnar collateral ligament
d. is a hinge joint that allows for flexion/extension of the forearm
Solution:
A
Exercise:
Problem: Which statement is true concerning the knee joint?
a. The lateral meniscus is an intrinsic ligament located on the lateral
side of the knee joint.
b. Hyperextension is resisted by the posterior cruciate ligament.
c. The anterior cruciate ligament supports the knee when it is flexed
and weight bearing.
d. The medial meniscus is attached to the tibial collateral ligament.
Solution:
D
Exercise:
Problem:The ankle joint
a. is also called the subtalar joint
b. allows for gliding movements that produce inversion/eversion of
the foot
c. is a uniaxial hinge joint
d. is supported by the tibial collateral ligament on the lateral side
Solution:
C
Exercise:
Problem:
Which region of the vertebral column has the greatest range of motion
for rotation?
a. cervical
b. thoracic
c. lumbar
d. sacral
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Discuss the structures that contribute to support of the shoulder joint.
Solution:
The shoulder joint allows for a large range of motion. The primary
support for the shoulder joint is provided by the four rotator cuff
muscles. These muscles serve as “dynamic ligaments” and thus can
modulate their strengths of contraction as needed to hold the head of
the humerus in position at the glenoid fossa. Additional but weaker
support comes from the coracohumeral ligament, an intrinsic ligament
that supports the superior aspect of the shoulder joint, and the
glenohumeral ligaments, which are intrinsic ligaments that support the
anterior side of the joint.
Exercise:
Problem:
Describe the sequence of injuries that may occur if the extended,
weight-bearing knee receives a very strong blow to the lateral side of
the knee.
Solution:
A strong blow to the lateral side of the extended knee will cause the
medial side of the knee joint to open, resulting in a sequence of three
injuries. First will be damage to the tibial collateral ligament. Since the
medial meniscus is attached to the tibial collateral ligament, the
meniscus is also injured. The third structure injured would be the
anterior cruciate ligament.
Glossary
acetabular labrum
lip of fibrocartilage that surrounds outer margin of the acetabulum on
the hip bone
annular ligament
intrinsic ligament of the elbow articular capsule that surrounds and
supports the head of the radius at the proximal radioulnar joint
anterior cruciate ligament
intracapsular ligament of the knee; extends from anterior, superior
surface of the tibia to the inner aspect of the lateral condyle of the
femur; resists hyperextension of knee
anterior talofibular ligament
intrinsic ligament located on the lateral side of the ankle joint, between
talus bone and lateral malleolus of fibula; supports talus at the
talocrural joint and resists excess inversion of the foot
atlantoaxial joint
series of three articulations between the atlas (C1) vertebra and the
axis (C2) vertebra, consisting of the joints between the inferior
articular processes of C1 and the superior articular processes of C2,
and the articulation between the dens of C2 and the anterior arch of C1
atlanto-occipital joint
articulation between the occipital condyles of the skull and the
superior articular processes of the atlas (C1 vertebra)
calcaneofibular ligament
intrinsic ligament located on the lateral side of the ankle joint, between
the calcaneus bone and lateral malleolus of the fibula; supports the
talus bone at the ankle joint and resists excess inversion of the foot
coracohumeral ligament
intrinsic ligament of the shoulder joint; runs from the coracoid process
of the scapula to the anterior humerus
deltoid ligament
broad intrinsic ligament located on the medial side of the ankle joint;
supports the talus at the talocrural joint and resists excess eversion of
the foot
elbow joint
humeroulnar joint
femoropatellar joint
portion of the knee joint consisting of the articulation between the
distal femur and the patella
fibular collateral ligament
extrinsic ligament of the knee joint that spans from the lateral
epicondyle of the femur to the head of the fibula; resists
hyperextension and rotation of the extended knee
glenohumeral joint
shoulder joint; articulation between the glenoid cavity of the scapula
and head of the humerus; multiaxial ball-and-socket joint that allows
for flexion/extension, abduction/adduction, circumduction, and
medial/lateral rotation of the humerus
glenohumeral ligament
one of the three intrinsic ligaments of the shoulder joint that strengthen
the anterior articular capsule
glenoid labrum
lip of fibrocartilage located around the outside margin of the glenoid
cavity of the scapula
humeroradial joint
articulation between the capitulum of the humerus and head of the
radius
humeroulnar joint
articulation between the trochlea of humerus and the trochlear notch of
the ulna; uniaxial hinge joint that allows for flexion/extension of the
forearm
iliofemoral ligament
intrinsic ligament spanning from the ilium of the hip bone to the
femur, on the superior-anterior aspect of the hip joint
ischiofemoral ligament
intrinsic ligament spanning from the ischium of the hip bone to the
femur, on the posterior aspect of the hip joint
lateral meniscus
C-shaped fibrocartilage articular disc located at the knee, between the
lateral condyle of the femur and the lateral condyle of the tibia
lateral tibiofemoral joint
portion of the knee consisting of the articulation between the lateral
condyle of the tibia and the lateral condyle of the femur; allows for
flexion/extension at the knee
ligament of the head of the femur
intracapsular ligament that runs from the acetabulum of the hip bone to
the head of the femur
medial meniscus
C-shaped fibrocartilage articular disc located at the knee, between the
medial condyle of the femur and medial condyle of the tibia
medial tibiofemoral joint
portion of the knee consisting of the articulation between the medial
condyle of the tibia and the medial condyle of the femur; allows for
flexion/extension at the knee
patellar ligament
ligament spanning from the patella to the anterior tibia; serves as the
final attachment for the quadriceps femoris muscle
posterior cruciate ligament
intracapsular ligament of the knee; extends from the posterior, superior
surface of the tibia to the inner aspect of the medial condyle of the
femur; prevents anterior displacement of the femur when the knee is
flexed and weight bearing
posterior talofibular ligament
intrinsic ligament located on the lateral side of the ankle joint, between
the talus bone and lateral malleolus of the fibula; supports the talus at
the talocrural joint and resists excess inversion of the foot
pubofemoral ligament
intrinsic ligament spanning from the pubis of the hip bone to the
femur, on the anterior-inferior aspect of the hip joint
radial collateral ligament
intrinsic ligament on the lateral side of the elbow joint; runs from the
lateral epicondyle of humerus to merge with the annular ligament
rotator cuff
strong connective tissue structure formed by the fusion of four rotator
cuff muscle tendons to the articular capsule of the shoulder joint;
surrounds and supports superior, anterior, lateral, and posterior sides of
the humeral head
subacromial bursa
bursa that protects the supraspinatus muscle tendon and superior end
of the humerus from rubbing against the acromion of the scapula
subscapular bursa
bursa that prevents rubbing of the subscapularis muscle tendon against
the scapula
subtalar joint
articulation between the talus and calcaneus bones of the foot; allows
motions that contribute to inversion/eversion of the foot
talocrural joint
ankle joint; articulation between the talus bone of the foot and medial
malleolus of the tibia, distal tibia, and lateral malleolus of the fibula; a
uniaxial hinge joint that allows only for dorsiflexion and plantar
flexion of the foot
temporomandibular joint (TMJ)
articulation between the condyle of the mandible and the mandibular
fossa and articular tubercle of the temporal bone of the skull; allows
for depression/elevation (opening/closing of mouth),
protraction/retraction, and side-to-side motions of the mandible
tibial collateral ligament
extrinsic ligament of knee joint that spans from the medial epicondyle
of the femur to the medial tibia; resists hyperextension and rotation of
extended knee
ulnar collateral ligament
intrinsic ligament on the medial side of the elbow joint; spans from the
medial epicondyle of the humerus to the medial ulna
zygapophysial joints
facet joints; plane joints between the superior and inferior articular
processes of adjacent vertebrae that provide for only limited motions
between the vertebrae
Overview of Muscle Tissues
By the end of this section, you will be able to:
e Describe the different types of muscle
e Explain contractibility and extensibility
Muscle is one of the four primary tissue types of the body, and the body
contains three types of muscle tissue: skeletal muscle, cardiac muscle, and
smooth muscle ([link]). All three muscle tissues have some properties in
common; they all exhibit a quality called excitability as their plasma
membranes can change their electrical states (from polarized to
depolarized) and send an electrical wave called an action potential along the
entire length of the membrane. While the nervous system can influence the
excitability of cardiac and smooth muscle to some degree, skeletal muscle
completely depends on signaling from the nervous system to work properly.
On the other hand, both cardiac muscle and smooth muscle can respond to
other stimuli, such as hormones and local stimuli.
The Three Types of Muscle Tissue
The body contains three types of
muscle tissue: (a) skeletal muscle,
(b) smooth muscle, and (c) cardiac
muscle. From top, LM x 1600,
LM x 1600, LM x 1600.
(Micrographs provided by the
Regents of University of Michigan
Medical School © 2012)
The muscles all begin the actual process of contracting (shortening) when a
protein called actin is pulled by a protein called myosin. This occurs in
striated muscle (skeletal and cardiac) after specific binding sites on the actin
have been exposed in response to the interaction between calcium ions
(Ca**) and proteins (troponin and tropomyosin) that “shield” the actin-
binding sites. Ca** also is required for the contraction of smooth muscle,
although its role is different: here Ca** activates enzymes, which in turn
activate myosin heads. All muscles require adenosine triphosphate (ATP) to
continue the process of contracting, and they all relax when the Ca** is
removed and the actin-binding sites are re-shielded.
A muscle can return to its original length when relaxed due to a quality of
muscle tissue called elasticity. It can recoil back to its original length due to
elastic fibers. Muscle tissue also has the quality of extensibility; it can
stretch or extend. Contractility allows muscle tissue to pull on its
attachment points and shorten with force.
Differences among the three muscle types include the microscopic
organization of their contractile proteins—actin and myosin. The actin and
myosin proteins are arranged very regularly in the cytoplasm of individual
muscle cells (referred to as fibers) in both skeletal muscle and cardiac
muscle, which creates a pattern, or stripes, called striations. The striations
are visible with a light microscope under high magnification (see [link]).
Skeletal muscle fibers are multinucleated structures that compose the
skeletal muscle. Cardiac muscle fibers each have one to two nuclei and are
physically and electrically connected to each other so that the entire heart
contracts as one unit (called a syncytium).
Because the actin and myosin are not arranged in such regular fashion in
smooth muscle, the cytoplasm of a smooth muscle fiber (which has only a
single nucleus) has a uniform, nonstriated appearance (resulting in the name
smooth muscle). However, the less organized appearance of smooth muscle
should not be interpreted as less efficient. Smooth muscle in the walls of
arteries is a critical component that regulates blood pressure necessary to
push blood through the circulatory system; and smooth muscle in the skin,
visceral organs, and internal passageways is essential for moving all
materials through the body.
Chapter Review
Muscle is the tissue in animals that allows for active movement of the body
or materials within the body. There are three types of muscle tissue: skeletal
muscle, cardiac muscle, and smooth muscle. Most of the body’s skeletal
muscle produces movement by acting on the skeleton. Cardiac muscle is
found in the wall of the heart and pumps blood through the circulatory
system.
Smooth muscle is found in the skin, where it is associated with hair
follicles; it also is found in the walls of internal organs, blood vessels, and
internal passageways, where it assists in moving materials.
Review Questions
Exercise:
Problem:
Muscle that has a striped appearance is described as being
a. elastic
b. nonstriated
c. excitable
d. striated
Solution:
D
Exercise:
Problem:
Which element is important in directly triggering contraction?
a. sodium (Na*)
b. calcium (Ca™)
c. potassium (K*)
d. chloride (CI)
Solution:
B
Exercise:
Problem:
Which of the following properties is not common to all three muscle
tissues?
a. excitability
b. the need for ATP
c. at rest, uses shielding proteins to cover actin-binding sites
d. elasticity
Solution:
C
Critical Thinking Questions
Exercise:
Problem: Why is elasticity an important quality of muscle tissue?
Solution:
It allows muscle to return to its original length during relaxation after
contraction.
Glossary
cardiac muscle
striated muscle found in the heart; joined to one another at intercalated
discs and under the regulation of pacemaker cells, which contract as
one unit to pump blood through the circulatory system. Cardiac muscle
is under involuntary control.
contractility
ability to shorten (contract) forcibly
elasticity
ability to stretch and rebound
excitability
ability to undergo neural stimulation
extensibility
ability to lengthen (extend)
skeletal muscle
striated, multinucleated muscle that requires signaling from the
nervous system to trigger contraction; most skeletal muscles are
referred to as voluntary muscles that move bones and produce
movement
smooth muscle
nonstriated, mononucleated muscle in the skin that is associated with
hair follicles; assists in moving materials in the walls of internal
organs, blood vessels, and internal passageways
Skeletal Muscle
By the end of this section, you will be able to:
¢ Describe the layers of connective tissues packaging skeletal muscle
e Explain how muscles work with tendons to move the body
e Identify areas of the skeletal muscle fibers
e Describe excitation-contraction coupling
The best-known feature of skeletal muscle is its ability to contract and
cause movement. Skeletal muscles act not only to produce movement but
also to stop movement, such as resisting gravity to maintain posture. Small,
constant adjustments of the skeletal muscles are needed to hold a body
upright or balanced in any position. Muscles also prevent excess movement
of the bones and joints, maintaining skeletal stability and preventing
skeletal structure damage or deformation. Joints can become misaligned or
dislocated entirely by pulling on the associated bones; muscles work to
keep joints stable. Skeletal muscles are located throughout the body at the
openings of internal tracts to control the movement of various substances.
These muscles allow functions, such as swallowing, urination, and
defecation, to be under voluntary control. Skeletal muscles also protect
internal organs (particularly abdominal and pelvic organs) by acting as an
external barrier or shield to external trauma and by supporting the weight of
the organs.
Skeletal muscles contribute to the maintenance of homeostasis in the body
by generating heat. Muscle contraction requires energy, and when ATP is
broken down, heat is produced. This heat is very noticeable during exercise,
when sustained muscle movement causes body temperature to rise, and in
cases of extreme cold, when shivering produces random skeletal muscle
contractions to generate heat.
Each skeletal muscle is an organ that consists of various integrated tissues.
These tissues include the skeletal muscle fibers, blood vessels, nerve fibers,
and connective tissue. Each skeletal muscle has three layers of connective
tissue (called “mysia”) that enclose it and provide structure to the muscle as
a whole, and also compartmentalize the muscle fibers within the muscle
({link]). Each muscle is wrapped in a sheath of dense, irregular connective
tissue called the epimysium, which allows a muscle to contract and move
powerfully while maintaining its structural integrity. The epimysium also
separates muscle from other tissues and organs in the area, allowing the
muscle to move independently.
The Three Connective Tissue Layers
Skeletal muscle Epimysium Muscle fascicles
Perimysium
Endomysium
Muscle fibers
Muscle fascicle
Muscle fiber
Sarcolemma
Bundles of muscle fibers, called fascicles, are
covered by the perimysium. Muscle fibers are
covered by the endomysium.
Inside each skeletal muscle, muscle fibers are organized into individual
bundles, each called a fascicle, by a middle layer of connective tissue called
the perimysium. This fascicular organization is common in muscles of the
limbs; it allows the nervous system to trigger a specific movement of a
muscle by activating a subset of muscle fibers within a bundle, or fascicle
of the muscle. Inside each fascicle, each muscle fiber is encased in a thin
connective tissue layer of collagen and reticular fibers called the
endomysium. The endomysium contains the extracellular fluid and
nutrients to support the muscle fiber. These nutrients are supplied via blood
to the muscle tissue.
In skeletal muscles that work with tendons to pull on bones, the collagen in
the three tissue layers (the mysia) intertwines with the collagen of a tendon.
At the other end of the tendon, it fuses with the periosteum coating the
bone. The tension created by contraction of the muscle fibers is then
transferred though the mysia, to the tendon, and then to the periosteum to
pull on the bone for movement of the skeleton. In other places, the mysia
may fuse with a broad, tendon-like sheet called an aponeurosis, or to
fascia, the connective tissue between skin and bones. The broad sheet of
connective tissue in the lower back that the latissimus dorsi muscles (the
“lats”) fuse into is an example of an aponeurosis.
Every skeletal muscle is also richly supplied by blood vessels for
nourishment, oxygen delivery, and waste removal. In addition, every
muscle fiber in a skeletal muscle is supplied by the axon branch of a
somatic motor neuron, which signals the fiber to contract. Unlike cardiac
and smooth muscle, the only way to functionally contract a skeletal muscle
is through signaling from the nervous system.
Skeletal Muscle Fibers
Because skeletal muscle cells are long and cylindrical, they are commonly
referred to as muscle fibers. Skeletal muscle fibers can be quite large for
human cells, with diameters up to 100 ym and lengths up to 30 cm (11.8 in)
in the Sartorius of the upper leg. During early development, embryonic
myoblasts, each with its own nucleus, fuse with up to hundreds of other
myoblasts to form the multinucleated skeletal muscle fibers. Multiple nuclei
mean multiple copies of genes, permitting the production of the large
amounts of proteins and enzymes needed for muscle contraction.
Some other terminology associated with muscle fibers is rooted in the
Greek sarco, which means “flesh.” The plasma membrane of muscle fibers
is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and
the specialized smooth endoplasmic reticulum, which stores, releases, and
retrieves calcium ions (Ca™*) is called the sarcoplasmic reticulum (SR)
({link]). As will soon be described, the functional unit of a skeletal muscle
fiber is the sarcomere, a highly organized arrangement of the contractile
myofilaments actin (thin filament) and myosin (thick filament), along with
other support proteins.
Muscle Fiber
Nucleus Muscle fiber a
——— eg _C
Mitochondrion
Sarcolemma
Light |
band
Dark A band
Sarcoplasmic Sarcomere
reticulum
Thin (actin) Thick (myosin)
filament Z disc H zone Z disc filament
| band A band | band M line
A skeletal muscle fiber is surrounded by a
plasma membrane called the sarcolemma,
which contains sarcoplasm, the cytoplasm of
muscle cells. A muscle fiber is composed of
many fibrils, which give the cell its striated
appearance.
The Sarcomere
The striated appearance of skeletal muscle fibers is due to the arrangement
of the myofilaments of actin and myosin in sequential order from one end
of the muscle fiber to the other. Each packet of these microfilaments and
their regulatory proteins, troponin and tropomyosin (along with other
proteins) is called a sarcomere.
Note:
-
openstax COLLEGE”
Watch this video to learn more about macro- and microstructures of
skeletal muscles. (a) What are the names of the “junction points” between
sarcomeres? (b) What are the names of the “subunits” within the
myofibrils that run the length of skeletal muscle fibers? (c) What is the
“double strand of pearls” described in the video? (d) What gives a skeletal
muscle fiber its striated appearance?
The sarcomere is the functional unit of the muscle fiber. The sarcomere
itself is bundled within the myofibril that runs the entire length of the
muscle fiber and attaches to the sarcolemma at its end. As myofibrils
contract, the entire muscle cell contracts. Because myofibrils are only
approximately 1.2 ym in diameter, hundreds to thousands (each with
thousands of sarcomeres) can be found inside one muscle fiber. Each
sarcomere is approximately 2 pm in length with a three-dimensional
cylinder-like arrangement and is bordered by structures called Z-discs (also
called Z-lines, because pictures are two-dimensional), to which the actin
myofilaments are anchored ({link]). Because the actin and its troponin-
tropomyosin complex (projecting from the Z-discs toward the center of the
sarcomere) form strands that are thinner than the myosin, it is called the
thin filament of the sarcomere. Likewise, because the myosin strands and
their multiple heads (projecting from the center of the sarcomere, toward
but not all to way to, the Z-discs) have more mass and are thicker, they are
called the thick filament of the sarcomere.
The Sarcomere
Sarcomere
H zone
Lighter! band Darker A band Lighter | band
Portion of a Portion of a
thick filament thin filament
| Troponin Actin — Tropomyosin
mi = 4 — ax
Actin-binding sites
Binding site Actin subunits
for myosin
ATP-binding site
Tail Heads
Myosin molecule Flexible hinge region
The sarcomere, the region from one Z-line
to the next Z-line, is the functional unit of a
skeletal muscle fiber.
The Neuromuscular Junction
Another specialization of the skeletal muscle is the site where a motor
neuron’s terminal meets the muscle fiber—called the neuromuscular
junction (NMJ). This is where the muscle fiber first responds to signaling
by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is
innervated by a motor neuron at the NMJ. Excitation signals from the
neuron are the only way to functionally activate the fiber to contract.
Note:
— :
mess Openstax COLLEGE
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Every skeletal muscle fiber is supplied by a motor neuron at the NMJ.
Watch this video to learn more about what happens at the NMJ. (a) What is
the definition of a motor unit? (b) What is the structural and functional
difference between a large motor unit and a small motor unit? (c) Can you
give an example of each? (d) Why is the neurotransmitter acetylcholine
degraded after binding to its receptor?
Excitation-Contraction Coupling
All living cells have membrane potentials, or electrical gradients across
their membranes. The inside of the membrane is usually around -60 to -90
mV, relative to the outside. This is referred to as a cell’s membrane
potential. Neurons and muscle cells can use their membrane potentials to
generate electrical signals. They do this by controlling the movement of
charged particles, called ions, across their membranes to create electrical
currents. This is achieved by opening and closing specialized proteins in the
membrane called ion channels. Although the currents generated by ions
moving through these channel proteins are very small, they form the basis
of both neural signaling and muscle contraction.
Both neurons and skeletal muscle cells are electrically excitable, meaning
that they are able to generate action potentials. An action potential is a
special type of electrical signal that can travel along a cell membrane as a
wave. This allows a signal to be transmitted quickly and faithfully over long
distances.
Although the term excitation-contraction coupling confuses or scares
some students, it comes down to this: for a skeletal muscle fiber to contract,
its membrane must first be “excited”—in other words, it must be stimulated
to fire an action potential. The muscle fiber action potential, which sweeps
along the sarcolemma as a wave, is “coupled” to the actual contraction
through the release of calcium ions (Ca**) from the SR. Once released, the
Ca** interacts with the shielding proteins, forcing them to move aside so
that the actin-binding sites are available for attachment by myosin heads.
The myosin then pulls the actin filaments toward the center, shortening the
muscle fiber.
In skeletal muscle, this sequence begins with signals from the somatic
motor division of the nervous system. In other words, the “excitation” step
in skeletal muscles is always triggered by signaling from the nervous
system ([link]).
Motor End-Plate and Innervation
Myelin sheath surrounding
axon of motor neuron
Axon terminal
Synaptic end bulb at the
Sarcolemma neuromuscular junction
Myofibril of
muscle fiber
Sarcoplasm
Synaptic end bulb
Synaptic vesicle
containing ACh Nerve impulse
(action potential)
Sarcolemma
Synaptic
cleft
Motor end-
plate
ACh
Synaptic
vesicle
Synaptic
vesicle
releases
ACh by e° :
exocytosis __ 9 = Synaptic cleft
ACh receptor
Motor
Binding of end-plate
ACh to its
receptor opens
the channel
At the NMJ, the axon terminal releases
ACh. The motor end-plate is the location
of the ACh-receptors in the muscle fiber
sarcolemma. When ACh molecules are
released, they diffuse across a minute
space called the synaptic cleft and bind to
the receptors.
The motor neurons that tell the skeletal muscle fibers to contract originate
in the spinal cord, with a smaller number located in the brainstem for
activation of skeletal muscles of the face, head, and neck. These neurons
have long processes, called axons, which are specialized to transmit action
potentials long distances— in this case, all the way from the spinal cord to
the muscle itself (which may be up to three feet away). The axons of
multiple neurons bundle together to form nerves, like wires bundled
together in a cable.
Signaling begins when a neuronal action potential travels along the axon
of a motor neuron, and then along the individual branches to terminate at
the NMJ. At the NMJ, the axon terminal releases a chemical messenger, or
neurotransmitter, called acetylcholine (ACh). The ACh molecules diffuse
across a minute space called the synaptic cleft and bind to ACh receptors
located within the motor end-plate of the sarcolemma on the other side of
the synapse. Once ACh binds, a channel in the ACh receptor opens and
positively charged ions can pass through into the muscle fiber, causing it to
depolarize, meaning that the membrane potential of the muscle fiber
becomes less negative (closer to zero.)
As the membrane depolarizes, another set of ion channels called voltage-
gated sodium channels are triggered to open. Sodium ions enter the
muscle fiber, and an action potential rapidly spreads (or “fires”) along the
entire membrane to initiate excitation-contraction coupling.
Things happen very quickly in the world of excitable membranes (just think
about how quickly you can snap your fingers as soon as you decide to do
it). Immediately following depolarization of the membrane, it repolarizes,
re-establishing the negative membrane potential. Meanwhile, the ACh in
the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE)
so that the ACh cannot rebind to a receptor and reopen its channel, which
would cause unwanted extended muscle excitation and contraction.
Propagation of an action potential along the sarcolemma is the excitation
portion of excitation-contraction coupling. Recall that this excitation
actually triggers the release of calcium ions (Ca**) from its storage in the
cell’s SR. For the action potential to reach the membrane of the SR, there
are periodic invaginations in the sarcolemma, called T-tubules (“T” stands
for “transverse”). You will recall that the diameter of a muscle fiber can be
up to 100 pm, so these T-tubules ensure that the membrane can get close to
the SR in the sarcoplasm. The arrangement of a T-tubule with the
membranes of SR on either side is called a triad ([link]). The triad
surrounds the cylindrical structure called a myofibril, which contains actin
and myosin.
The T-tubule
Sarcolemma
Sarcoplasmic
reticulum
Terminal cisternae
Narrow T-tubules permit the
conduction of electrical impulses. The
SR functions to regulate intracellular
levels of calcium. Two terminal
cisternae (where enlarged SR connects
to the T-tubule) and one T-tubule
comprise a triad—a “threesome” of
membranes, with those of SR on two
sides and the T-tubule sandwiched
between them.
The T-tubules carry the action potential into the interior of the cell, which
triggers the opening of calcium channels in the membrane of the adjacent
SR, causing Ca** to diffuse out of the SR and into the sarcoplasm. It is the
arrival of Ca** in the sarcoplasm that initiates contraction of the muscle
fiber by its contractile units, or sarcomeres.
Chapter Review
Skeletal muscles contain connective tissue, blood vessels, and nerves. There
are three layers of connective tissue: epimysium, perimysium, and
endomysium. Skeletal muscle fibers are organized into groups called
fascicles. Blood vessels and nerves enter the connective tissue and branch
in the cell. Muscles attach to bones directly or through tendons or
aponeuroses. Skeletal muscles maintain posture, stabilize bones and joints,
control internal movement, and generate heat.
Skeletal muscle fibers are long, multinucleated cells. The membrane of the
cell is the sarcolemma; the cytoplasm of the cell is the sarcoplasm. The
sarcoplasmic reticulum (SR) is a form of endoplasmic reticulum. Muscle
fibers are composed of myofibrils. The striations are created by the
organization of actin and myosin resulting in the banding pattern of
myofibrils.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn more about macro- and microstructures of
skeletal muscles. (a) What are the names of the “junction points”
between sarcomeres? (b) What are the names of the “subunits” within
the myofibrils that run the length of skeletal muscle fibers? (c) What is
the “double strand of pearls” described in the video? (d) What gives a
skeletal muscle fiber its striated appearance?
Solution:
(a) Z-lines. (b) Sarcomeres. (c) This is the arrangement of the actin and
myosin filaments in a sarcomere. (d) The alternating strands of actin
and myosin filaments.
Exercise:
Problem:
Every skeletal muscle fiber is supplied by a motor neuron at the NMJ.
Watch this video to learn more about what happens at the
neuromuscular junction. (a) What is the definition of a motor unit? (b)
What is the structural and functional difference between a large motor
unit and a small motor unit? Can you give an example of each? (c)
Why is the neurotransmitter acetylcholine degraded after binding to its
receptor?
Solution:
(a) It is the number of skeletal muscle fibers supplied by a single motor
neuron. (b) A large motor unit has one neuron supplying many skeletal
muscle fibers for gross movements, like the Temporalis muscle, where
1000 fibers are supplied by one neuron. A small motor has one neuron
supplying few skeletal muscle fibers for very fine movements, like the
extraocular eye muscles, where six fibers are supplied by one neuron.
(c) To avoid prolongation of muscle contraction.
Review Questions
Exercise:
Problem:
The correct order for the smallest to the largest unit of organization in
muscle tissue is
a. fascicle, filament, muscle fiber, myofibril
b. filament, myofibril, muscle fiber, fascicle
c. muscle fiber, fascicle, filament, myofibril
d. myofibril, muscle fiber, filament, fascicle
Solution:
B
Exercise:
Problem: Depolarization of the sarcolemma means
a. the inside of the membrane has become less negative as sodium
ions accumulate
b. the outside of the membrane has become less negative as sodium
ions accumulate
c. the inside of the membrane has become more negative as sodium
ions accumulate
d. the sarcolemma has completely lost any electrical charge
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
What would happen to skeletal muscle if the epimysium were
destroyed?
Solution:
Muscles would lose their integrity during powerful movements,
resulting in muscle damage.
Exercise:
Problem: Describe how tendons facilitate body movement.
Solution:
When a muscle contracts, the force of movement is transmitted
through the tendon, which pulls on the bone to produce skeletal
movement.
Exercise:
Problem: What are the five primary functions of skeletal muscle?
Solution:
Produce movement of the skeleton, maintain posture and body
position, support soft tissues, encircle openings of the digestive,
urinary, and other tracts, and maintain body temperature.
Exercise:
Problem:
What are the opposite roles of voltage-gated sodium channels and
voltage-gated potassium channels?
Solution:
The opening of voltage-gated sodium channels, followed by the influx
of Na’, transmits an Action Potential after the membrane has
sufficiently depolarized. The delayed opening of potassium channels
allows K" to exit the cell, to repolarize the membrane.
Glossary
acetylcholine (ACh)
neurotransmitter that binds at a motor end-plate to trigger
depolarization
actin
protein that makes up most of the thin myofilaments in a sarcomere
muscle fiber
action potential
change in voltage of a cell membrane in response to a stimulus that
results in transmission of an electrical signal; unique to neurons and
muscle fibers
aponeurosis
broad, tendon-like sheet of connective tissue that attaches a skeletal
muscle to another skeletal muscle or to a bone
depolarize
to reduce the voltage difference between the inside and outside of a
cell’s plasma membrane (the sarcolemma for a muscle fiber), making
the inside less negative than at rest
endomysium
loose, and well-hydrated connective tissue covering each muscle fiber
in a skeletal muscle
epimysium
outer layer of connective tissue around a skeletal muscle
excitation-contraction coupling
sequence of events from motor neuron signaling to a skeletal muscle
fiber to contraction of the fiber’s sarcomeres
fascicle
bundle of muscle fibers within a skeletal muscle
motor end-plate
sarcolemma of muscle fiber at the neuromuscular junction, with
receptors for the neurotransmitter acetylcholine
myofibril
long, cylindrical organelle that runs parallel within the muscle fiber
and contains the sarcomeres
myosin
protein that makes up most of the thick cylindrical myofilament within
a sarcomere muscle fiber
neuromuscular junction (NMJ)
synapse between the axon terminal of a motor neuron and the section
of the membrane of a muscle fiber with receptors for the acetylcholine
released by the terminal
neurotransmitter
signaling chemical released by nerve terminals that bind to and
activate receptors on target cells
perimysium
connective tissue that bundles skeletal muscle fibers into fascicles
within a skeletal muscle
sarcomere
longitudinally, repeating functional unit of skeletal muscle, with all of
the contractile and associated proteins involved in contraction
sarcolemma
plasma membrane of a skeletal muscle fiber
sarcoplasm
cytoplasm of a muscle cell
sarcoplasmic reticulum (SR)
specialized smooth endoplasmic reticulum, which stores, releases, and
retrieves Ca**
synaptic cleft
space between a nerve (axon) terminal and a motor end-plate
T-tubule
projection of the sarcolemma into the interior of the cell
thick filament
the thick myosin strands and their multiple heads projecting from the
center of the sarcomere toward, but not all to way to, the Z-discs
thin filament
thin strands of actin and its troponin-tropomyosin complex projecting
from the Z-discs toward the center of the sarcomere
triad
the grouping of one T-tubule and two terminal cisternae
troponin
regulatory protein that binds to actin, tropomyosin, and calcium
tropomyosin
regulatory protein that covers myosin-binding sites to prevent actin
from binding to myosin
voltage-gated sodium channels
membrane proteins that open sodium channels in response to a
sufficient voltage change, and initiate and transmit the action potential
as Na* enters through the channel
Types of Muscle Fibers
By the end of this section, you will be able to:
e Describe the types of skeletal muscle fibers
e Explain fast and slow muscle fibers
Two criteria to consider when classifying the types of muscle fibers are how
fast some fibers contract relative to others, and how fibers produce ATP.
Using these criteria, there are three main types of skeletal muscle fibers.
Slow oxidative (SO) fibers contract relatively slowly and use aerobic
respiration (oxygen and glucose) to produce ATP. Fast oxidative (FO)
fibers have fast contractions and primarily use aerobic respiration, but
because they may switch to anaerobic respiration (glycolysis), can fatigue
more quickly than SO fibers. Lastly, fast glycolytic (FG) fibers have fast
contractions and primarily use anaerobic glycolysis. The FG fibers fatigue
more quickly than the others. Most skeletal muscles in a human contain(s)
all three types, although in varying proportions.
The speed of contraction is dependent on how quickly myosin’s ATPase
hydrolyzes ATP to produce cross-bridge action. Fast fibers hydrolyze ATP
approximately twice as quickly as slow fibers, resulting in much quicker
cross-bridge cycling (which pulls the thin filaments toward the center of the
sarcomeres at a faster rate). The primary metabolic pathway used by a
muscle fiber determines whether the fiber is classified as oxidative or
glycolytic. If a fiber primarily produces ATP through aerobic pathways it is
oxidative. More ATP can be produced during each metabolic cycle, making
the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP
through anaerobic glycolysis, which produces less ATP per cycle. As a
result, glycolytic fibers fatigue at a quicker rate.
The oxidative fibers contain many more mitochondria than the glycolytic
fibers, because aerobic metabolism, which uses oxygen (O>) in the
metabolic pathway, occurs in the mitochondria. The SO fibers possess a
large number of mitochondria and are capable of contracting for longer
periods because of the large amount of ATP they can produce, but they have
a relatively small diameter and do not produce a large amount of tension.
SO fibers are extensively supplied with blood capillaries to supply Oz from
the red blood cells in the bloodstream. The SO fibers also possess
myoglobin, an O»-carrying molecule similar to Oj-carrying hemoglobin in
the red blood cells. The myoglobin stores some of the needed O> within the
fibers themselves (and gives SO fibers their red color). All of these features
allow SO fibers to produce large quantities of ATP, which can sustain
muscle activity without fatiguing for long periods of time.
The fact that SO fibers can function for long periods without fatiguing
makes them useful in maintaining posture, producing isometric
contractions, stabilizing bones and joints, and making small movements that
happen often but do not require large amounts of energy. They do not
produce high tension, and thus they are not used for powerful, fast
movements that require high amounts of energy and rapid cross-bridge
cycling.
FO fibers are sometimes called intermediate fibers because they possess
characteristics that are intermediate between fast fibers and slow fibers.
They produce ATP relatively quickly, more quickly than SO fibers, and thus
can produce relatively high amounts of tension. They are oxidative because
they produce ATP aerobically, possess high amounts of mitochondria, and
do not fatigue quickly. However, FO fibers do not possess significant
myoglobin, giving them a lighter color than the red SO fibers. FO fibers are
used primarily for movements, such as walking, that require more energy
than postural control but less energy than an explosive movement, such as
sprinting. FO fibers are useful for this type of movement because they
produce more tension than SO fibers but they are more fatigue-resistant
than FG fibers.
FG fibers primarily use anaerobic glycolysis as their ATP source. They
have a large diameter and possess high amounts of glycogen, which is used
in glycolysis to generate ATP quickly to produce high levels of tension.
Because they do not primarily use aerobic metabolism, they do not possess
substantial numbers of mitochondria or significant amounts of myoglobin
and therefore have a white color. FG fibers are used to produce rapid,
forceful contractions to make quick, powerful movements. These fibers
fatigue quickly, permitting them to only be used for short periods. Most
muscles possess a mixture of each fiber type. The predominant fiber type in
a muscle is determined by the primary function of the muscle.
Chapter Review
ATP provides the energy for muscle contraction. The three mechanisms for
ATP regeneration are creatine phosphate, anaerobic glycolysis, and aerobic
metabolism. Creatine phosphate provides about the first 15 seconds of ATP
at the beginning of muscle contraction. Anaerobic glycolysis produces
small amounts of ATP in the absence of oxygen for a short period. Aerobic
metabolism utilizes oxygen to produce much more ATP, allowing a muscle
to work for longer periods. Muscle fatigue, which has many contributing
factors, occurs when muscle can no longer contract. An oxygen debt is
created as a result of muscle use. The three types of muscle fiber are slow
oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibers use
aerobic metabolism to produce low power contractions over long periods
and are slow to fatigue. FO fibers use aerobic metabolism to produce ATP
but produce higher tension contractions than SO fibers. FG fibers use
anaerobic metabolism to produce powerful, high-tension contractions but
fatigue quickly.
Review Questions
Exercise:
Problem: Muscle fatigue is caused by
a. buildup of ATP and lactic acid levels
b. exhaustion of energy reserves and buildup of lactic acid levels
c. buildup of ATP and pyruvic acid levels
d. exhaustion of energy reserves and buildup of pyruvic acid levels
Solution:
B
Exercise:
Problem:
A sprinter would experience muscle fatigue sooner than a marathon
runner due to
a. anaerobic metabolism in the muscles of the sprinter
b. anaerobic metabolism in the muscles of the marathon runner
c. aerobic metabolism in the muscles of the sprinter
d. glycolysis in the muscles of the marathon runner
Solution:
A
Exercise:
Problem:
What aspect of creatine phosphate allows it to supply energy to
muscles?
a. ATPase activity
b. phosphate bonds
c. carbon bonds
d. hydrogen bonds
Solution:
B
Exercise:
Problem:
Drug X blocks ATP regeneration from ADP and phosphate. How will
muscle cells respond to this drug?
a. by absorbing ATP from the bloodstream
b. by using ADP as an energy source
c. by using glycogen as an energy source
d. none of the above
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Why do muscle cells use creatine phosphate instead of glycolysis to
supply ATP for the first few seconds of muscle contraction?
Solution:
Creatine phosphate is used because creatine phosphate and ADP are
converted very quickly into ATP by creatine kinase. Glycolysis cannot
generate ATP as quickly as creatine phosphate.
Exercise:
Problem:
Is aerobic respiration more or less efficient than glycolysis? Explain
your answer.
Solution:
Aerobic respiration is much more efficient than anaerobic glycolysis,
yielding 36 ATP per molecule of glucose, as opposed to two ATP
produced by glycolysis.
Glossary
fast glycolytic (FG)
muscle fiber that primarily uses anaerobic glycolysis
fast oxidative (FO)
intermediate muscle fiber that is between slow oxidative and fast
glycolytic fibers
slow oxidative (SO)
muscle fiber that primarily uses aerobic respiration
Exercise and Muscle Performance
By the end of this section, you will be able to:
e Describe hypertrophy and atrophy
e Explain how resistance exercise builds muscle
e Explain how performance-enhancing substances affect muscle
Physical training alters the appearance of skeletal muscles and can produce
changes in muscle performance. Conversely, a lack of use can result in
decreased performance and muscle appearance. Although muscle cells can
change in size, new cells are not formed when muscles grow. Instead,
structural proteins are added to muscle fibers in a process called
hypertrophy, so cell diameter increases. The reverse, when structural
proteins are lost and muscle mass decreases, is called atrophy. Age-related
muscle atrophy is called sarcopenia. Cellular components of muscles can
also undergo changes in response to changes in muscle use.
Endurance Exercise
Slow fibers are predominantly used in endurance exercises that require little
force but involve numerous repetitions. The aerobic metabolism used by
slow-twitch fibers allows them to maintain contractions over long periods.
Endurance training modifies these slow fibers to make them even more
efficient by producing more mitochondria to enable more aerobic
metabolism and more ATP production. Endurance exercise can also
increase the amount of myoglobin in a cell, as increased aerobic respiration
increases the need for oxygen. Myoglobin is found in the sarcoplasm and
acts as an oxygen storage supply for the mitochondria.
The training can trigger the formation of more extensive capillary networks
around the fiber, a process called angiogenesis, to supply oxygen and
remove metabolic waste. To allow these capillary networks to supply the
deep portions of the muscle, muscle mass does not greatly increase in order
to maintain a smaller area for the diffusion of nutrients and gases. All of
these cellular changes result in the ability to sustain low levels of muscle
contractions for greater periods without fatiguing.
The proportion of SO muscle fibers in muscle determines the suitability of
that muscle for endurance, and may benefit those participating in endurance
activities. Postural muscles have a large number of SO fibers and relatively
few FO and FG fibers, to keep the back straight ({link]). Endurance athletes,
like marathon-runners also would benefit from a larger proportion of SO
fibers, but it is unclear if the most-successful marathoners are those with
naturally high numbers of SO fibers, or whether the most successful
marathon runners develop high numbers of SO fibers with repetitive
training. Endurance training can result in overuse injuries such as stress
fractures and joint and tendon inflammation.
Marathoners
a
Long-distance runners have a large
number of SO fibers and relatively few
FO and FG fibers. (credit:
“Tseo2”/Wikimedia Commons)
Resistance Exercise
Resistance exercises, as opposed to endurance exercise, require large
amounts of FG fibers to produce short, powerful movements that are not
repeated over long periods. The high rates of ATP hydrolysis and cross-
bridge formation in FG fibers result in powerful muscle contractions.
Muscles used for power have a higher ratio of FG to SO/FO fibers, and
trained athletes possess even higher levels of FG fibers in their muscles.
Resistance exercise affects muscles by increasing the formation of
myofibrils, thereby increasing the thickness of muscle fibers. This added
structure causes hypertrophy, or the enlargement of muscles, exemplified by
the large skeletal muscles seen in body builders and other athletes ([link]).
Because this muscular enlargement is achieved by the addition of structural
proteins, athletes trying to build muscle mass often ingest large amounts of
protein.
Hypertrophy
Body builders have a large number of
FG fibers and relatively few FO and
SO fibers. (credit: Lin Mei/flickr)
Except for the hypertrophy that follows an increase in the number of
sarcomeres and myofibrils in a skeletal muscle, the cellular changes
observed during endurance training do not usually occur with resistance
training. There is usually no significant increase in mitochondria or
capillary density. However, resistance training does increase the
development of connective tissue, which adds to the overall mass of the
muscle and helps to contain muscles as they produce increasingly powerful
contractions. Tendons also become stronger to prevent tendon damage, as
the force produced by muscles is transferred to tendons that attach the
muscle to bone.
For effective strength training, the intensity of the exercise must continually
be increased. For instance, continued weight lifting without increasing the
weight of the load does not increase muscle size. To produce ever-greater
results, the weights lifted must become increasingly heavier, making it more
difficult for muscles to move the load. The muscle then adapts to this
heavier load, and an even heavier load must be used if even greater muscle
mass is desired.
If done improperly, resistance training can lead to overuse injuries of the
muscle, tendon, or bone. These injuries can occur if the load is too heavy or
if the muscles are not given sufficient time between workouts to recover or
if joints are not aligned properly during the exercises. Cellular damage to
muscle fibers that occurs after intense exercise includes damage to the
sarcolemma and myofibrils. This muscle damage contributes to the feeling
of soreness after strenuous exercise, but muscles gain mass as this damage
is repaired, and additional structural proteins are added to replace the
damaged ones. Overworking skeletal muscles can also lead to tendon
damage and even skeletal damage if the load is too great for the muscles to
bear.
Performance-Enhancing Substances
Some athletes attempt to boost their performance by using various agents
that may enhance muscle performance. Anabolic steroids are one of the
more widely known agents used to boost muscle mass and increase power
output. Anabolic steroids are a form of testosterone, a male sex hormone
that stimulates muscle formation, leading to increased muscle mass.
Endurance athletes may also try to boost the availability of oxygen to
muscles to increase aerobic respiration by using substances such as
erythropoietin (EPO), a hormone normally produced in the kidneys, which
triggers the production of red blood cells. The extra oxygen carried by these
blood cells can then be used by muscles for aerobic respiration. Human
growth hormone (hGH) is another supplement, and although it can facilitate
building muscle mass, its main role is to promote the healing of muscle and
other tissues after strenuous exercise. Increased hGH may allow for faster
recovery after muscle damage, reducing the rest required after exercise, and
allowing for more sustained high-level performance.
Although performance-enhancing substances often do improve
performance, most are banned by governing bodies in sports and are illegal
for nonmedical purposes. Their use to enhance performance raises ethical
issues of cheating because they give users an unfair advantage over
nonusers. A greater concern, however, is that their use carries serious health
risks. The side effects of these substances are often significant,
nonreversible, and in some cases fatal. The physiological strain caused by
these substances is often greater than what the body can handle, leading to
effects that are unpredictable and dangerous. Anabolic steroid use has been
linked to infertility, aggressive behavior, cardiovascular disease, and brain
cancer.
Similarly, some athletes have used creatine to increase power output.
Creatine phosphate provides quick bursts of ATP to muscles in the initial
stages of contraction. Increasing the amount of creatine available to cells is
thought to produce more ATP and therefore increase explosive power
output, although its effectiveness as a supplement has been questioned.
Note:
Everyday Connection
Aging and Muscle Tissue
Although atrophy due to disuse can often be reversed with exercise, muscle
atrophy with age, referred to as sarcopenia, is irreversible. This is a
primary reason why even highly trained athletes succumb to declining
performance with age. This decline is noticeable in athletes whose sports
require strength and powerful movements, such as sprinting, whereas the
effects of age are less noticeable in endurance athletes such as marathon
runners or long-distance cyclists. As muscles age, muscle fibers die, and
they are replaced by connective tissue and adipose tissue ([link]). Because
those tissues cannot contract and generate force as muscle can, muscles
lose the ability to produce powerful contractions. The decline in muscle
mass causes a loss of strength, including the strength required for posture
and mobility. This may be caused by a reduction in FG fibers that
hydrolyze ATP quickly to produce short, powerful contractions. Muscles in
older people sometimes possess greater numbers of SO fibers, which are
responsible for longer contractions and do not produce powerful
movements. There may also be a reduction in the size of motor units,
resulting in fewer fibers being stimulated and less muscle tension being
produced.
Atrophy
{ )
Atrophied
Muscle
Normal
Muscle
Muscle mass is reduced as muscles
atrophy with disuse.
Sarcopenia can be delayed to some extent by exercise, as training adds
structural proteins and causes cellular changes that can offset the effects of
atrophy. Increased exercise can produce greater numbers of cellular
mitochondria, increase capillary density, and increase the mass and
strength of connective tissue. The effects of age-related atrophy are
especially pronounced in people who are sedentary, as the loss of muscle
cells is displayed as functional impairments such as trouble with
locomotion, balance, and posture. This can lead to a decrease in quality of
life and medical problems, such as joint problems because the muscles that
stabilize bones and joints are weakened. Problems with locomotion and
balance can also cause various injuries due to falls.
Chapter Review
Hypertrophy is an increase in muscle mass due to the addition of structural
proteins. The opposite of hypertrophy is atrophy, the loss of muscle mass
due to the breakdown of structural proteins. Endurance exercise causes an
increase in cellular mitochondria, myoglobin, and capillary networks in SO
fibers. Endurance athletes have a high level of SO fibers relative to the
other fiber types. Resistance exercise causes hypertrophy. Power-producing
muscles have a higher number of FG fibers than of slow fibers. Strenuous
exercise causes muscle cell damage that requires time to heal. Some
athletes use performance-enhancing substances to enhance muscle
performance. Muscle atrophy due to age is called sarcopenia and occurs as
muscle fibers die and are replaced by connective and adipose tissue.
Review Questions
Exercise:
Problem:
The muscles of a professional sprinter are most likely to have
a. 80 percent fast-twitch muscle fibers and 20 percent slow-twitch
muscle fibers
b. 20 percent fast-twitch muscle fibers and 80 percent slow-twitch
muscle fibers
c. 50 percent fast-twitch muscle fibers and 50 percent slow-twitch
muscle fibers
d. 40 percent fast-twitch muscle fibers and 60 percent slow-twitch
muscle fibers
Solution:
A
Exercise:
Problem:
The muscles of a professional marathon runner are most likely to have
a. 80 percent fast-twitch muscle fibers and 20 percent slow-twitch
muscle fibers
b. 20 percent fast-twitch muscle fibers and 80 percent slow-twitch
muscle fibers
c. 50 percent fast-twitch muscle fibers and 50 percent slow-twitch
muscle fibers
d. 40 percent fast-twitch muscle fibers and 60 percent slow-twitch
muscle fibers
Solution:
B
Exercise:
Problem: Which of the following statements is true?
a. Fast fibers have a small diameter.
b. Fast fibers contain loosely packed myofibrils.
c. Fast fibers have large glycogen reserves.
d. Fast fibers have many mitochondria.
Solution:
Cc
Exercise:
Problem: Which of the following statements is false?
a. Slow fibers have a small network of capillaries.
b. Slow fibers contain the pigment myoglobin.
c. Slow fibers contain a large number of mitochondria.
d. Slow fibers contract for extended periods.
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
What changes occur at the cellular level in response to endurance
training?
Solution:
Endurance training modifies slow fibers to make them more efficient
by producing more mitochondria to enable more aerobic metabolism
and more ATP production. Endurance exercise can also increase the
amount of myoglobin in a cell and formation of more extensive
capillary networks around the fiber.
Exercise:
Problem:
What changes occur at the cellular level in response to resistance
training?
Solution:
Resistance exercises affect muscles by causing the formation of more
actin and myosin, increasing the structure of muscle fibers.
Glossary
angiogenesis
formation of blood capillary networks
atrophy
loss of structural proteins from muscle fibers
hypertrophy
addition of structural proteins to muscle fibers
Sarcopenia
age-related muscle atrophy
Cardiac Muscle Tissue
By the end of this section, you will be able to:
¢ Describe intercalated discs and gap junctions
e Describe a desmosome
Cardiac muscle tissue is only found in the heart. Highly coordinated
contractions of cardiac muscle pump blood into the vessels of the
circulatory system. Similar to skeletal muscle, cardiac muscle is striated and
organized into sarcomeres, possessing the same banding organization as
skeletal muscle ({link]). However, cardiac muscle fibers are shorter than
skeletal muscle fibers and usually contain only one nucleus, which is
located in the central region of the cell. Cardiac muscle fibers also possess
many mitochondria and myoglobin, as ATP is produced primarily through
aerobic metabolism. Cardiac muscle fibers cells also are extensively
branched and are connected to one another at their ends by intercalated
discs. An intercalated disc allows the cardiac muscle cells to contract in a
wave-like pattern so that the heart can work as a pump.
Cardiac Muscle Tissue
Cardiac muscle tissue is only found in the
heart. LM x 1600. (Micrograph provided
by the Regents of University of Michigan
Medical School © 2012)
Note:
==
mmm ~OPENStaX COLLEGE’
—
—
. a
.
Dy)
r |
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
Intercalated discs are part of the sarcolemma and contain two structures
important in cardiac muscle contraction: gap junctions and desmosomes. A
gap junction forms channels between adjacent cardiac muscle fibers that
allow the depolarizing current produced by cations to flow from one cardiac
muscle cell to the next. This joining is called electric coupling, and in
cardiac muscle it allows the quick transmission of action potentials and the
coordinated contraction of the entire heart. This network of electrically
connected cardiac muscle cells creates a functional unit of contraction
called a syncytium. The remainder of the intercalated disc is composed of
desmosomes. A desmosome is a cell structure that anchors the ends of
cardiac muscle fibers together so the cells do not pull apart during the stress
of individual fibers contracting ([link]).
Cardiac Muscle
Capillary Intercalated discs
Desmosome
Gap junction
Nucleus Cardiac
muscle fiber
Intercalated discs are part of the cardiac muscle
sarcolemma and they contain gap junctions and
desmosomes.
Contractions of the heart (heartbeats) are controlled by specialized cardiac
muscle cells called pacemaker cells that directly control heart rate.
Although cardiac muscle cannot be consciously controlled, the pacemaker
cells respond to signals from the autonomic nervous system (ANS) to speed
up or slow down the heart rate. The pacemaker cells can also respond to
various hormones that modulate heart rate to control blood pressure.
The wave of contraction that allows the heart to work as a unit, called a
functional syncytium, begins with the pacemaker cells. This group of cells
is self-excitable and able to depolarize to threshold and fire action potentials
on their own, a feature called autorhythmicity; they do this at set intervals
which determine heart rate. Because they are connected with gap junctions
to surrounding muscle fibers and the specialized fibers of the heart’s
conduction system, the pacemaker cells are able to transfer the
depolarization to the other cardiac muscle fibers in a manner that allows the
heart to contract in a coordinated manner.
Another feature of cardiac muscle is its relatively long action potentials in
its fibers, having a sustained depolarization “plateau.” The plateau is
produced by Ca** entry though voltage-gated calcium channels in the
sarcolemma of cardiac muscle fibers. This sustained depolarization (and
Ca** entry) provides for a longer contraction than is produced by an action
potential in skeletal muscle. Unlike skeletal muscle, a large percentage of
the Ca™* that initiates contraction in cardiac muscles comes from outside
the cell rather than from the SR.
Chapter Review
Cardiac muscle is striated muscle that is present only in the heart. Cardiac
muscle fibers have a single nucleus, are branched, and joined to one another
by intercalated discs that contain gap junctions for depolarization between
cells and desmosomes to hold the fibers together when the heart contracts.
Contraction in each cardiac muscle fiber is triggered by Ca™ ions in a
similar manner as skeletal muscle, but here the Ca** ions come from SR
and through voltage-gated calcium channels in the sarcolemma. Pacemaker
cells stimulate the spontaneous contraction of cardiac muscle as a
functional unit, called a syncytium.
Review Questions
Exercise:
Problem:
Cardiac muscles differ from skeletal muscles in that they
a. are striated
b. utilize aerobic metabolism
c. contain myofibrils
d. contain intercalated discs
Solution:
D
Exercise:
Problem:
If cardiac muscle cells were prevented from undergoing aerobic
metabolism, they ultimately would
a. undergo glycolysis
b. synthesize ATP
c. stop contracting
d. start contracting
Solution:
Critical Thinking Questions
Exercise:
Problem:
What would be the drawback of cardiac contractions being the same
duration as skeletal muscle contractions?
Solution:
An action potential could reach a cardiac muscle cell before it has
entered the relaxation phase, resulting in the sustained contractions of
tetanus. If this happened, the heart would not beat regularly.
Exercise:
Problem:
How are cardiac muscle cells similar to and different from skeletal
muscle cells?
Solution:
Cardiac and skeletal muscle cells both contain ordered myofibrils and
are striated. Cardiac muscle cells are branched and contain intercalated
discs, which skeletal muscles do not have.
Glossary
autorhythmicity
heart’s ability to control its own contractions
desmosome
cell structure that anchors the ends of cardiac muscle fibers to allow
contraction to occur
intercalated disc
part of the sarcolemma that connects cardiac tissue, and contains gap
junctions and desmosomes
Smooth Muscle
By the end of this section, you will be able to:
e Describe a dense body
e Explain how smooth muscle works with internal organs and
passageways through the body
e Explain how smooth muscles differ from skeletal and cardiac muscles
e Explain the difference between single-unit and multi-unit smooth
muscle
Smooth muscle (so-named because the cells do not have striations) is
present in the walls of hollow organs like the urinary bladder, uterus,
stomach, intestines, and in the walls of passageways, such as the arteries
and veins of the circulatory system, and the tracts of the respiratory, urinary,
and reproductive systems ({link]ab). Smooth muscle is also present in the
eyes, where it functions to change the size of the iris and alter the shape of
the lens; and in the skin where it causes hair to stand erect in response to
cold temperature or fear.
Smooth Muscle Tissue
Autonomic
neurons
Nucleus
Muscle
fibers
(a)
(b)
Smooth muscle tissue is found around organs in the digestive,
respiratory, reproductive tracts and the iris of the eye. LM x
1600. (Micrograph provided by the Regents of University of
Michigan Medical School © 2012)
Note:
— 7
mess Openstax COLLEGE
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at
both ends, somewhat like a football) and have a single nucleus; they range
from about 30 to 200 pm (thousands of times shorter than skeletal muscle
fibers), and they produce their own connective tissue, endomysium.
Although they do not have striations and sarcomeres, smooth muscle fibers
do have actin and myosin contractile proteins, and thick and thin filaments.
These thin filaments are anchored by dense bodies. A dense body is
analogous to the Z-discs of skeletal and cardiac muscle fibers and is
fastened to the sarcolemma. Calcium ions are supplied by the SR in the
fibers and by sequestration from the extracellular fluid through membrane
indentations called calveoli.
Because smooth muscle cells do not contain troponin, cross-bridge
formation is not regulated by the troponin-tropomyosin complex but instead
by the regulatory protein calmodulin. In a smooth muscle fiber, external
Ca™* ions passing through opened calcium channels in the sarcolemma, and
additional Ca** released from SR, bind to calmodulin. The Ca**-
calmodulin complex then activates an enzyme called myosin (light chain)
kinase, which, in turn, activates the myosin heads by phosphorylating them
(converting ATP to ADP and P;, with the P; attaching to the head). The
heads can then attach to actin-binding sites and pull on the thin filaments.
The thin filaments also are anchored to the dense bodies; the structures
invested in the inner membrane of the sarcolemma (at adherens junctions)
that also have cord-like intermediate filaments attached to them. When the
thin filaments slide past the thick filaments, they pull on the dense bodies,
structures tethered to the sarcolemma, which then pull on the intermediate
filaments networks throughout the sarcoplasm. This arrangement causes the
entire muscle fiber to contract in a manner whereby the ends are pulled
toward the center, causing the midsection to bulge in a corkscrew motion
([link]).
Muscle Contraction
Relaxed muscle cell Contracted muscle cell
—> =
Intermediate filaments Dense bodies
The dense bodies and intermediate filaments are networked
through the sarcoplasm, which cause the muscle fiber to
contract.
Although smooth muscle contraction relies on the presence of Ca** ions,
smooth muscle fibers have a much smaller diameter than skeletal muscle
cells. T-tubules are not required to reach the interior of the cell and
therefore not necessary to transmit an action potential deep into the fiber.
Smooth muscle fibers have a limited calcium-storing SR but have calcium
channels in the sarcolemma (similar to cardiac muscle fibers) that open
during the action potential along the sarcolemma. The influx of
extracellular Ca** ions, which diffuse into the sarcoplasm to reach the
calmodulin, accounts for most of the Ca** that triggers contraction of a
smooth muscle cell.
Muscle contraction continues until ATP-dependent calcium pumps actively
transport Ca** ions back into the SR and out of the cell. However, a low
concentration of calcium remains in the sarcoplasm to maintain muscle
tone. This remaining calcium keeps the muscle slightly contracted, which is
important in certain tracts and around blood vessels.
Because most smooth muscles must function for long periods without rest,
their power output is relatively low, but contractions can continue without
using large amounts of energy. Some smooth muscle can also maintain
contractions even as Ca** is removed and myosin kinase is
inactivated/dephosphorylated. This can happen as a subset of cross-bridges
between myosin heads and actin, called latch-bridges, keep the thick and
thin filaments linked together for a prolonged period, and without the need
for ATP. This allows for the maintaining of muscle “tone” in smooth muscle
that lines arterioles and other visceral organs with very little energy
expenditure.
Smooth muscle is not under voluntary control; thus, it is called involuntary
muscle. The triggers for smooth muscle contraction include hormones,
neural stimulation by the ANS, and local factors. In certain locations, such
as the walls of visceral organs, stretching the muscle can trigger its
contraction (the stress-relaxation response).
Axons of neurons in the ANS do not form the highly organized NMJs with
smooth muscle, as seen between motor neurons and skeletal muscle fibers.
Instead, there is a series of neurotransmitter-filled bulges called varicosities
as an axon courses through smooth muscle, loosely forming motor units
({link]). A varicosity releases neurotransmitters into the synaptic cleft.
Also, visceral muscle in the walls of the hollow organs (except the heart)
contains pacesetter cells. A pacesetter cell can spontaneously trigger action
potentials and contractions in the muscle.
Motor Units
Varicosity
Vesicles with
neurotransmitters .
Autonomic neuron
—— Smooth muscle cells
A series of axon-like swelling, called varicosities or “boutons,”
from autonomic neurons form motor units through the smooth
muscle.
Smooth muscle is organized in two ways: as single-unit smooth muscle,
which is much more common; and as multiunit smooth muscle. The two
types have different locations in the body and have different characteristics.
Single-unit muscle has its muscle fibers joined by gap junctions so that the
muscle contracts as a single unit. This type of smooth muscle is found in the
walls of all visceral organs except the heart (which has cardiac muscle in its
walls), and so it is commonly called visceral muscle. Because the muscle
fibers are not constrained by the organization and stretchability limits of
sarcomeres, visceral smooth muscle has a stress-relaxation response. This
means that as the muscle of a hollow organ is stretched when it fills, the
mechanical stress of the stretching will trigger contraction, but this is
immediately followed by relaxation so that the organ does not empty its
contents prematurely. This is important for hollow organs, such as the
stomach or urinary bladder, which continuously expand as they fill. The
smooth muscle around these organs also can maintain a muscle tone when
the organ empties and shrinks, a feature that prevents “flabbiness” in the
empty organ. In general, visceral smooth muscle produces slow, steady
contractions that allow substances, such as food in the digestive tract, to
move through the body.
Multiunit smooth muscle cells rarely possess gap junctions, and thus are not
electrically coupled. As a result, contraction does not spread from one cell
to the next, but is instead confined to the cell that was originally stimulated.
Stimuli for multiunit smooth muscles come from autonomic nerves or
hormones but not from stretching. This type of tissue is found around large
blood vessels, in the respiratory airways, and in the eyes.
Hyperplasia in Smooth Muscle
Similar to skeletal and cardiac muscle cells, smooth muscle can undergo
hypertrophy to increase in size. Unlike other muscle, smooth muscle can
also divide to produce more cells, a process called hyperplasia. This can
most evidently be observed in the uterus at puberty, which responds to
increased estrogen levels by producing more uterine smooth muscle fibers,
and greatly increases the size of the myometrium.
Sections Summary
Smooth muscle is found throughout the body around various organs and
tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped.
Smooth muscle cells can undergo hyperplasia, mitotically dividing to
produce new cells. The smooth cells are nonstriated, but their sarcoplasm is
filled with actin and myosin, along with dense bodies in the sarcolemma to
anchor the thin filaments and a network of intermediate filaments involved
in pulling the sarcolemma toward the fiber’s middle, shortening it in the
process. Ca** ions trigger contraction when they are released from SR and
enter through opened voltage-gated calcium channels. Smooth muscle
contraction is initiated when the Ca‘™ binds to intracellular calmodulin,
which then activates an enzyme called myosin kinase that phosphorylates
myosin heads so they can form the cross-bridges with actin and then pull on
the thin filaments. Smooth muscle can be stimulated by pacesetter cells, by
the autonomic nervous system, by hormones, spontaneously, or by
stretching. The fibers in some smooth muscle have latch-bridges, cross-
bridges that cycle slowly without the need for ATP; these muscles can
maintain low-level contractions for long periods. Single-unit smooth muscle
tissue contains gap junctions to synchronize membrane depolarization and
contractions so that the muscle contracts as a single unit. Single-unit
smooth muscle in the walls of the viscera, called visceral muscle, has a
stress-relaxation response that permits muscle to stretch, contract, and relax
as the organ expands. Multiunit smooth muscle cells do not possess gap
junctions, and contraction does not spread from one cell to the next.
Multiple Choice
Exercise:
Problem:
Smooth muscles differ from skeletal and cardiac muscles in that they
a. lack myofibrils
b. are under voluntary control
c. lack myosin
d. lack actin
Solution:
A
Exercise:
Problem:
Which of the following statements describes smooth muscle cells?
a. They are resistant to fatigue.
b. They have a rapid onset of contractions.
c. They cannot exhibit tetanus.
d. They primarily use anaerobic metabolism.
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Why can smooth muscles contract over a wider range of resting
lengths than skeletal and cardiac muscle?
Solution:
Smooth muscles can contract over a wider range of resting lengths
because the actin and myosin filaments in smooth muscle are not as
rigidly organized as those in skeletal and cardiac muscle.
Exercise:
Problem:
Describe the differences between single-unit smooth muscle and
multiunit smooth muscle.
Solution:
Single-unit smooth muscle is found in the walls of hollow organs;
multiunit smooth muscle is found in airways to the lungs and large
arteries. Single-unit smooth muscle cells contract synchronously, they
are coupled by gap junctions, and they exhibit spontaneous action
potential. Multiunit smooth cells lack gap junctions, and their
contractions are not synchronous.
Glossary
calmodulin
regulatory protein that facilitates contraction in smooth muscles
dense body
sarcoplasmic structure that attaches to the sarcolemma and shortens the
muscle as thin filaments slide past thick filaments
hyperplasia
process in which one cell splits to produce new cells
latch-bridges
subset of a cross-bridge in which actin and myosin remain locked
together
pacesetter cell
cell that triggers action potentials in smooth muscle
stress-relaxation response
relaxation of smooth muscle tissue after being stretched
varicosity
enlargement of neurons that release neurotransmitters into synaptic
clefts
visceral muscle
smooth muscle found in the walls of visceral organs
Development and Regeneration of Muscle Tissue
By the end of this section, you will be able to:
e Describe the function of satellite cells
e Define fibrosis
e Explain which muscle has the greatest regeneration ability
Most muscle tissue of the body arises from embryonic mesoderm. Paraxial
mesodermal cells adjacent to the neural tube form blocks of cells called
somites. Skeletal muscles, excluding those of the head and limbs, develop
from mesodermal somites, whereas skeletal muscle in the head and limbs
develop from general mesoderm. Somites give rise to myoblasts. A
myoblast is a muscle-forming stem cell that migrates to different regions in
the body and then fuse(s) to form a syncytium, or myotube. As a myotube
is formed from many different myoblast cells, it contains many nuclei, but
has a continuous cytoplasm. This is why skeletal muscle cells are
multinucleate, as the nucleus of each contributing myoblast remains intact
in the mature skeletal muscle cell. However, cardiac and smooth muscle
cells are not multinucleate because the myoblasts that form their cells do
not fuse.
Gap junctions develop in the cardiac and single-unit smooth muscle in the
early stages of development. In skeletal muscles, ACh receptors are initially
present along most of the surface of the myoblasts, but spinal nerve
innervation causes the release of growth factors that stimulate the formation
of motor end-plates and NMJs. As neurons become active, electrical signals
that are sent through the muscle influence the distribution of slow and fast
fibers in the muscle.
Although the number of muscle cells is set during development, satellite
cells help to repair skeletal muscle cells. A satellite cell is similar to a
myoblast because it is a type of stem cell; however, satellite cells are
incorporated into muscle cells and facilitate the protein synthesis required
for repair and growth. These cells are located outside the sarcolemma and
are stimulated to grow and fuse with muscle cells by growth factors that are
released by muscle fibers under certain forms of stress. Satellite cells can
regenerate muscle fibers to a very limited extent, but they primarily help to
repair damage in living cells. If a cell is damaged to a greater extent than
can be repaired by satellite cells, the muscle fibers are replaced by scar
tissue in a process called fibrosis. Because scar tissue cannot contract,
muscle that has sustained significant damage loses strength and cannot
produce the same amount of power or endurance as it could before being
damaged.
Smooth muscle tissue can regenerate from a type of stem cell called a
pericyte, which is found in some small blood vessels. Pericytes allow
smooth muscle cells to regenerate and repair much more readily than
skeletal and cardiac muscle tissue. Similar to skeletal muscle tissue, cardiac
muscle does not regenerate to a great extent. Dead cardiac muscle tissue is
replaced by scar tissue, which cannot contract. As scar tissue accumulates,
the heart loses its ability to pump because of the loss of contractile power.
However, some minor regeneration may occur due to stem cells found in
the blood that occasionally enter cardiac tissue.
Note:
Career Connections
Physical Therapist
As muscle cells die, they are not regenerated but instead are replaced by
connective tissue and adipose tissue, which do not possess the contractile
abilities of muscle tissue. Muscles atrophy when they are not used, and
over time if atrophy is prolonged, muscle cells die. It is therefore important
that those who are susceptible to muscle atrophy exercise to maintain
muscle function and prevent the complete loss of muscle tissue. In extreme
cases, when movement is not possible, electrical stimulation can be
introduced to a muscle from an external source. This acts as a substitute for
endogenous neural stimulation, stimulating the muscle to contract and
preventing the loss of proteins that occurs with a lack of use.
Physiotherapists work with patients to maintain muscles. They are trained
to target muscles susceptible to atrophy, and to prescribe and monitor
exercises designed to stimulate those muscles. There are various causes of
atrophy, including mechanical injury, disease, and age. After breaking a
limb or undergoing surgery, muscle use is impaired and can lead to disuse
atrophy. If the muscles are not exercised, this atrophy can lead to long-term
muscle weakness. A stroke can also cause muscle impairment by
interrupting neural stimulation to certain muscles. Without neural inputs,
these muscles do not contract and thus begin to lose structural proteins.
Exercising these muscles can help to restore muscle function and minimize
functional impairments. Age-related muscle loss is also a target of physical
therapy, as exercise can reduce the effects of age-related atrophy and
improve muscle function.
The goal of a physiotherapist is to improve physical functioning and
reduce functional impairments; this is achieved by understanding the cause
of muscle impairment and assessing the capabilities of a patient, after
which a program to enhance these capabilities is designed. Some factors
that are assessed include strength, balance, and endurance, which are
continually monitored as exercises are introduced to track improvements in
muscle function. Physiotherapists can also instruct patients on the proper
use of equipment, such as crutches, and assess whether someone has
sufficient strength to use the equipment and when they can function
without it.
Chapter Review
Muscle tissue arises from embryonic mesoderm. Somites give rise to
myoblasts and fuse to form a myotube. The nucleus of each contributing
myoblast remains intact in the mature skeletal muscle cell, resulting in a
mature, multinucleate cell. Satellite cells help to repair skeletal muscle
cells. Smooth muscle tissue can regenerate from stem cells called pericytes,
whereas dead cardiac muscle tissue is replaced by scar tissue. Aging causes
muscle mass to decrease and be replaced by noncontractile connective
tissue and adipose tissue.
Review Questions
Exercise:
Problem:
From which embryonic cell type does muscle tissue develop?
a. ganglion cells
b. myotube cells
c. myoblast cells
d. satellite cells
Solution:
C
Exercise:
Problem: Which cell type helps to repair injured muscle fibers?
a. ganglion cells
b. myotube cells
c. myoblast cells
d. satellite cells
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Why is muscle that has sustained significant damage unable to produce
the same amount of power as it could before being damaged?
Solution:
If the damage exceeds what can be repaired by satellite cells, the
damaged tissue is replaced by scar tissue, which cannot contract.
Exercise:
Problem:
Which muscle type(s) (skeletal, smooth, or cardiac) can regenerate
new muscle cells/fibers? Explain your answer.
Solution:
Smooth muscle tissue can regenerate from stem cells called pericytes,
cells found in some small blood vessels. These allow smooth muscle
cells to regenerate and repair much more readily than skeletal and
cardiac muscle tissue.
Glossary
fibrosis
replacement of muscle fibers by scar tissue
myoblast
muscle-forming stem cell
myotube
fusion of many myoblast cells
pericyte
stem cell that regenerates smooth muscle cells
satellite cell
stem cell that helps to repair muscle cells
somites
blocks of paraxial mesoderm cells
Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their
Lever Systems
By the end of this section, you will be able to:
e Compare and contrast agonist and antagonist muscles
e Describe how fascicles are arranged within a skeletal muscle
e Explain the major events of a skeletal muscle contraction within a
muscle in generating force
To move the skeleton, the tension created by the contraction of the fibers in
most skeletal muscles is transferred to the tendons. The tendons are strong
bands of dense, regular connective tissue that connect muscles to bones.
The bone connection is why this muscle tissue is called skeletal muscle.
Interactions of Skeletal Muscles in the Body
To pull on a bone, that is, to change the angle at its synovial joint, which
essentially moves the skeleton, a skeletal muscle must also be attached to a
fixed part of the skeleton. The moveable end of the muscle that attaches to
the bone being pulled is called the muscle’s insertion, and the end of the
muscle attached to a fixed (stabilized) bone is called the origin. During
forearm flexion—bending the elbow—the brachioradialis assists the
brachialis.
Although a number of muscles may be involved in an action, the principal
muscle involved is called the prime mover, or agonist. To lift a cup, a
muscle called the biceps brachii is actually the prime mover; however,
because it can be assisted by the brachialis, the brachialis is called a
synergist in this action ([link]). A synergist can also be a fixator that
stabilizes the bone that is the attachment for the prime mover’s origin.
Prime Movers and Synergists
Biceps brachii
(prime mover)
Brachioradialis
(synergist)
Biceps brachii
(dissected)
Brachialis
(synergist)
Brachioradialis
The biceps brachii flex the lower arm. The
brachoradialis, in the forearm, and brachialis,
located deep to the biceps in the upper arm, are
both synergists that aid in this motion.
A muscle with the opposite action of the prime mover is called an
antagonist. Antagonists play two important roles in muscle function: (1)
they maintain body or limb position, such as holding the arm out or
standing erect; and (2) they control rapid movement, as in shadow boxing
without landing a punch or the ability to check the motion of a limb.
For example, to extend the knee, a group of four muscles called the
quadriceps femoris in the anterior compartment of the thigh are activated
(and would be called the agonists of knee extension). However, to flex the
knee joint, an opposite or antagonistic set of muscles called the hamstrings
is activated.
As you can see, these terms would also be reversed for the opposing action.
If you consider the first action as the knee bending, the hamstrings would
be called the agonists and the quadriceps femoris would then be called the
antagonists. See [link] for a list of some agonists and antagonists.
Agonist and Antagonist Skeletal Muscle Pairs
Agonist
Biceps brachii:
in the anterior
compartment of
the arm
Hamstrings:
group of three
muscles in the
posterior
compartment of
the thigh
Flexor
digitorum
superficialis
and flexor
digitorum
profundus: in
the anterior
compartment of
the forearm
Antagonist
Triceps
brachii: in
the posterior
compartment
of the arm
Quadriceps
femoris:
group of
four muscles
in the
anterior
compartment
of the thigh
Extensor
digitorum: in
the posterior
compartment
of the
forearm
Movement
The biceps brachii flexes the
forearm, whereas the triceps
brachii extends it.
The hamstrings flex the leg,
whereas the quadriceps
femoris extend it.
The flexor digitorum
superficialis and flexor
digitorum profundus flex the
fingers and the hand at the
wrist, whereas the extensor
digitorum extends the fingers
and the hand at the wrist.
There are also skeletal muscles that do not pull against the skeleton for
movements. For example, there are the muscles that produce facial
expressions. The insertions and origins of facial muscles are in the skin, so
that certain individual muscles contract to form a smile or frown, form
sounds or words, and raise the eyebrows. There also are skeletal muscles in
the tongue, and the external urinary and anal sphincters that allow for
voluntary regulation of urination and defecation, respectively. In addition,
the diaphragm contracts and relaxes to change the volume of the pleural
cavities but it does not move the skeleton to do this.
Note:
Everyday Connections
Exercise and Stretching
When exercising, it is important to first warm up the muscles. Stretching
pulls on the muscle fibers and it also results in an increased blood flow to
the muscles being worked. Without a proper warm-up, it is possible that
you may either damage some of the muscle fibers or pull a tendon. A
pulled tendon, regardless of location, results in pain, swelling, and
diminished function; if it is moderate to severe, the injury could
immobilize you for an extended period.
Recall the discussion about muscles crossing joints to create movement.
Most of the joints you use during exercise are synovial joints, which have
synovial fluid in the joint space between two bones. Exercise and
stretching may also have a beneficial effect on synovial joints. Synovial
fluid is a thin, but viscous film with the consistency of egg whites. When
you first get up and start moving, your joints feel stiff for a number of
reasons. After proper stretching and warm-up, the synovial fluid may
become less viscous, allowing for better joint function.
Patterns of Fascicle Organization
Skeletal muscle is enclosed in connective tissue scaffolding at three levels.
Each muscle fiber (cell) is covered by endomysium and the entire muscle is
covered by epimysium. When a group of muscle fibers is “bundled” as a
unit within the whole muscle by an additional covering of a connective
tissue called perimysium, that bundled group of muscle fibers is called a
fascicle. Fascicle arrangement by perimysia is correlated to the force
generated by a muscle; it also affects the range of motion of the muscle.
Based on the patterns of fascicle arrangement, skeletal muscles can be
classified in several ways. What follows are the most common fascicle
arrangements.
Parallel muscles have fascicles that are arranged in the same direction as
the long axis of the muscle ([link]). The majority of skeletal muscles in the
body have this type of organization. Some parallel muscles are flat sheets
that expand at the ends to make broad attachments. Other parallel muscles
are rotund with tendons at one or both ends. Muscles that seem to be plump
have a large mass of tissue located in the middle of the muscle, between the
insertion and the origin, which is known as the central body. A more
common name for this muscle is belly. When a muscle contracts, the
contractile fibers shorten it to an even larger bulge. For example, extend and
then flex your biceps brachii muscle; the large, middle section is the belly
({link]). When a parallel muscle has a central, large belly that is spindle-
shaped, meaning it tapers as it extends to its origin and insertion, it
sometimes is called fusiform.
Muscle Shapes and Fiber Alignment
Orbicularis oris
Circular
Deltoid
Multipennate
Convergent
Extensor digitorum
Biceps brachii (posterior view)
To origin
Parallel- Belly
fusiform Unipennate
To insertion ——
Rectus femoris
Bipennate
\\
Parallel
( (non-fusiform)
ma
{
The skeletal muscles of the body typically come in
seven different general shapes.
Biceps Brachii Muscle Contraction
The large mass at the center of a muscle is
called the belly. Tendons emerge from
both ends of the belly and connect the
muscle to the bones, allowing the skeleton
to move. The tendons of the bicep connect
to the upper arm and the forearm. (credit:
Victoria Garcia)
Circular muscles are also called sphincters (see [link]). When they relax,
the sphincters’ concentrically arranged bundles of muscle fibers increase
the size of the opening, and when they contract, the size of the opening
shrinks to the point of closure. The orbicularis oris muscle is a circular
muscle that goes around the mouth. When it contracts, the oral opening
becomes smaller, as when puckering the lips for whistling. Another
example is the orbicularis oculi, one of which surrounds each eye. Consider,
for example, the names of the two orbicularis muscles (orbicularis oris and
oribicularis oculi), where part of the first name of both muscles is the same.
The first part of orbicularis, orb (orb = “circular”), is a reference to a round
or circular structure; it may also make one think of orbit, such as the moon’s
path around the earth. The word oris (oris = “oral”) refers to the oral cavity,
or the mouth. The word oculi (ocular = “eye’”) refers to the eye.
There are other muscles throughout the body named by their shape or
location. The deltoid is a large, triangular-shaped muscle that covers the
shoulder. It is so-named because the Greek letter delta looks like a triangle.
The rectus abdomis (rector = “straight”) is the straight muscle in the
anterior wall of the abdomen, while the rectus femoris is the straight muscle
in the anterior compartment of the thigh.
When a muscle has a widespread expansion over a sizable area, but then the
fascicles come to a single, common attachment point, the muscle is called
convergent. The attachment point for a convergent muscle could be a
tendon, an aponeurosis (a flat, broad tendon), or a raphe (a very slender
tendon). The large muscle on the chest, the pectoralis major, is an example
of a convergent muscle because it converges on the greater tubercle of the
humerus via a tendon. The temporalis muscle of the cranium is another.
Pennate muscles (penna = “feathers”) blend into a tendon that runs through
the central region of the muscle for its whole length, somewhat like the
quill of a feather with the muscle arranged similar to the feathers. Due to
this design, the muscle fibers in a pennate muscle can only pull at an angle,
and as a result, contracting pennate muscles do not move their tendons very
far. However, because a pennate muscle generally can hold more muscle
fibers within it, it can produce relatively more tension for its size. There are
three subtypes of pennate muscles.
In a unipennate muscle, the fascicles are located on one side of the tendon.
The extensor digitorum of the forearm is an example of a unipennate
muscle. A bipennate muscle has fascicles on both sides of the tendon. In
some pennate muscles, the muscle fibers wrap around the tendon,
sometimes forming individual fascicles in the process. This arrangement is
referred to as multipennate. A common example is the deltoid muscle of
the shoulder, which covers the shoulder but has a single tendon that inserts
on the deltoid tuberosity of the humerus.
Because of fascicles, a portion of a multipennate muscle like the deltoid can
be stimulated by the nervous system to change the direction of the pull. For
example, when the deltoid muscle contracts, the arm abducts (moves away
from midline in the sagittal plane), but when only the anterior fascicle is
stimulated, the arm will abduct and flex (move anteriorly at the shoulder
joint).
The Lever System of Muscle and Bone Interactions
Skeletal muscles do not work by themselves. Muscles are arranged in pairs
based on their functions. For muscles attached to the bones of the skeleton,
the connection determines the force, speed, and range of movement. These
characteristics depend on each other and can explain the general
organization of the muscular and skeletal systems.
The skeleton and muscles act together to move the body. Have you ever
used the back of a hammer to remove a nail from wood? The handle acts as
a lever and the head of the hammer acts as a fulcrum, the fixed point that
the force is applied to when you pull back or push down on the handle. The
effort applied to this system is the pulling or pushing on the handle to
remove the nail, which is the load, or “resistance” to the movement of the
handle in the system. Our musculoskeletal system works in a similar
manner, with bones being stiff levers and the articular endings of the bones
—encased in synovial joints—acting as fulcrums. The load would be an
object being lifted or any resistance to a movement (your head is a load
when you are lifting it), and the effort, or applied force, comes from
contracting skeletal muscle.
Chapter Review
Skeletal muscles each have an origin and an insertion. The end of the
muscle that attaches to the bone being pulled is called the muscle’s insertion
and the end of the muscle attached to a fixed, or stabilized, bone is called
the origin. The muscle primarily responsible for a movement is called the
prime mover, and muscles that assist in this action are called synergists. A
synergist that makes the insertion site more stable is called a fixator.
Meanwhile, a muscle with the opposite action of the prime mover is called
an antagonist. Several factors contribute to the force generated by a skeletal
muscle. One is the arrangement of the fascicles in the skeletal muscle.
Fascicles can be parallel, circular, convergent, pennate, fusiform, or
triangular. Each arrangement has its own range of motion and ability to do
work.
Review Questions
Exercise:
Problem:
Which of the following is unique to the muscles of facial expression?
a. They all originate from the scalp musculature.
b. They insert onto the cartilage found around the face.
c. They only insert onto the facial bones.
d. They insert into the skin.
Solution:
D
Exercise:
Problem: Which of the following helps an agonist work?
a. a Synergist
b. a fixator
c. an insertion
d. an antagonist
Solution:
A
Exercise:
Problem:
Which of the following statements is correct about what happens
during flexion?
a. The angle between bones is increased.
b. The angle between bones is decreased.
c. The bone moves away from the body.
d. The bone moves toward the center of the body.
Solution:
B
Exercise:
Problem: Which is moved the /east during muscle contraction?
a. the origin
b. the insertion
c. the ligaments
d. the joints
Solution:
A
Exercise:
Problem: Which muscle has a convergent pattern of fascicles?
a. biceps brachii
b. gluteus maximus
c. pectoralis major
d. rectus femoris
Solution:
C
Exercise:
Problem:
A muscle that has a pattern of fascicles running along the long axis of
the muscle has which of the following fascicle arrangements?
a. Circular
b. pennate
c. parallel
d. rectus
Solution:
C
Exercise:
Problem: Which arrangement best describes a bipennate muscle?
a. The muscle fibers feed in on an angle to a long tendon from both
sides.
b. The muscle fibers feed in on an angle to a long tendon from all
directions.
c. The muscle fibers feed in on an angle to a long tendon from one
side.
d. The muscle fibers on one side of a tendon feed into it at a certain
angle and muscle fibers on the other side of the tendon feed into it
at the opposite angle.
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
What effect does fascicle arrangement have on a muscle’s action?
Solution:
Fascicle arrangements determine what type of movement a muscle can
make. For instance, circular muscles act as sphincters, closing orifices.
Exercise:
Problem:
Movements of the body occur at joints. Describe how muscles are
arranged around the joints of the body.
Solution:
Muscles work in pairs to facilitate movement of the bones around the
joints. Agonists are the prime movers while antagonists oppose or
resist the movements of the agonists. Synergists assist the agonists, and
fixators stabilize a muscle’s origin.
Exercise:
Problem:Explain how a synergist assists an agonist by being a fixator.
Solution:
Agonists are the prime movers while antagonists oppose or resist the
movements of the agonists. Synergists assist the agonists, and fixators
stabilize a muscle’s origin.
Glossary
abduct
move away from midline in the sagittal plane
agonist
(also, prime mover) muscle whose contraction is responsible for
producing a particular motion
antagonist
muscle that opposes the action of an agonist
belly
bulky central body of a muscle
bipennate
pennate muscle that has fascicles that are located on both sides of the
tendon
circular
(also, sphincter) fascicles that are concentrically arranged around an
opening
convergent
fascicles that extend over a broad area and converge on a common
attachment site
fascicle
muscle fibers bundled by perimysium into a unit
fixator
synergist that assists an agonist by preventing or reducing movement at
another joint, thereby stabilizing the origin of the agonist
flexion
movement that decreases the angle of a joint
fusiform
muscle that has fascicles that are spindle-shaped to create large bellies
insertion
end of a skeletal muscle that is attached to the structure (usually a
bone) that is moved when the muscle contracts
multipennate
pennate muscle that has a tendon branching within it
origin
end of a skeletal muscle that is attached to another structure (usually a
bone) in a fixed position
parallel
fascicles that extend in the same direction as the long axis of the
muscle
pennate
fascicles that are arranged differently based on their angles to the
tendon
prime mover
(also, agonist) principle muscle involved in an action
synergist
muscle whose contraction helps a prime mover in an action
unipennate
pennate muscle that has fascicles located on one side of the tendon
Naming Skeletal Muscles
By the end of this section, you will be able to:
e Describe the criteria used to name skeletal muscles
e Explain how understanding the muscle names helps describe shapes,
location, and actions of various muscles
The Greeks and Romans conducted the first studies done on the human
body in Western culture. The educated class of subsequent societies studied
Latin and Greek, and therefore the early pioneers of anatomy continued to
apply Latin and Greek terminology or roots when they named the skeletal
muscles. The large number of muscles in the body and unfamiliar words
can make learning the names of the muscles in the body seem daunting, but
understanding the etymology can help. Etymology is the study of how the
root of a particular word entered a language and how the use of the word
evolved over time. Taking the time to learn the root of the words is crucial
to understanding the vocabulary of anatomy and physiology. When you
understand the names of muscles it will help you remember where the
muscles are located and what they do ({link], [link], and [link]).
Pronunciation of words and terms will take a bit of time to master, but after
you have some basic information; the correct names and pronunciations
will become easier.
Overview of the Muscular System
‘ Occipitofrontalis
Sternocleidomastoid i ———— (frontal belly)
Deltoid
Trapezius
Pectoralis major Pectoralis minor
ae Serratus anterior
Rectus abdominis
Biceps brachii
Abdominal Brachialis
external oblique
Brachioradialis
: Pronator teres
Pectineus
Flexor carpi radialis
Adductor Tensor fasciae latae
longus
Sartorius
llliopsoas
Rectus femoris Js
Gracilis
Vastus lateralis Vastus medialis
Soleus and
Fibularis longus :
gastrocnemius
Tibialis anterior
a) ip
Major muscles of the body.
Right side: superficial; left side:
deep (anterior view)
Occipitofrontalis
(occipital belly) Epicranial aponeurosis
Splenius capitis
Levator scapulae
Supraspinatus Rhomboids
i Trapezius
Teres minor
Deltoid
Infraspinatus
f Latissimus dorsi
Teres major
: ri Brachioradialis
Triceps brachii ee
: Extensor carpi radialis
Serratus posterior
inferior Extensor digitorum
External oblique Extensor carpi ulnaris
Gluteus medius Flexor carpi ulnaris
(dissected)
Gluteus maximus 77]
(dissected)
Semimembranosus
“> Gluteus minimus
i \ + Gemellus muscles
“YN
Biceps femoris
Semitendinosus
Gracilis
Gastrocnemius (dissected)
Peroneus longus
Tibialis posterior Soleus
Major muscles of the body.
Right side: superficial; left side:
deep (posterior view)
On the anterior and posterior views of the
muscular system above, superficial
muscles (those at the surface) are shown
on the right side of the body while deep
muscles (those underneath the superficial
muscles) are shown on the left half of the
body. For the legs, superficial muscles are
shown in the anterior view while the
posterior view shows both superficial and
deep muscles.
Understanding a Muscle Name from the Latin
abductor ab = away from duct = to move a Ieee
moves away from
A muscle that
abductor F
diciti diaiti digitus = digit refers to a finger moves the
gmt 9 9 9 or toe little finger or
minimi
oh: toe away
pa minimus = :
minimi ase little
mini, tiny
adductor ad = to, toward duct = to move enindscle te
moves towards
Sdduictor A muscle that
diciti diaiti diaitus = diait refers to a finger moves the
ont g 9 9 or toe little finger or
minimi
aE toe toward
8 cent minimus = \
minimi ae little
mini, tiny
Mnemonic Device for Latin Roots
Latin or
Greek
Example Translation Mnemonic Device
ad to; toward ADvance toward your goal
ab away from n/a
sub under SUBmarines move under water.
something A conDUCTOR makes a train
ductor
that moves move.
If you are antisocial, you are
anti against against engaging in social
activities.
epi on top of n/a
apo to the side of n/a
Mnemonic Device for Latin Roots
Example
longissimus
longus
brevis
maximus
medius
minimus
rectus
multi
uni
bi/di
tri
quad
Latin or
Greek
Translation
longest
long
short
large
medium
tiny; little
straight
many
one
two
three
four
Mnemonic Device
“Longissimus” is longer than the
word “long.”
long
brief
max
“Medius” and “medium” both
begin with “med.”
mini
To RECTify a situation is to
straighten it out.
If something is MULTIcolored, it
has many colors.
A UNIcorn has one horn.
If a ring is DIcast, it is made of
two metals.
TRIple the amount of money is
three times as much.
QUADruplets are four children
born at one birth.
Mnemonic Device for Latin Roots
Latin or
Greek
Example Translation Mnemonic Device
externus outside EXternal
internus inside INternal
Anatomists name the skeletal muscles according to a number of criteria,
each of which describes the muscle in some way. These include naming the
muscle after its shape, its size compared to other muscles in the area, its
location in the body or the location of its attachments to the skeleton, how
many origins it has, or its action.
The skeletal muscle’s anatomical location or its relationship to a particular
bone often determines its name. For example, the frontalis muscle is located
on top of the frontal bone of the skull. Similarly, the shapes of some
muscles are very distinctive and the names, such as orbicularis, reflect the
shape. For the buttocks, the size of the muscles influences the names:
gluteus maximus (largest), gluteus medius (medium), and the gluteus
minimus (smallest). Names were given to indicate length—brevis (short),
longus (long)—and to identify position relative to the midline: lateralis (to
the outside away from the midline), and medialis (toward the midline). The
direction of the muscle fibers and fascicles are used to describe muscles
relative to the midline, such as the rectus (straight) abdominis, or the
oblique (at an angle) muscles of the abdomen.
Some muscle names indicate the number of muscles in a group. One
example of this is the quadriceps, a group of four muscles located on the
anterior (front) thigh. Other muscle names can provide information as to
how many origins a particular muscle has, such as the biceps brachii. The
prefix bi indicates that the muscle has two origins and tri indicates three
origins.
The location of a muscle’s attachment can also appear in its name. When
the name of a muscle is based on the attachments, the origin is always
named first. For instance, the sternocleidomastoid muscle of the neck has a
dual origin on the sternum (sterno) and clavicle (cleido), and it inserts on
the mastoid process of the temporal bone. The last feature by which to
name a muscle is its action. When muscles are named for the movement
they produce, one can find action words in their name. Some examples are
flexor (decreases the angle at the joint), extensor (increases the angle at the
joint), abductor (moves the bone away from the midline), or adductor
(moves the bone toward the midline).
Chapter Review
Muscle names are based on many characteristics. The location of a muscle
in the body is important. Some muscles are named based on their size and
location, such as the gluteal muscles of the buttocks. Other muscle names
can indicate the location in the body or bones with which the muscle is
associated, such as the tibialis anterior. The shapes of some muscles are
distinctive; for example, the direction of the muscle fibers is used to
describe muscles of the body midline. The origin and/or insertion can also
be features used to name a muscle; examples are the biceps brachii, triceps
brachii, and the pectoralis major.
Review Questions
Exercise:
Problem:
The location of a muscle’s insertion and origin can determine
a. action
b. the force of contraction
c. muscle name
d. the load a muscle can carry
Solution:
A
Exercise:
Problem: Where is the temporalis muscle located?
a. on the forehead
b. in the neck
c. on the side of the head
d. on the chin
Solution:
c
Exercise:
Problem:Which muscle name does not make sense?
a. extensor digitorum
b. gluteus minimus
c. biceps femoris
d. extensor minimus longus
Solution:
D
Exercise:
Problem:
Which of the following terms would be used in the name of a muscle
that moves the leg away from the body?
a. flexor
b. adductor
c. extensor
d. abductor
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Describe the different criteria that contribute to how skeletal muscles
are named.
Solution:
In anatomy and physiology, many word roots are Latin or Greek.
Portions, or roots, of the word give us clues about the function, shape,
action, or location of a muscle.
Glossary
abductor
moves the bone away from the midline
adductor
moves the bone toward the midline
bi
two
brevis
short
extensor
muscle that increases the angle at the joint
flexor
muscle that decreases the angle at the joint
lateralis
to the outside
longus
long
maximus
largest
medialis
to the inside
medius
medium
minimus
smallest
oblique
at an angle
rectus
straight
tri
three
Muscles of the Head, Neck, and Back
By the end of this section, you will be able to:
e Identify the axial muscles of the face, head, and neck
e Identify the movement and function of the face, head, and neck muscles
The skeletal muscles are divided into axial (muscles of the trunk and head) and appendicular (muscles of
the arms and legs) categories. This system reflects the bones of the skeleton system, which are also
arranged in this manner. The axial muscles are grouped based on location, function, or both. Some of the
axial muscles may seem to blur the boundaries because they cross over to the appendicular skeleton. The
first grouping of the axial muscles you will review includes the muscles of the head and neck, then you
will review the muscles of the vertebral column, and finally you will review the oblique and rectus
muscles.
Muscles That Create Facial Expression
The origins of the muscles of facial expression are on the surface of the skull (remember, the origin of a
muscle does not move). The insertions of these muscles have fibers intertwined with connective tissue and
the dermis of the skin. Because the muscles insert in the skin rather than on bone, when they contract, the
skin moves to create facial expression ([link]).
Muscles of Facial Expression
Epicranial aponeurosis
Occipitofrontalis
(frontal belly)
Corrugator supercilii
Orbicularis oculi
Occipitofrontalis
(occipital belly)
Orbicularis oris
Facial muscles (anterior view) Facial muscles (lateral view)
Many of the muscles of facial expression insert into the
skin surrounding the eyelids, nose and mouth, producing
facial expressions by moving the skin rather than bones.
The orbicularis oris is a circular muscle that moves the lips, and the orbicularis oculi is a circular muscle
that closes the eye. The occipitofrontalis muscle moves up the scalp and eyebrows. The muscle has a
frontal belly and an occipital (near the occipital bone on the posterior part of the skull) belly. In other
words, there is a muscle on the forehead (frontalis) and one on the back of the head (occipitalis), but there
is no muscle across the top of the head. Instead, the two bellies are connected by a broad tendon called the
epicranial aponeurosis, or galea aponeurosis (galea = “apple”). The physicians originally studying human
anatomy thought the skull looked like an apple.
A large portion of the face is composed of the buccinator muscle, which compresses the cheek. This
muscle allows you to whistle, blow, and suck; and it contributes to the action of chewing. There are
several small facial muscles, one of which is the corrugator supercilii, which is the prime mover of the
eyebrows. Place your finger on your eyebrows at the point of the bridge of the nose. Raise your eyebrows
as if you were surprised and lower your eyebrows as if you were frowning. With these movements, you
can feel the action of the corrugator supercilli. Additional muscles of facial expression are presented in
[link].
Muscles in Facial Expression
Target motion
direction
Brow
Furrowing Skin of scalp Anterior Occipito- Epicraneal Underneath
brow frontalis, aponeurosis skin of
frontal belly forehead
Unfurrowing Skin of scalp Posterior Occipito- Occipital bone; Epicraneal
brow frontalis, mastoid process aponeurosis
occipital belly (temporal bone)
Lowering Skin Inferior Corrugator Frontal bone Skin
eyebrows underneath supercilii underneath
(e.g., scowling, eyebrows eyebrow
frowning)
Nose
Movement Target Prime mover Origin Insertion
Flaring nostrils Nasal cartilage Inferior Nasal bone
(pushes nostrils | compression;
open when posterior
cartilage is compression
compressed)
Raising Upper lip Elevation Levator labii Maxilla Underneath
upper lip superioris skin at corners
of the mouth;
orbicularis oris
Lowering Lower lip Depression Depressor Mandible Underneath
lower lip labii inferioris skin of lower lip
Opening mouth | Lower jaw Depression, Depressor Mandible Underneath
and sliding lateral angulus oris skin at corners
lower jaw left of mouth
and right
Smiling Corners of Lateral Zygomaticus Zygomatic bone Underneath
skin at corners
of mouth
(dimple area);
orbicularis oris
Shaping of lips Lips Orbicularis Tissue Underneath
(as during oris surrounding lips skin at corners
speech) of the mouth
Lateral Cheeks Lateral Buccinator Maxilla, mandible; | Orbicularis
movement of sphenoid bone (via | oris
cheeks (e.g., pterygomandibular
sucking on a raphae)
straw; also used
to compress air
in mouth while
blowing)
Pursing of lips Corners of Lateral Risorius Fascia of parotid Underneath
by straightening | mouth salivary gland skin at corners
them laterally of the mouth
Mandible Underneath
skin of chin
mouth elevation major
Protrusion of Lower lip and Protraction Mentalis
lower lip (e.g., skin of chin
pouting
expression)
Muscles That Move the Eyes
The movement of the eyeball is under the control of the extrinsic eye muscles, which originate outside the
eye and insert onto the outer surface of the white of the eye. These muscles are located inside the eye
socket and cannot be seen on any part of the visible eyeball ({link] and [link]). If you have ever been to a
doctor who held up a finger and asked you to follow it up, down, and to both sides, he or she is checking
to make sure your eye muscles are acting in a coordinated pattern.
Muscles of the Eyes
Superior oblique
Levator palpebrae superioris
Superior oblique
Superior 4
rectus ] Lateral
rectus
Sphenoid
bone
~ eS
S33 » Medial
: oe ge %
we Lateral \ ey Ee “= Ye, rectus
i rectus Ss aeet
Inferior rectus & 3
ge
Sa =
Medial rectus Inferior oblique Inferior oblique Inferior rectus
(a) Right eye (lateral view) (b) Right eye (anterior view)
(a) The extrinsic eye muscles originate outside of the eye on
the skull. (b) Each muscle inserts onto the eyeball.
Muscles of the Eyes
Target
motion Prime
Movement Target direction mover
Moves eyes up
and toward Superior
nose; rotates Eyeballs (elevates); Superior
eyes from 1 medial rectus
o’clock to 3 (adducts)
o’clock
Moves eyes
down and Inferior
toward nose; Eyeballs (depresses); Inferior
rotates eyes medial rectus
from 6 o’clock (adducts)
to 3 o’clock
mea Lateral Lateral
away from Eyeballs
‘gee (abducts) rectus
Origin
Common
tendinous
ring (ring
attaches to
optic
foramen)
Common
tendinous
ring (ring
attaches to
optic
foramen)
Common
tendinous
ring (ring
attaches to
optic
foramen)
Insertion
Superior
surface of
eyeball
Inferior
surface of
eyeball
Lateral
surface of
eyeball
Muscles of the Eyes
Movement
Moves eyes
toward nose
Moves eyes up
and away from
nose; rotates
eyeball from
12 o’clock to 9
o’clock
Moves eyes
down and
away from
nose; rotates
eyeball from 6
o’clock to 9
o’clock
Opens eyes
Closes eyelids
Target
Eyeballs
Eyeballs
Eyeballs
Upper
eyelid
Eyelid
skin
Target
motion
direction
Medial
(adducts)
Superior
(elevates);
lateral
(abducts)
Superior
(elevates);
lateral
(abducts)
Superior
(elevates)
Compression
along
superior—
inferior axis
Muscles That Move the Lower Jaw
Prime
mover
Medial
rectus
Inferior
oblique
Superior
oblique
Levator
palpabrae
superioris
Orbicularis
oculi
Origin
Common
tendinous
ring (ring
attaches to
optic
foramen)
Floor of
orbit
(maxilla)
Sphenoid
bone
Roof of
orbit
(sphenoid
bone)
Medial
bones
composing
the orbit
Insertion
Medial
surface of
eyeball
Surface of
eyeball
between
inferior rectus
and lateral
rectus
Suface of
eyeball
between
superior rectus
and lateral
rectus
Skin of upper
eyelids
Circumference
of orbit
In anatomical terminology, chewing is called mastication. Muscles involved in chewing must be able to
exert enough pressure to bite through and then chew food before it is swallowed ([link] and [link]). The
masseter muscle is the main muscle used for chewing because it elevates the mandible (lower jaw) to
close the mouth, and it is assisted by the temporalis muscle, which retracts the mandible. You can feel the
temporalis move by putting your fingers to your temple as you chew.
Muscles That Move the Lower Jaw
Lateral
pterygoid
Area of
superficial
muscle
dissection
Medial
pterygoid
Chewing muscles (superficial) Chewing muscles (deep)
The muscles that move the lower jaw are typically
located within the cheek and originate from processes
in the skull. This provides the jaw muscles with the
large amount of leverage needed for chewing.
Muscles of the Lower Jaw
Target
motion Prime
Movement Target direction mover Origin Insertion
Maxilla
. : arch;
ea ou Mandible supenor Masseter zygomatic Mandible
aids chewing (elevates)
arch (for
masseter)
Closes mouth; Superior
pulls Lower jaw Mandible (elevates); Temporalis Jemporal Mandible
in under upper posterior bone
jaw (retracts)
Inferior
Opens mouth; (depresses);
pushes lower posterior Pterygoid
jaw out under Mandible (protracts); Lateral process of Mandible
upper jaw; lateral pterygoid sphenoid
moves lower (abducts); bone
jaw side-to-side medial
(adducts)
Muscles of the Lower Jaw
Target
motion Prime
Movement Target direction mover Origin Insertion
Superior
Closes mouth; (elevates);
pushes lower posterior . Mandible;
: . Sphenoid
jaw out under : (protracts); Medial : temporo-
: Mandible : bone; :
upper jaw; lateral pterygoid faawilla mandibular
moves lower (abducts); joint
jaw side-to-side medial
(adducts)
Although the masseter and temporalis are responsible for elevating and closing the jaw to break food into
digestible pieces, the medial pterygoid and lateral pterygoid muscles provide assistance in chewing and
moving food within the mouth.
Muscles That Move the Tongue
Although the tongue is obviously important for tasting food, it is also necessary for mastication,
deglutition (swallowing), and speech ({link] and [link]). Because it is so moveable, the tongue facilitates
complex speech patterns and sounds.
Muscles that Move the Tongue
Styloglossus ~~
Pharyngopalatine arch
Dorsal surface
Palatine tonsil
of tongue
Palatoglossus
Buccinator
Hyoglossus
i Fungiform
Valate Z ; i papilla
papilla Fe
Mandible bone
Genioglossus
(a) Extrinsic tongue muscles (b) Palatoglossus and surface of tongue
Muscles for Tongue Movement, Swallowing, and Speech
Target motion Prime
ene ae | Tgecnaren | Rome | onan | insertion |
Tongue
Moves tongue down; sticks Tongue Inferior (depresses); | Genioglossus | Mandible Tongue
tongue out of mouth anterior (protracts) undersurface;
hyoid bone
Moves tongue up; retracts Tongue Superior (elevates); | Styloglossus Temporal Tongue
tongue back into mouth posterior (retracts) bone (styloid | undersurface
process) and sides
Flattens tongue Tongue Inferior (depresses) | Hyoglossus Hyoid bone Sides of
tongue
Bulges tongue Tongue Superior (elevation) | Palatoglossus | Soft palate
tongue
Swallowing and speaking
Raises the hyoid bone in a way Hyoid bone; | Superior (elevates) Digastric Mandible; Hyoid bone
that also raises the larynx, larynx temporal
allowing the epiglottis to cover bone
the glottis during deglutition;
also assists in opening the
mouth by depressing the
mandible
Raises and retracts the hyoid Hyoid bone | Superior (elevates); | Stylohyoid Temporal Hyoid bone
bone in a way that elongates posterior (retracts) bone (styloid
the oral cavity during deglutition process)
Mylohyoid Mandible Hyoid bone;
Raises hyoid bone in a way Hyoid bone | Superior (elevates)
that presses tongue against
the roof of the mouth, pushing
food back into the pharynx
during deglutition
Raises and moves hyoid bone Hyoid bone | Superior (elevates); | Geniohyoid Mandible Hyoid bone
forward, widening pharynx anterior (protracts)
during deglutition
Retracts hyoid bone and Hyoid bone | Inferior (depresses); | Omohyoid Scapula Hyoid bone
moves it down during later posterior (retracts)
phases of deglutition
Depresses the hyoid bone Hyoid bone | Inferior (depresses) | Sternohyoid Clavicle Hyoid bone
during swallowing and speaking
Shrinks distance between Hyoid bone; | Hyoid bone: inferior | Thyrohyoid Thyroid Hyoid bone
thyroid cartilage and hyoid thyroid (depresses); thyroid cartilage
bone, allowing production of cartilage cartilage: superior
high-pitch vocalizations (elevates)
Depresses larynx, thyroid Larynx; Inferior (depresses) | Sternothyroid | Sternum Thyroid
cartilage, and hyoid bone to thyroid cartilage
create different vocal tones cartilage;
hyoid bone
median raphe
Rotates and tilts head to he Skull; Individually: medial Sternocleid- Sternum; Temporal bone
side; tilts head forward cervical rotation; lateral omastoid; clavicle (mastoid
vertebrae flexion; bilaterally: semispinalis Process);
anterior (flexes) capitis occipital bone
Rotates and tilts head to the Skull; Individually: lateral Splenius
side; tilts head backwards cervical rotation; lateral capitis;
vertebrae flexion; bilaterally: longissimus
anterior (flexes) capitis
Tongue muscles can be extrinsic or intrinsic. Extrinsic tongue muscles insert into the tongue from outside
origins, and the intrinsic tongue muscles insert into the tongue from origins within it. The extrinsic
muscles move the whole tongue in different directions, whereas the intrinsic muscles allow the tongue to
change its shape (such as, curling the tongue in a loop or flattening it).
The extrinsic muscles all include the word root glossus (glossus = “tongue’”’), and the muscle names are
derived from where the muscle originates. The genioglossus (genio = “chin”) originates on the mandible
and allows the tongue to move downward and forward. The styloglossus originates on the styloid bone,
and allows upward and backward motion. The palatoglossus originates on the soft palate to elevate the
back of the tongue, and the hyoglossus originates on the hyoid bone to move the tongue downward and
flatten it.
Note:
Everyday Connections
Anesthesia and the Tongue Muscles
Before surgery, a patient must be made ready for general anesthesia. The normal homeostatic controls of
the body are put “on hold” so that the patient can be prepped for surgery. Control of respiration must be
switched from the patient’s homeostatic control to the control of the anesthesiologist. The drugs used for
anesthesia relax a majority of the body’s muscles.
Among the muscles affected during general anesthesia are those that are necessary for breathing and
moving the tongue. Under anesthesia, the tongue can relax and partially or fully block the airway, and the
muscles of respiration may not move the diaphragm or chest wall. To avoid possible complications, the
safest procedure to use on a patient is called endotracheal intubation. Placing a tube into the trachea
allows the doctors to maintain a patient’s (open) airway to the lungs and seal the airway off from the
oropharynx. Post-surgery, the anesthesiologist gradually changes the mixture of the gases that keep the
patient unconscious, and when the muscles of respiration begin to function, the tube is removed. It still
takes about 30 minutes for a patient to wake up, and for breathing muscles to regain control of respiration.
After surgery, most people have a sore or scratchy throat for a few days.
Muscles of the Anterior Neck
The muscles of the anterior neck assist in deglutition (swallowing) and speech by controlling the positions
of the larynx (voice box), and the hyoid bone, a horseshoe-shaped bone that functions as a solid
foundation on which the tongue can move. The muscles of the neck are categorized according to their
position relative to the hyoid bone ([{link]). Suprahyoid muscles are superior to it, and the infrahyoid
muscles are located inferiorly.
Muscles of the Anterior Neck
Suprahyoid
muscles:
Geniohyoid
Digastric
Mylohyoid
Stylohyoid
Inferior edge
of mandible
Styloglossus
Hyoid bone
Infrahyoid
muscles:
Thyroid cartilage
Thyrohyoid
of larynx
Omohyoid
Thyroid gland J Sternohyoid
Sternothyroid
Trachea
Right and left
—e_cclavicles
L— ~ Sternum
The anterior muscles of the neck facilitate
swallowing and speech. The suprahyoid
muscles originate from above the hyoid bone
in the chin region. The infrahyoid muscles
originate below the hyoid bone in the lower
neck.
Scapula
The suprahyoid muscles raise the hyoid bone, the floor of the mouth, and the larynx during deglutition.
These include the digastric muscle, which has anterior and posterior bellies that work to elevate the hyoid
bone and larynx when one swallows; it also depresses the mandible. The stylohyoid muscle moves the
hyoid bone posteriorly, elevating the larynx, and the mylohyoid muscle lifts it and helps press the tongue
to the top of the mouth. The geniohyoid depresses the mandible in addition to raising and pulling the
hyoid bone anteriorly.
The strap-like infrahyoid muscles generally depress the hyoid bone and control the position of the larynx.
The omohyoid muscle, which has superior and inferior bellies, depresses the hyoid bone in conjunction
with the sternohyoid and thyrohyoid muscles. The thyrohyoid muscle also elevates the larynx’s thyroid
cartilage, whereas the sternothyroid depresses it to create different tones of voice.
Muscles That Move the Head
The head, attached to the top of the vertebral column, is balanced, moved, and rotated by the neck muscles
([link]). When these muscles act unilaterally, the head rotates. When they contract bilaterally, the head
flexes or extends. The major muscle that laterally flexes and rotates the head is the sternocleidomastoid.
In addition, both muscles working together are the flexors of the head. Place your fingers on both sides of
the neck and turn your head to the left and to the right. You will feel the movement originate there. This
muscle divides the neck into anterior and posterior triangles when viewed from the side ([link]).
Posterior and Lateral Views of the Neck
(> = = ; Suboccipital muscles
= < ” Splenius
a \S capitis (cut)
Sternocleidomastoid Levator
Levator scapulae
| . Longissimus
Multifidus . i capitis
muscles ‘ \
Acromion
process of Semispinalis
capitis
1st thoracic
vertebrae
Sma Scalenes
Neck muscles Superficial neck muscles: Deep neck muscles: left
(left lateral view) tight side trapezius removed side semispinalis capitis
(posterior view) removed (posterior view)
The superficial and deep muscles of the neck are responsible for moving the
head, cervical vertebrae, and scapulas.
Muscles That Move the Head
Target
motion
Movement Target direction Prime mover Origin Insertion
Rotates Individually: Temporal
and tilts rotates head bone
head to the Skull; to opposite Grcmuleidemacuid Sternum; (mastoid
side; tilts vertebrae side; clavicle process);
head bilaterally: occipital
forward flexion bone
Muscles That Move the Head
Target
motion
Movement Target direction Prime mover Origin Insertion
Individually: Transverse
laterally and
Rotates flexes and articular
and tilts Skull; rotates head Semispinalis capitis processes Occipital
head vertebrae to same of cervical bone
backward side; and
bilaterally: thoracic
extension vertebra
Individually:
Rotates laterally Spinous Temporal
and tilts flexes and processes bone
head to the Skull; rotates head Sian niccanits of cervical (mastoid
side; tilts vertebrae to same P P and process);
head side; thoracic occipital
backward bilaterally: vertebra bone
extension
Individually: Transverse
Rotates laterally and
and tilts flexes and articular Temporal
head to the Skull; rotates head Longissimus capitis processes bone
side; tilts vertebrae to same of cervical (mastoid
head side; and process)
backward bilaterally: thoracic
extension vertebra
Muscles of the Posterior Neck and the Back
The posterior muscles of the neck are primarily concerned with head movements, like extension. The back
muscles stabilize and move the vertebral column, and are grouped according to the lengths and direction
of the fascicles.
The splenius muscles originate at the midline and run laterally and superiorly to their insertions. From the
sides and the back of the neck, the splenius capitis inserts onto the head region, and the splenius cervicis
extends onto the cervical region. These muscles can extend the head, laterally flex it, and rotate it ([link]).
Muscles of the Neck and Back
Sternocleidomastoid
Trapezius
Splenius capitis
Splenius
Splenius cervicis
Levator
scapulae P
Rhomboides
minor
Rhomboides
major
Trapezius
Medial
scalene
Anterior
scalene
Muscles of the neck (left lateral view) Superficial (left side) and deep
(right side) muscles of the neck and
upper back (posterior view)
Semispinalis capitis 4 Longissimus capitis
(joined with deep
spinalis capitis) ql lliocostalis cervicis
Semispinalis S lliocostalis thoracis
cervicis ,
ee Longissimus thoracis
Longissimus
cervicis
lliocostalis lumborum
Spinalis
thoracis Transverse
processes
of vertebrae
Semispinalis i / Rotator
thoracis H brevis
Rotator
longus
Interspinales
Short
rotator
Intertransversarii
Deep muscles of the back Deep spinal muscles
(posterior view) (multifidus removed)
The large, complex muscles of the neck and back move the
head, shoulders, and vertebral column.
The erector spinae group forms the majority of the muscle mass of the back and it is the primary extensor
of the vertebral column. It controls flexion, lateral flexion, and rotation of the vertebral column, and
maintains the lumbar curve. The erector spinae comprises the iliocostalis (laterally placed) group, the
longissimus (intermediately placed) group, and the spinalis (medially placed) group.
The iliocostalis group includes the iliocostalis cervicis, associated with the cervical region; the
iliocostalis thoracis, associated with the thoracic region; and the iliocostalis lumborum, associated with
the lumbar region. The three muscles of the longissimus group are the longissimus capitis, associated
with the head region; the longissimus cervicis, associated with the cervical region; and the longissimus
thoracis, associated with the thoracic region. The third group, the spinalis group, comprises the spinalis
capitis (head region), the spinalis cervicis (cervical region), and the spinalis thoracis (thoracic region).
The transversospinales muscles run from the transverse processes to the spinous processes of the
vertebrae. Similar to the erector spinae muscles, the semispinalis muscles in this group are named for the
areas of the body with which they are associated. The semispinalis muscles include the semispinalis
capitis, the semispinalis cervicis, and the semispinalis thoracis. The multifidus muscle of the lumbar
region helps extend and laterally flex the vertebral column.
Important in the stabilization of the vertebral column is the segmental muscle group, which includes the
interspinales and intertransversarii muscles. These muscles bring together the spinous and transverse
processes of each consecutive vertebra. Finally, the scalene muscles work together to flex, laterally flex,
and rotate the head. They also contribute to deep inhalation. The scalene muscles include the anterior
scalene muscle (anterior to the middle scalene), the middle scalene muscle (the longest, intermediate
between the anterior and posterior scalenes), and the posterior scalene muscle (the smallest, posterior to
the middle scalene).
Chapter Review
Muscles are either axial muscles or appendicular. The axial muscles are grouped based on location,
function, or both. Some axial muscles cross over to the appendicular skeleton. The muscles of the head
and neck are all axial. The muscles in the face create facial expression by inserting into the skin rather than
onto bone. Muscles that move the eyeballs are extrinsic, meaning they originate outside of the eye and
insert onto it. Tongue muscles are both extrinsic and intrinsic. The genioglossus depresses the tongue and
moves it anteriorly; the styloglossus lifts the tongue and retracts it; the palatoglossus elevates the back of
the tongue; and the hyoglossus depresses and flattens it. The muscles of the anterior neck facilitate
swallowing and speech, stabilize the hyoid bone and position the larynx. The muscles of the neck stabilize
and move the head. The sternocleidomastoid divides the neck into anterior and posterior triangles.
The muscles of the back and neck that move the vertebral column are complex, overlapping, and can be
divided into five groups. The splenius group includes the splenius capitis and the splenius cervicis. The
erector spinae has three subgroups. The iliocostalis group includes the iliocostalis cervicis, the iliocostalis
thoracis, and the iliocostalis lumborum. The longissimus group includes the longissimus capitis, the
longissimus cervicis, and the longissimus thoracis. The spinalis group includes the spinalis capitis, the
spinalis cervicis, and the spinalis thoracis. The transversospinales include the semispinalis capitis,
semispinalis cervicis, semispinalis thoracis, multifidus, and rotatores. The segmental muscles include the
interspinales and intertransversarii. Finally, the scalenes include the anterior scalene, middle scalene, and
posterior scalene.
Review Questions
Exercise:
Problem: Which of the following is a prime mover in head flexion?
a. occipitofrontalis
b. corrugator supercilii
c. sternocleidomastoid
d. masseter
Solution:
C
Exercise:
Problem: Where is the inferior oblique muscle located?
a. in the abdomen
b. in the eye socket
c. in the anterior neck
d. in the face
Solution:
B
Exercise:
Problem: What is the action of the masseter?
a. swallowing
b. chewing
c. moving the lips
d. closing the eye
Solution:
B
Exercise:
Problem:The names of the extrinsic tongue muscles commonly end in
a. -glottis
b. -glossus
c. -gluteus
d. -hyoid
Solution:
B
Exercise:
Problem: What is the function of the erector spinae?
a. movement of the arms
b. stabilization of the pelvic girdle
c. postural support
d. rotating of the vertebral column
Solution:
C
Critical Thinking Questions
Exercise:
Problem:Explain the difference between axial and appendicular muscles.
Solution:
Axial muscles originate on the axial skeleton (the bones in the head, neck, and core of the body),
whereas appendicular muscles originate on the bones that make up the body’s limbs.
Exercise:
Problem: Describe the muscles of the anterior neck.
Solution:
The muscles of the anterior neck are arranged to facilitate swallowing and speech. They work on the
hyoid bone, with the suprahyoid muscles pulling up and the infrahyoid muscles pulling down.
Exercise:
Problem: Why are the muscles of the face different from typical skeletal muscle?
Solution:
Most skeletal muscles create movement by actions on the skeleton. Facial muscles are different in
that they create facial movements and expressions by pulling on the skin—no bone movements are
involved.
Glossary
anterior scalene
a muscle anterior to the middle scalene
appendicular
of the arms and legs
axial
of the trunk and head
buccinator
muscle that compresses the cheek
corrugator supercilii
prime mover of the eyebrows
deglutition
swallowing
digastric
muscle that has anterior and posterior bellies and elevates the hyoid bone and larynx when one
swallows; it also depresses the mandible
epicranial aponeurosis
(also, galea aponeurosis) flat broad tendon that connects the frontalis and occipitalis
erector spinae group
large muscle mass of the back; primary extensor of the vertebral column
extrinsic eye muscles
originate outside the eye and insert onto the outer surface of the white of the eye, and create eyeball
movement
frontalis
front part of the occipitofrontalis muscle
genioglossus
muscle that originates on the mandible and allows the tongue to move downward and forward
geniohyoid
muscle that depresses the mandible, and raises and pulls the hyoid bone anteriorly
hyoglossus
muscle that originates on the hyoid bone to move the tongue downward and flatten it
iliocostalis cervicis
muscle of the iliocostalis group associated with the cervical region
iliocostalis group
laterally placed muscles of the erector spinae
iliocostalis lumborum
muscle of the iliocostalis group associated with the lumbar region
iliocostalis thoracis
muscle of the iliocostalis group associated with the thoracic region
infrahyoid muscles
anterior neck muscles that are attached to, and inferior to the hyoid bone
lateral pterygoid
muscle that moves the mandible from side to side
longissimus capitis
muscle of the longissimus group associated with the head region
longissimus cervicis
muscle of the longissimus group associated with the cervical region
longissimus group
intermediately placed muscles of the erector spinae
longissimus thoracis
muscle of the longissimus group associated with the thoracic region
masseter
main muscle for chewing that elevates the mandible to close the mouth
mastication
chewing
medial pterygoid
muscle that moves the mandible from side to side
middle scalene
longest scalene muscle, located between the anterior and posterior scalenes
multifidus
muscle of the lumbar region that helps extend and laterally flex the vertebral column
mylohyoid
muscle that lifts the hyoid bone and helps press the tongue to the top of the mouth
occipitalis
posterior part of the occipitofrontalis muscle
occipitofrontalis
muscle that makes up the scalp with a frontal belly and an occipital belly
omohyoid
muscle that has superior and inferior bellies and depresses the hyoid bone
orbicularis oculi
circular muscle that closes the eye
orbicularis oris
circular muscle that moves the lips
palatoglossus
muscle that originates on the soft palate to elevate the back of the tongue
posterior scalene
smallest scalene muscle, located posterior to the middle scalene
scalene muscles
flex, laterally flex, and rotate the head; contribute to deep inhalation
segmental muscle group
interspinales and intertransversarii muscles that bring together the spinous and transverse processes of
each consecutive vertebra
semispinalis capitis
transversospinales muscle associated with the head region
semispinalis cervicis
transversospinales muscle associated with the cervical region
semispinalis thoracis
transversospinales muscle associated with the thoracic region
spinalis capitis
muscle of the spinalis group associated with the head region
spinalis cervicis
muscle of the spinalis group associated with the cervical region
spinalis group
medially placed muscles of the erector spinae
spinalis thoracis
muscle of the spinalis group associated with the thoracic region
splenius
posterior neck muscles; includes the splenius capitis and splenius cervicis
splenius capitis
neck muscle that inserts into the head region
splenius cervicis
neck muscle that inserts into the cervical region
sternocleidomastoid
major muscle that laterally flexes and rotates the head
sternohyoid
muscle that depresses the hyoid bone
sternothyroid
muscle that depresses the larynx’s thyroid cartilage
styloglossus
muscle that originates on the styloid bone, and allows upward and backward motion of the tongue
stylohyoid
muscle that elevates the hyoid bone posteriorly
suprahyoid muscles
neck muscles that are superior to the hyoid bone
temporalis
muscle that retracts the mandible
thyrohyoid
muscle that depresses the hyoid bone and elevates the larynx’s thyroid cartilage
transversospinales
muscles that originate at the transverse processes and insert at the spinous processes of the vertebrae
Muscles of the Abdominal Wall and Thorax
By the end of this section, you will be able to:
e Identify the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal
wall and thorax
e Identify the movement and function of the intrinsic skeletal muscles of the back and neck, and the
skeletal muscles of the abdominal wall and thorax
It is a complex job to balance the body on two feet and walk upright. The muscles of the vertebral column,
thorax, and abdominal wall extend, flex, and stabilize different parts of the body’s trunk. The deep muscles of
the core of the body help maintain posture as well as carry out other functions. The brain sends out electrical
impulses to these various muscle groups to control posture by alternate contraction and relaxation. This is
necessary so that no single muscle group becomes fatigued too quickly. If any one group fails to function,
body posture will be compromised.
Muscles of the Abdomen
There are four pairs of abdominal muscles that cover the anterior and lateral abdominal region and meet at the
anterior midline. These muscles of the anterolateral abdominal wall can be divided into four groups: the
external obliques, the internal obliques, the transversus abdominis, and the rectus abdominis ([link] and
(link).
Muscles of the Abdomen
Pectoralis major External oblique
Latissimus dorsi Rectus Transversus
sheath abdominis
Anterior serratus muscles
External oblique
Linea alba (of the
rectus sheath)
Rectus abdominis
(enclosed within
rectus sheath)
Tendinous intersections
(between the anterior \ ea
segments of the rectus } Rectus abdominis ;
abdominis) Aponeurosis of
internal oblique
Internal oblique
Quadratus lumborum
llia of hip bones
lliacus
Sacrum
Psoas major
youy
(a) The anterior abdominal muscles include the medially
located rectus abdominis, which is covered by a sheet of
connective tissue called the rectus sheath. On the flanks of
the body, medial to the rectus abdominis, the abdominal
wall is composed of three layers. The external oblique
muscles form the superficial layer, while the internal
oblique muscles form the middle layer, and the transverses
abdominus forms the deepest layer. (b) The muscles of the
lower back move the lumbar spine but also assist in femur
movements.
Muscles of the Abdomen
Target
motion Prime
Movement Target direction mover Origin Insertion
Satan External Ribs Ribs 7—
Twisting at waist; also Vertebral P . obliques; ; 10; linea
: ; lateral ; 5-12;
bending to the side column ; internal Ba alba;
flexion ° ilium a
obliques ilium
Squeezing abdomen nea Sternum;
; F Tlium; :
during forceful Abdominal ; Transversus . linea
: ; : Compression ; ribs 5—
exhalations, defecation, cavity abdominus 10 alba;
urination, and childbirth pubis
Sternum;
Ea ] é R F f ;
Sitting up vee Flexion oe ; Pubis ribs 5
column abdominis
and 7
. . Vertebral Lateral Quadratus lium; Ri
Bending to the side : ribs 5— vertebrae
column flexion lumborum 10 LLL4
There are three flat skeletal muscles in the antero-lateral wall of the abdomen. The external oblique, closest to
the surface, extend inferiorly and medially, in the direction of sliding one’s four fingers into pants pockets.
Perpendicular to it is the intermediate internal oblique, extending superiorly and medially, the direction the
thumbs usually go when the other fingers are in the pants pocket. The deep muscle, the transversus
abdominis, is arranged transversely around the abdomen, similar to the front of a belt on a pair of pants. This
arrangement of three bands of muscles in different orientations allows various movements and rotations of the
trunk. The three layers of muscle also help to protect the internal abdominal organs in an area where there is
no bone.
The linea alba is a white, fibrous band that is made of the bilateral rectus sheaths that join at the anterior
midline of the body. These enclose the rectus abdominis muscles (a pair of long, linear muscles, commonly
called the “sit-up” muscles) that originate at the pubic crest and symphysis, and extend the length of the body’s
trunk. Each muscle is segmented by three transverse bands of collagen fibers called the tendinous
intersections. This results in the look of “six-pack abs,” as each segment hypertrophies on individuals at the
gym who do many sit-ups.
The posterior abdominal wall is formed by the lumbar vertebrae, parts of the ilia of the hip bones, psoas major
and iliacus muscles, and quadratus lumborum muscle. This part of the core plays a key role in stabilizing the
rest of the body and maintaining posture.
Note:
Career Connections
Physical Therapists
Those who have a muscle or joint injury will most likely be sent to a physical therapist (PT) after seeing their
regular doctor. PTs have a master’s degree or doctorate, and are highly trained experts in the mechanics of
body movements. Many PTs also specialize in sports injuries.
If you injured your shoulder while you were kayaking, the first thing a physical therapist would do during
your first visit is to assess the functionality of the joint. The range of motion of a particular joint refers to the
normal movements the joint performs. The PT will ask you to abduct and adduct, circumduct, and flex and
extend the arm. The PT will note the shoulder’s degree of function, and based on the assessment of the injury,
will create an appropriate physical therapy plan.
The first step in physical therapy will probably be applying a heat pack to the injured site, which acts much
like a warm-up to draw blood to the area, to enhance healing. You will be instructed to do a series of exercises
to continue the therapy at home, followed by icing, to decrease inflammation and swelling, which will
continue for several weeks. When physical therapy is complete, the PT will do an exit exam and send a
detailed report on the improved range of motion and return of normal limb function to your doctor. Gradually,
as the injury heals, the shoulder will begin to function correctly. A PT works closely with patients to help
them get back to their normal level of physical activity.
Muscles of the Thorax
The muscles of the chest serve to facilitate breathing by changing the size of the thoracic cavity ([link]). When
you inhale, your chest rises because the cavity expands. Alternately, when you exhale, your chest falls because
the thoracic cavity decreases in size.
Muscles of the Thorax
Target motion Prime
Movement Target direction mover Origin Insertion
Sternum;
Inhalation; Thoracic Compression; . ribs 6-12; Central
: : ; Diaphragm
exhalation cavity expansion lumbar tendon
vertebrae
Muscles of the Thorax
Target motion Prime
Movement Target direction mover Origin Insertion
Rib Rib
Elevation superior inferior to
F : d External
Inhalation;exhalation Ribs (expands : to each each
: : intercostals é ;
thoracic cavity) intercostal intercostal
muscle muscle
Movement Rib Rib
along inferi :
superior/inferior Internal cna cia eaieg
Forced exhalation Ribs : : : each to each
axis to bring intercostals : :
. intercostal intercostal
ribs closer
muscle muscle
together
The Diaphragm
The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation
of the diaphragm ((link]). It separates the thoracic and abdominal cavities, and is dome-shaped at rest. The
superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity. The inferior
surface is concave, creating the curved roof of the abdominal cavity.
Muscles of the Diaphragm
Central tendon
of diaphragm - —— Sternum
Vena cava
passing through
caval opening
Esophagus
passing through
esophageal hiatus
Aorta passing
through aortic
hiatus
12th (floating) ribs
Left psoas major
Left quadratus
lumborum
Vertebrae
Diaphragm (inferior view)
The diaphragm separates the thoracic and
abdominal cavities.
Defecating, urination, and even childbirth involve cooperation between the diaphragm and abdominal muscles
(this cooperation is referred to as the “Valsalva maneuver”). You hold your breath by a steady contraction of
the diaphragm; this stabilizes the volume and pressure of the peritoneal cavity. When the abdominal muscles
contract, the pressure cannot push the diaphragm up, so it increases pressure on the intestinal tract
(defecation), urinary tract (urination), or reproductive tract (childbirth).
The inferior surface of the pericardial sac and the inferior surfaces of the pleural membranes (parietal pleura)
fuse onto the central tendon of the diaphragm. To the sides of the tendon are the skeletal muscle portions of the
diaphragm, which insert into the tendon while having a number of origins including the xiphoid process of the
sternum anteriorly, the inferior six ribs and their cartilages laterally, and the lumbar vertebrae and 12th ribs
posteriorly.
The diaphragm also includes three openings for the passage of structures between the thorax and the abdomen.
The inferior vena cava passes through the caval opening, and the esophagus and attached nerves pass through
the esophageal hiatus. The aorta, thoracic duct, and azygous vein pass through the aortic hiatus of the posterior
diaphragm.
The Intercostal Muscles
There are three sets of muscles, called intercostal muscles, which span each of the intercostal spaces. The
principal role of the intercostal muscles is to assist in breathing by changing the dimensions of the rib cage
(link).
Intercostal Muscles
Clavicle Gi - af
Ribs =
Pectoralis minor
Innermost
intercostal
Pectoralis major
(dissected)
Internal
intercostal
Sternum
Serratus
anterior
External —_ oe
intercostals —
alt
intercostal
Internal
intercostal
The external intercostals are located laterally on the sides of the body.
The internal intercostals are located medially near the sternum. The
innermost intercostals are located deep to both the internal and external
intercostals.
The 11 pairs of superficial external intercostal muscles aid in inspiration of air during breathing because
when they contract, they raise the rib cage, which expands it. The 11 pairs of internal intercostal muscles,
just under the externals, are used for expiration because they draw the ribs together to constrict the rib cage.
The innermost intercostal muscles are the deepest, and they act as synergists for the action of the internal
intercostals.
Muscles of the Pelvic Floor and Perineum
The pelvic floor is a muscular sheet that defines the inferior portion of the pelvic cavity. The pelvic
diaphragm, spanning anteriorly to posteriorly from the pubis to the coccyx, comprises the levator ani and the
ischiococcygeus. Its openings include the anal canal and urethra, and the vagina in women.
The large levator ani consists of two skeletal muscles, the pubococcygeus and the iliococcygeus ((link]). The
levator ani is considered the most important muscle of the pelvic floor because it supports the pelvic viscera. It
resists the pressure produced by contraction of the abdominal muscles so that the pressure is applied to the
colon to aid in defecation and to the uterus to aid in childbirth (assisted by the ischiococcygeus, which pulls
the coccyx anteriorly). This muscle also creates skeletal muscle sphincters at the urethra and anus.
Muscles of the Pelvic Floor
Pubic crest
Vaginal canal
(females only)
Rectal canal
Sacrum
lliac crests
Pelvic diaphragm (superior view)
The pelvic floor muscles support the pelvic
organs, resist intra-abdominal pressure, and work
as sphincters for the urethra, rectum, and vagina.
The perineum is the diamond-shaped space between the pubic symphysis (anteriorly), the coccyx
(posteriorly), and the ischial tuberosities (laterally), lying just inferior to the pelvic diaphragm (levator ani and
coccygeus). Divided transversely into triangles, the anterior is the urogenital triangle, which includes the
external genitals. The posterior is the anal triangle, which contains the anus ([link]). The perineum is also
divided into superficial and deep layers with some of the muscles common to men and women ([(link]).
Women also have the compressor urethrae and the sphincter urethrovaginalis, which function to close the
vagina. In men, there is the deep transverse perineal muscle that plays a role in ejaculation.
Muscles of the Perineum
Penis
Clitoris
Ischiocavernosus
Bulbospongiosus (aka
bulbocavernosus)
Urethra
Vagina
Transverse perineal
muscles
Anus
External anal sphincter
Levator ani
Coccyx
Gluteus maximus
Male perineal muscles: inferior view Female perineal muscles: inferior view
The perineum muscles play roles in urination in both sexes,
ejaculation in men, and vaginal contraction in women.
Muscles of the Perineum Common to Men and Women
Target motion ;
[tarot [Taman | Primemover | ova | insertion |
Movement
Defecation; Abdominal Superior Levator ani Pubis; ischium Urethra; anal
urination; birth; cavity (resists pubococcygeus; canal; perineal
coughing pressure levator ani body; coccyx
during iliococcygeus
abdominal
compression)
Superficial muscles
None— Perineal body Superficial Ischium Perineal body
supports transverse
perineal body perineal
maintaining
anus at center
of perineum
Involuntary Urethra Compression Bulbospongiosus | Perineal body Perineal
response that membrane;
compresses corpus
urethra when spongiosum
excreting urine of penis; deep
in both sexes or fascia of penis;
while ejaculating clitoris in
in males; also female
aids in erection of
penis in males
Compresses Veins of penis Compression Ischiocavernosus | Ischium; ischial | Pubic
veins to maintain | and clitoris rami; pubic rami | symphysis;
erection of penis corpus
in males; erection cavernosum of
of clitoris in penis in male;
females clitoris of
female
Deep muscles
Voluntarily Urethra Compression External urethral Ischial rami; Male: median
compresses sphincter pubic rami raphe; female:
urethra during vaginal wall
urination
External anal
sphincter
Closes anus Sphincter Anoccoccygeal | Perineal body
ligament
Chapter Review
Made of skin, fascia, and four pairs of muscle, the anterior abdominal wall protects the organs located in the
abdomen and moves the vertebral column. These muscles include the rectus abdominis, which extends through
the entire length of the trunk, the external oblique, the internal oblique, and the transversus abdominus. The
quadratus lumborum forms the posterior abdominal wall.
The muscles of the thorax play a large role in breathing, especially the dome-shaped diaphragm. When it
contracts and flattens, the volume inside the pleural cavities increases, which decreases the pressure within
them. As a result, air will flow into the lungs. The external and internal intercostal muscles span the space
between the ribs and help change the shape of the rib cage and the volume-pressure ratio inside the pleural
cavities during inspiration and expiration.
The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in
women. The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as
sphincters for the urethra, rectum, and vagina.
Review Questions
Exercise:
Problem: Which of the following abdominal muscles is not a part of the anterior abdominal wall?
a. quadratus lumborum
b. rectus abdominis
c. interior oblique
d. exterior oblique
Solution:
A
Exercise:
Problem: Which muscle pair plays a role in respiration?
a. intertransversarii, interspinales
b. semispinalis cervicis, semispinalis thoracis
c. trapezius, rhomboids
d. diaphragm, scalene
Solution:
D
Exercise:
Problem: What is the linea alba?
a. a small muscle that helps with compression of the abdominal organs
b. a long tendon that runs down the middle of the rectus abdominis
c. a long band of collagen fibers that connects the hip to the knee
d. another name for the tendinous inscription
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Describe the fascicle arrangement in the muscles of the abdominal wall. How do they relate to each
other?
Solution:
Arranged into layers, the muscles of the abdominal wall are the internal and external obliques, which run
on diagonals, the rectus abdominis, which runs straight down the midline of the body, and the transversus
abdominis, which wraps across the trunk of the body.
Exercise:
Problem: What are some similarities and differences between the diaphragm and the pelvic diaphragm?
Solution:
Both diaphragms are thin sheets of skeletal muscle that horizontally span areas of the trunk. The
diaphragm separating the thoracic and abdominal cavities is the primary muscle of breathing. The pelvic
diaphragm, consisting of two paired muscles, the coccygeus and the levator ani, forms the pelvic floor at
the inferior end of the trunk.
Glossary
anal triangle
posterior triangle of the perineum that includes the anus
caval opening
opening in the diaphragm that allows the inferior vena cava to pass through; foramen for the vena cava
compressor urethrae
deep perineal muscle in women
deep transverse perineal
deep perineal muscle in men
diaphragm
skeletal muscle that separates the thoracic and abdominal cavities and is dome-shaped at rest
external intercostal
superficial intercostal muscles that raise the rib cage
external oblique
superficial abdominal muscle with fascicles that extend inferiorly and medially
iliococcygeus
muscle that makes up the levator ani along with the pubococcygeus
innermost intercostal
the deepest intercostal muscles that draw the ribs together
intercostal muscles
muscles that span the spaces between the ribs
internal intercostal
muscles the intermediate intercostal muscles that draw the ribs together
internal oblique
flat, intermediate abdominal muscle with fascicles that run perpendicular to those of the external oblique
ischiococcygeus
muscle that assists the levator ani and pulls the coccyx anteriorly
levator ani
pelvic muscle that resists intra-abdominal pressure and supports the pelvic viscera
linea alba
white, fibrous band that runs along the midline of the trunk
pelvic diaphragm
muscular sheet that comprises the levator ani and the ischiococcygeus
perineum
diamond-shaped region between the pubic symphysis, coccyx, and ischial tuberosities
pubococcygeus
muscle that makes up the levator ani along with the iliococcygeus
quadratus lumborum
posterior part of the abdominal wall that helps with posture and stabilization of the body
rectus abdominis
long, linear muscle that extends along the middle of the trunk
rectus sheaths
tissue that makes up the linea alba
sphincter urethrovaginalis
deep perineal muscle in women
tendinous intersections
three transverse bands of collagen fibers that divide the rectus abdominis into segments
transversus abdominis
deep layer of the abdomen that has fascicles arranged transversely around the abdomen
urogenital triangle
anterior triangle of the perineum that includes the external genitals
Muscles of the Pectoral Girdle and Upper Limbs
By the end of this section, you will be able to:
e Identify the muscles of the pectoral girdle and upper limbs
e Identify the movement and function of the pectoral girdle and upper limbs
Muscles of the shoulder and upper limb can be divided into four groups: muscles that stabilize and position the
pectoral girdle, muscles that move the arm, muscles that move the forearm, and muscles that move the wrists,
hands, and fingers. The pectoral girdle, or shoulder girdle, consists of the lateral ends of the clavicle and scapula,
along with the proximal end of the humerus, and the muscles covering these three bones to stabilize the shoulder
joint. The girdle creates a base from which the head of the humerus, in its ball-and-socket joint with the glenoid
fossa of the scapula, can move the arm in multiple directions.
Muscles That Position the Pectoral Girdle
Muscles that position the pectoral girdle are located either on the anterior thorax or on the posterior thorax ([link]
and [link]). The anterior muscles include the subclavius, pectoralis minor, and serratus anterior. The posterior
muscles include the trapezius, rhomboid major, and rhomboid minor. When the rhomboids are contracted, your
scapula moves medially, which can pull the shoulder and upper limb posteriorly.
Muscles That Position the Pectoral Girdle
Deltoid (cut)
Coracoid process
of scapula
Pectoralis major
(cut) Acromion process Rhomboid
Scapula of scapula minor
Subclavius
Pectoralis minor Rhomboid
major
Sternum Serratus :
anterior
Deltoid
Trapezius
Pectoral girdle muscle (left anterior lateral view) Pectoral girdle muscles (posterior view)
The muscles that stabilize the pectoral girdle make it a steady
base on which other muscles can move the arm. Note that the
pectoralis major and deltoid, which move the humerus, are cut
here to show the deeper positioning muscles.
Muscles that Position the Pectoral Girdle
Position Target
in the motion Prime
thorax Movement Target direction mover Origin Insertion
Stabilizes ;
Anterior clavicle durin ia
6 Clavicle Depression Subclavius First rib surface of
thorax movement by Ears
depressing it
Muscles that Position the Pectoral Girdle
Position
in the
thorax
Anterior
thorax
Anterior
thorax
Posterior
thorax
Posterior
thorax
Posterior
thorax
Movement
Rotates
shoulder
anteriorly
(throwing
motion); assists
with inhalation
Moves arm
from side of
body to front
of body; assists
with inhalation
Elevates
shoulders
(shrugging);
pulls shoulder
blades
together; tilts
head
backwards
Stabilizes
scapula during
pectoral girdle
movement
Stabilizes
scapula during
pectoral girdle
movement
Target
Scapula;
ribs
Scapula;
ribs
Scapula;
cervical
spine
Scapula
Scapula
Muscles That Move the Humerus
Similar to the muscles that position the pectoral girdle, muscles that cross the shoulder joint and move the humerus
Target
motion
direction
Scapula:
depresses;
ribs: elevates
Scapula:
protracts;
ribs: elevates
Scapula:
rotests
inferiorly,
retracts,
elevates, and
depresses;
spine:
extends
Retracts;
rotates
inferiorly
Retracts;
rotates
inferiorly
Prime
mover
Pectoralis
minor
Serratus
anterior
Trapezius
Rhomboid
major
Rhomboid
minor
Origin
Anterior
surfaces
of
certain
ribs (2-4
or 3-5)
Muscle
slips
from
certain
ribs (1-8
or 1-9)
Skull;
vertebral
column
Thoracic
vertebrae
(T2-T5)
Cervical
and
thoracic
vertebrae
(C7 and
T1)
Insertion
Coracoid
process of
scapula
Anterior
surface of
vertebral
border of
scapula
Acromion
and spine
of
scapula;
clavicle
Medial
border of
scapula
Medial
border of
scapula
bone of the arm include both axial and scapular muscles ((link] and [link]). The two axial muscles are the
pectoralis major and the latissimus dorsi. The pectoralis major is thick and fan-shaped, covering much of the
superior portion of the anterior thorax. The broad, triangular latissimus dorsi is located on the inferior part of the
back, where it inserts into a thick connective tissue shealth called an aponeurosis.
Muscles That Move the Humerus
Pectoralis
major
Latissimus dorsi
AM
(a) Pectoralis major and latissimus dorsi
(left anterior lateral view)
(b) Left deltoid and left latissimus dorsi
(posterior view)
Teres minor Supraspinatus
Deltoid
(cut) Spine of
NN scapula
|
Deltoid (cut)
Coracoid process
of scapula
Infraspinatus
Pectoralis Humerus
major (cut)
Subscapularis
Teres major
Teres major Latissimus dorsi
(near its origin)
Serratus Triceps brachii: long head
anterior
Triceps brachii: lateral head
B Ya
(d) Deep muscles of the left shoulder
(posterior view)
(c) Deep muscles of the left shoulder
(anterior lateral view)
(a, c) The muscles that move the humerus anteriorly are
generally located on the anterior side of the body and
originate from the sternum (e.g., pectoralis major) or the
anterior side of the scapula (e.g., subscapularis). (b) The
muscles that move the humerus superiorly generally
originate from the superior surfaces of the scapula and/or
the clavicle (e.g., deltoids). The muscles that move the
humerus inferiorly generally originate from middle or
lower back (e.g., latissiumus dorsi). (d) The muscles that
move the humerus posteriorly are generally located on the
posterior side of the body and insert into the scapula (e.g.,
infraspinatus).
Muscles That Move the Humerus
Target motion
direction
Movement Target Prime mover Origin Insertion
Axial muscles
Brings elbows Humerus Flexion; Pectoralis Clavicle; sternum; | Greater
together; moves adduction; major cartilage of certain | tubercle of
elbow up (as medial ribs (1-6 or 1-7); | humerus
during an uppercut rotation aponeurosis of
punch) external oblique
muscle
Moves elbow back Humerus; Humerus: Latissimus Thoracic Intertubercular
(as in elbowing scapula extension, dorsi vertebrae sulcus of
someone standing adduction, and (T7-T12); lumbar | humerus
behind you); medial rotation; vertebrae; lower
spreads elbows scapula: ribs (9-12);
apart depression iliac crest
Scapular muscles
Lifts arms at Humerus Abduction; Deltoid Trapezius; Deltoid
shoulder flexion; clavicle; tuberosity
extension; acromion; of humerus
medial and spine of scapula
lateral rotation
Assists pectoralis
major in bringing
elbows together
Humerus Medial Subscapularis Subscapular Lesser
rotation fossa of tubercle of
scapula humerus
shoulder joint during
movement of the
pectoral girdle
and stabilizes
Rotates elbow Humerus Abduction Supraspinatus Supraspinous Greater
outwards, as during fossa of scapula | tubercle of
a tennis swing humerus
Rotates elbow Humerus Extension; Infraspinatus Infraspinous fossa | Greater
outwards, as during adduction of scapula tubercle of
a tennis swing humerus
Assists infraspinatus | Humerus Extension; Teres major Posterior surface | Intertubercular
in rotating elbow adduction of scapula sulcus of
outwards humerus
Assists infraspinatus | Humerus Extension; Teres minor Lateral border of Greater
in rotating elbow adduction dorsal scapular tubercle of
outwards surface humerus
Moves elbow up Humerus Flexion; Coracobra Coracoid process | Medial surface
and across body, adduction chialis of scapula of humerus,
as when putting shaft
hand on chest
The rest of the shoulder muscles originate on the scapula. The anatomical and ligamental structure of the shoulder
joint and the arrangements of the muscles covering it, allows the arm to carry out different types of movements.
The deltoid, the thick muscle that creates the rounded lines of the shoulder is the major abductor of the arm, but it
also facilitates flexing and medial rotation, as well as extension and lateral rotation. The subscapularis originates
on the anterior scapula and medially rotates the arm. Named for their locations, the supraspinatus (superior to the
spine of the scapula) and the infraspinatus (inferior to the spine of the scapula) abduct the arm, and laterally rotate
the arm, respectively. The thick and flat teres major is inferior to the teres minor and extends the arm, and assists
in adduction and medial rotation of it. The long teres minor laterally rotates and extends the arm. Finally, the
coracobrachialis flexes and adducts the arm.
The tendons of the deep subscapularis, supraspinatus, infraspinatus, and teres minor connect the scapula to the
humerus, forming the rotator cuff (musculotendinous cuff), the circle of tendons around the shoulder joint. When
baseball pitchers undergo shoulder surgery it is usually on the rotator cuff, which becomes pinched and inflamed,
and may tear away from the bone due to the repetitive motion of bring the arm overhead to throw a fast pitch.
Muscles That Move the Forearm
The forearm, made of the radius and ulna bones, has four main types of action at the hinge of the elbow joint:
flexion, extension, pronation, and supination. The forearm flexors include the biceps brachii, brachialis, and
brachioradialis. The extensors are the triceps brachii and anconeus. The pronators are the pronator teres and the
pronator quadratus, and the supinator is the only one that turns the forearm anteriorly. When the forearm faces
anteriorly, it is supinated. When the forearm faces posteriorly, it is pronated.
The biceps brachii, brachialis, and brachioradialis flex the forearm. The two-headed biceps brachii crosses the
shoulder and elbow joints to flex the forearm, also taking part in supinating the forearm at the radioulnar joints and
flexing the arm at the shoulder joint. Deep to the biceps brachii, the brachialis provides additional power in
flexing the forearm. Finally, the brachioradialis can flex the forearm quickly or help lift a load slowly. These
muscles and their associated blood vessels and nerves form the anterior compartment of the arm (anterior flexor
compartment of the arm) ([link] and [link]).
Muscles That Move the Forearm
Biceps brachii
(short head) Triceps brachii
. ¥ (lateral head)
Biceps brachii
(long head)
Triceps brachii
(long head)
Left upper arm muscles (anterior lateral view) Left upper arm muscles (posterior view)
Triceps brachii
Brachioradialis
Extensor carpi radialis longus
Extensor carpi radialis brevis
Lateral
epicondyle
of humerus
Pronator teres
Abductor pollicis longus
Extensor pollicis brevis
Extensor pollicis
Flexor carpi
radialis
. Anconeus
Palmaris longus
longus Extensor carpi
Flexor carpi ulnaris ulnaris
Extensor digitorum
Flexor digitorum superficialis Extensor digiti
i minimi
g
Left forearm superficial muscles (palmar view) Left forearm superficial muscles (dorsal view)
Lateral epicondyle of humerus
Medial epicondyle of humerus
Supinator
Flexor pollicis longus
Brachialis Abductor pollicis longus .
(cut) \ Medial
epicondyle
Pronator quadratus of humerus
Flexor digitorum Extensor pollicis longus
profundus Fl
4 4 jexor
Extensor pollicis Be digitorum
brevis é : profundus
Flexor carpi ulnaris
Flexor retinaculum
(cut)
Extensor indicis
y Extensor
rz - retinaculum
Left forearm deep muscles (palmar view) Left forearm deep muscles (dorsal view)
The muscles originating in the upper arm flex, extend, pronate, and
supinate the forearm. The muscles originating in the forearm move the
wrists, hands, and fingers.
Muscles That Move the Forearm
Target motion ‘
aie | taroet | Tagetcnane | Primemover | ovgin | insertion |
Anterior muscles (flexion)
Performs a bicep Forearm Flexion; Biceps brachii Coracoid process; | Radial
curl; also allows supination tubercle above tuberosity
palm of hand to glenoid cavity
point toward body
while flexing
Forearm Flexion Brachialis Front of distal Coronoid
humerus process of ulna
Assists and Forearm Flexion Brachioradialis Lateral Base of styloid
stabilizes elbow supracondylar process of
during bicep-curl tidge at distal end | radius
motion of humerus
Posterior muscles (extension)
Extends forearm, Forearm Extension Triceps brachii Infraglenoid Olecranon
as during a punch tubercle of process of ulna
scapula; posterior
shaft of humerus;
posterior humeral
shaft distal to
radial groove
Assists in extending | Forearm Extension; Anconeus Lateral epicondyle | Lateral aspect
forearm; also allows abduction of humerus of olecranon
forearm to extend process of ulna
away from body
Anterior muscles (pronation)
Turns hand Forearm Pronation Pronator teres
palm-down
Assists in Forearm Pronation Pronator Distal portion Distal surface
turning hand quadratus of anterior ulnar of anterior
palm-down shaft radius
Posterior muscles (supination)
Medial epicondyle | Lateral radius
of humerus;
coronoid process
of ulna
Turns hand Forearm Supination Supinator Lateral epicondyle | Proximal end
palm-up of humerus; of radius
proximal ulna
Muscles That Move the Wrist, Hand, and Fingers
Wrist, hand, and finger movements are facilitated by two groups of muscles. The forearm is the origin of the
extrinsic muscles of the hand. The palm is the origin of the intrinsic muscles of the hand.
Muscles of the Arm That Move the Wrists, Hands, and Fingers
The muscles in the anterior compartment of the forearm (anterior flexor compartment of the forearm) originate
on the humerus and insert onto different parts of the hand. These make up the bulk of the forearm. From lateral to
medial, the superficial anterior compartment of the forearm includes the flexor carpi radialis, palmaris
longus, flexor carpi ulnaris, and flexor digitorum superficialis. The flexor digitorum superficialis flexes the
hand as well as the digits at the knuckles, which allows for rapid finger movements, as in typing or playing a
musical instrument (see [link] and [link]). However, poor ergonomics can irritate the tendons of these muscles as
they slide back and forth with the carpal tunnel of the anterior wrist and pinch the median nerve, which also travels
through the tunnel, causing Carpal Tunnel Syndrome. The deep anterior compartment produces flexion and
bends fingers to make a fist. These are the flexor pollicis longus and the flexor digitorum profundus.
The muscles in the superficial posterior compartment of the forearm (superficial posterior extensor
compartment of the forearm) originate on the humerus. These are the extensor radialis longus, extensor carpi
radialis brevis, extensor digitorum, extensor digiti minimi, and the extensor carpi ulnaris.
The muscles of the deep posterior compartment of the forearm (deep posterior extensor compartment of the
forearm) originate on the radius and ulna. These include the abductor pollicis longus, extensor pollicis brevis,
extensor pollicis longus, and extensor indicis (see [link]).
Muscles That Move the Wrist, Hands, and Forearm
Target motion
Target direction
Superficial anterior compartment of forearm
Eine mene | ori
Insertion
Bends wrist toward body; tilts | Wrist;
hand to side away from body | hand
Flexion;
abduction
Assists in bending hand up Flexion
toward shoulder
Flexor carpi
radialis
Medial epicondyle
of humerus
Palmaris
longus
Medial epicondyle
of humerus
Base of second
and third
metacarpals
Palmar
aponeurosis; skin
and fascia of palm
Assists in bending hand up
toward shoulder; tilts hand to
side away from body;
stabilizes wrist
Flexion,
abduction
also bends wrist toward body | fingers
Bends fingers to make a fist | Wrist; Flexion
fingers
Deep anterior compartment of forearm
Bends tip of Thumb Flexion
thumb
Bends fingers to make a fist; | Wrist; Flexion
Superficial posterior compartment of forearm
Wrist Extension;
abduction
Straightens wrist away from
body; tilts hand to side away
from body
Assists extensor radialis
Extension,
longus in extending and abduction
Flexor carpi
ulnaris
Medial epicondyle
of humerus; olecranon
process; posterior
surface of ulna
Flexor digitorum
superficialis
Medial epicondyle of
humerus; coronoid
process of ulna; shaft
of radius
Anterior surface of
radius; interosseous
membrane
Flexor pollicis
longus
Flexor digitorum | Coronoid process;
profundus anteromedial surface of
ulna; interosseous
membrane
Extensor Lateral supracondylar
radialis longus ridge of humerus
Extensor carpi
radialis brevis
Lateral epicondyle
of humerus
abducting wrist; also
stabilizes hand during finger
flexion.
Opens fingers and moves Wrist; Extension;
them sideways away from fingers abduction
the body
Extends little finger Little Extension
finger
Straightens wrist away from | Wrist Extension;
body; tilts hand to side adduction
toward body
Deep posterior compartment of forearm
Thumb:
abduction,
Moves thumb sideways
toward body; extends thumb;
moves hand sideways
toward body
extension;
wrist: abduction
Extends thumb Thumb Extension
Extends thumb Thumb Extension
Extends index finger; Wrist; Extension
straightens wrist away from index
body finger
Extensor Lateral epicondyle
digitorum of humerus
Extensor Lateral epicondyle
digiti minimi of humerus
Extensor carpi
ulnaris
Lateral epicondyle of
humerus; posterior
border of ulna
Abductor Posterior surface
pollicis longus of radius and ulna;
interosseous
membrane
Extensor Dorsal shaft of radius
pollicis brevis and ulna; interosseous
membrane
Extensor Dorsal shaft of radius
pollicis longus and ulna; interosseous
membrane
Posterior surface
of distal ulna;
interosseous membrane
Extensor indicis
Pisiform, hamate
bones, and base of
fifth metacarpal
Middle phalanges
of fingers 2-5
Distal phalanx
of thumb
Distal phalanges
of fingers 2-5
Base of second
metacarpal
Base of third
metacarpal
Extensor
expansions;
distal phalanges
of fingers
Extensor
expansion;
distal phalanx of
finger 5
Base of fifth
metacarpal
Base of first
metacarpal;
trapezium
Base of proximal
phalanx of thumb
Base of distal
phalanx of thumb
Tendon of extensor
digitorum of index
finger
The tendons of the forearm muscles attach to the wrist and extend into the hand. Fibrous bands called retinacula
sheath the tendons at the wrist. The flexor retinaculum extends over the palmar surface of the hand while the
extensor retinaculum extends over the dorsal surface of the hand.
Intrinsic Muscles of the Hand
The intrinsic muscles of the hand both originate and insert within it ([link]). These muscles allow your fingers to
also make precise movements for actions, such as typing or writing. These muscles are divided into three groups.
The thenar muscles are on the radial aspect of the palm. The hypothenar muscles are on the medial aspect of the
palm, and the intermediate muscles are midpalmar.
The thenar muscles include the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and the
adductor pollicis. These muscles form the thenar eminence, the rounded contour of the base of the thumb, and
all act on the thumb. The movements of the thumb play an integral role in most precise movements of the hand.
The hypothenar muscles include the abductor digiti minimi, flexor digiti minimi brevis, and the opponens
digiti minimi. These muscles form the hypothenar eminence, the rounded contour of the little finger, and as
such, they all act on the little finger. Finally, the intermediate muscles act on all the fingers and include the
lumbrical, the palmar interossei, and the dorsal interossei.
Intrinsic Muscles of the Hand
Vee
Opponens pollicis
Abductor digiti
minimi Abductor pollicis
brevl
Flexor digiti ie
minimi brevis
Pisometacarpal
Flexor pollicis ligament
brevis
FLX ‘I : Opponens digiti
; 2 minimi
! Adductor pollicis oe
f
y
{ |
Lumbricalis
muscles U
Superficial muscles of left hand (palmar)
Dorsal interossei
muscles
Palmar interossei
muscles
Interossei muscles of left hand (palmar view) Interossei muscles of left hand (dorsal view)
The intrinsic muscles of the hand both originate and
insert within the hand. These muscles provide the fine
motor control of the fingers by flexing, extending,
abducting, and adducting the more distal finger and
thumb segments.
Intrinsic Muscles of the Hand
Target
motion Prime
Muscle Movement Target direction mover
Thenar Moves thumb . eeu
Thumb Abduction pollicis
muscles toward body .
brevis
Moves thumb
Thenar across palm to ve Opponens
Th ice
muscles touch other nae Pepesiien pollicis
fingers
Origin
Flexor
retinaculum;
and nearby
carpals
Flexor
retinaculum;
trapezium
Insertion
Lateral
base of
proximal
phalanx o
thumb
Anterior ¢
first
metacarpe
Intrinsic Muscles of the Hand
Muscle
Thenar
muscles
Thenar
muscles
Hypothenar
muscles
Hypothenar
muscles
Hypothenar
muscles
Intermediate
muscles
Movement
Flexes thumb
Moves thumb
away from
body
Moves little
finger toward
body
Flexes little
finger
Moves little
finger across
palm to touch
thumb
Flexes each
finger at
metacarpo-
phalangeal
joints; extends
each finger at
interphalangeal
joints
Target
Thumb
Thumb
Little
finger
Little
finger
Little
finger
Fingers
Target
motion
direction
Flexion
Adduction
Abduction
Flexion
Opposition
Flexion
Prime
mover
Flexor
pollicis
brevis
Adductor
pollicis
Abductor
digiti
minimi
Flexor
digiti
minimi
brevis
Opponens
digiti
minimi
Lumbricals
Origin
Flexor
retinaculum;
trapezium
Capitate
bone; bases
of
metacarpals
2-4; front of
metacarpal
3
Pisiform
bone
Hamate
bone; flexor
retinaculum
Hamate
bone; flexor
retinaculum
Palm
(lateral sides
of tendons
in flexor
digitorum
profundus)
Insertion
Lateral
base of
proximal
phalanx o
thumb
Medial
base of
proximal
phalanx o
thumb
Medial
side of
proximal
phalanx o
little finge
Medial
side of
proximal
phalanx o
little finge
Medial
side of
fifth
metacarpé
Fingers 2-
5 (lateral
edges of
extension:
expansion
on first
phalanges
Intrinsic Muscles of the Hand
Target
motion Prime
Muscle Movement Target direction mover Origin Insertion
Adducts and : Peters!
Side of each expansion
flexes each :
; metacarpal on first
Bngenat that faces hal
oe phalanx o
‘ metacarpo- Adduction; f
Intermediate : an Palmar metacarpal each finge
phalangeal Fingers flexion; . ;
muscles eeeeen : interossei 3 (absent (except
joints; extends extension ;
é from finger 3)
each finger at :
; metacarpal on side
interphalangeal 3) facing
join :
JoMts finger 3
Abducts and ee ae
flexes the three foi eee i
middle fingers Sian
at metacarpo- finger
5 halangeal Ab ion; : :
Intermediate pe nge ‘ dnctic Dorsal Sides of extensor
joints; extends Fingers flexion; . . :
muscles : interossei metacarpals expansion
the three extension sen tict
middle fingers phalanx o
a ;
a side
interphalangeal opposite
join 3
jeans finger 3
Chapter Review
The clavicle and scapula make up the pectoral girdle, which provides a stable origin for the muscles that move the
humerus. The muscles that position and stabilize the pectoral girdle are located on the thorax. The anterior thoracic
muscles are the subclavius, pectoralis minor, and the serratus anterior. The posterior thoracic muscles are the
trapezius, levator scapulae, rhomboid major, and rhomboid minor. Nine muscles cross the shoulder joint to move
the humerus. The ones that originate on the axial skeleton are the pectoralis major and the latissimus dorsi. The
deltoid, subscapularis, supraspinatus, infraspinatus, teres major, teres minor, and coracobrachialis originate on the
scapula.
The forearm flexors include the biceps brachii, brachialis, and brachioradialis. The extensors are the triceps brachii
and anconeus. The pronators are the pronator teres and the pronator quadratus. The supinator is the only one that
turns the forearm anteriorly.
The extrinsic muscles of the hands originate along the forearm and insert into the hand in order to facilitate crude
movements of the wrists, hands, and fingers. The superficial anterior compartment of the forearm produces
flexion. These muscles are the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and the flexor digitorum
superficialis. The deep anterior compartment produces flexion as well. These are the flexor pollicis longus and the
flexor digitorum profundus. The rest of the compartments produce extension. The extensor carpi radialis longus,
extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris are the
muscles found in the superficial posterior compartment. The deep posterior compartment includes the abductor
longus, extensor pollicis brevis, extensor pollicis longus, and the extensor indicis.
Finally, the intrinsic muscles of the hands allow our fingers to make precise movements, such as typing and
writing. They both originate and insert within the hand. The thenar muscles, which are located on the lateral part of
the palm, are the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and adductor pollicis. The
hypothenar muscles, which are located on the medial part of the palm, are the abductor digiti minimi, flexor digiti
minimi brevis, and opponens digiti minimi. The intermediate muscles, located in the middle of the palm, are the
lumbricals, palmar interossei, and dorsal interossei.
Review Questions
Exercise:
Problem:The rhomboid major and minor muscles are deep to the
a. rectus abdominis
b. scalene muscles
c. trapezius
d. ligamentum nuchae
Solution:
C
Exercise:
Problem:Which muscle extends the forearm?
a. biceps brachii
b. triceps brachii
c. brachialis
d. deltoid
Solution:
B
Exercise:
Problem: What is the origin of the wrist flexors?
a. the lateral epicondyle of the humerus
b. the medial epicondyle of the humerus
c. the carpal bones of the wrist
d. the deltoid tuberosity of the humerus
Solution:
B
Exercise:
Problem:Which muscles stabilize the pectoral girdle?
a. axial and scapular
b. axial
c. appendicular
d. axial and appendicular
Solution:
A
Critical Thinking Questions
Exercise:
Problem:The tendons of which muscles form the rotator cuff? Why is the rotator cuff important?
Solution:
Tendons of the infraspinatus, supraspinatus, teres minor, and the subscapularis form the rotator cuff, which
forms a foundation on which the arms and shoulders can be stabilized and move.
Exercise:
Problem: List the general muscle groups of the shoulders and upper limbs as well as their subgroups.
Solution:
The muscles that make up the shoulders and upper limbs include the muscles that position the pelvic girdle,
the muscles that move the humerus, the muscles that move the forearm, and the muscles that move the wrists,
hands, and fingers.
Glossary
abductor digiti minimi
muscle that abducts the little finger
adductor pollicis
muscle that adducts the thumb
abductor pollicis brevis
muscle that abducts the thumb
abductor pollicis longus
muscle that inserts into the first metacarpal
anconeus
small muscle on the lateral posterior elbow that extends the forearm
anterior compartment of the arm
(anterior flexor compartment of the arm) the biceps brachii, brachialis, brachioradialis, and their associated
blood vessels and nerves
anterior compartment of the forearm
(anterior flexor compartment of the forearm) deep and superficial muscles that originate on the humerus and
insert into the hand
biceps brachii
two-headed muscle that crosses the shoulder and elbow joints to flex the forearm while assisting in supinating
it and flexing the arm at the shoulder
brachialis
muscle deep to the biceps brachii that provides power in flexing the forearm.
brachioradialis
muscle that can flex the forearm quickly or help lift a load slowly
coracobrachialis
muscle that flexes and adducts the arm
deep anterior compartment
flexor pollicis longus, flexor digitorum profundus, and their associated blood vessels and nerves
deep posterior compartment of the forearm
(deep posterior extensor compartment of the forearm) the abductor pollicis longus, extensor pollicis brevis,
extensor pollicis longus, extensor indicis, and their associated blood vessels and nerves
deltoid
shoulder muscle that abducts the arm as well as flexes and medially rotates it, and extends and laterally
rotates it
dorsal interossei
muscles that abduct and flex the three middle fingers at the metacarpophalangeal joints and extend them at the
interphalangeal joints
extensor carpi radialis brevis
muscle that extends and abducts the hand at the wrist
extensor carpi ulnaris
muscle that extends and adducts the hand
extensor digiti minimi
muscle that extends the little finger
extensor digitorum
muscle that extends the hand at the wrist and the phalanges
extensor indicis
muscle that inserts onto the tendon of the extensor digitorum of the index finger
extensor pollicis brevis
muscle that inserts onto the base of the proximal phalanx of the thumb
extensor pollicis longus
muscle that inserts onto the base of the distal phalanx of the thumb
extensor radialis longus
muscle that extends and abducts the hand at the wrist
extensor retinaculum
band of connective tissue that extends over the dorsal surface of the hand
extrinsic muscles of the hand
muscles that move the wrists, hands, and fingers and originate on the arm
flexor carpi radialis
muscle that flexes and abducts the hand at the wrist
flexor carpi ulnaris
muscle that flexes and adducts the hand at the wrist
flexor digiti minimi brevis
muscle that flexes the little finger
flexor digitorum profundus
muscle that flexes the phalanges of the fingers and the hand at the wrist
flexor digitorum superficialis
muscle that flexes the hand and the digits
flexor pollicis brevis
muscle that flexes the thumb
flexor pollicis longus
muscle that flexes the distal phalanx of the thumb
flexor retinaculum
band of connective tissue that extends over the palmar surface of the hand
hypothenar
group of muscles on the medial aspect of the palm
hypothenar eminence
rounded contour of muscle at the base of the little finger
infraspinatus
muscle that laterally rotates the arm
intermediate
group of midpalmar muscles
intrinsic muscles of the hand
muscles that move the wrists, hands, and fingers and originate in the palm
latissimus dorsi
broad, triangular axial muscle located on the inferior part of the back
lumbrical
muscle that flexes each finger at the metacarpophalangeal joints and extend each finger at the interphalangeal
joints
opponens digiti minimi
muscle that brings the little finger across the palm to meet the thumb
opponens pollicis
muscle that moves the thumb across the palm to meet another finger
palmar interossei
muscles that abduct and flex each finger at the metacarpophalangeal joints and extend each finger at the
interphalangeal joints
palmaris longus
muscle that provides weak flexion of the hand at the wrist
pectoral girdle
shoulder girdle, made up of the clavicle and scapula
pectoralis major
thick, fan-shaped axial muscle that covers much of the superior thorax
pectoralis minor
muscle that moves the scapula and assists in inhalation
pronator quadratus
pronator that originates on the ulna and inserts on the radius
pronator teres
pronator that originates on the humerus and inserts on the radius
retinacula
fibrous bands that sheath the tendons at the wrist
rhomboid major
muscle that attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae
rhomboid minor
muscle that attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae
rotator cuff
(also, musculotendinous cuff) the circle of tendons around the shoulder joint
serratus anterior
large and flat muscle that originates on the ribs and inserts onto the scapula
subclavius
muscle that stabilizes the clavicle during movement
subscapularis
muscle that originates on the anterior scapula and medially rotates the arm
superficial anterior compartment of the forearm
flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, and their associated
blood vessels and nerves
superficial posterior compartment of the forearm
extensor radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor
carpi ulnaris, and their associated blood vessels and nerves
supinator
muscle that moves the palm and forearm anteriorly
supraspinatus
muscle that abducts the arm
teres major
muscle that extends the arm and assists in adduction and medial rotation of it
teres minor
muscle that laterally rotates and extends the arm
thenar
group of muscles on the lateral aspect of the palm
thenar eminence
rounded contour of muscle at the base of the thumb
trapezius
muscle that stabilizes the upper part of the back
triceps brachii
three-headed muscle that extends the forearm
Muscles of the Pelvic Girdle and Lower Limbs
By the end of this section, you will be able to:
e Identify the appendicular muscles of the pelvic girdle and lower limb
e Identify the movement and function of the pelvic girdle and lower limb
The appendicular muscles of the lower body position and stabilize the
pelvic girdle, which serves as a foundation for the lower limbs.
Comparatively, there is much more movement at the pectoral girdle than at
the pelvic girdle. There is very little movement of the pelvic girdle because
of its connection with the sacrum at the base of the axial skeleton. The
pelvic girdle is less range of motion because it was designed to stabilize and
support the body.
Muscles of the Thigh
What would happen if the pelvic girdle, which attaches the lower limbs to
the torso, were capable of the same range of motion as the pectoral girdle?
For one thing, walking would expend more energy if the heads of the
femurs were not secured in the acetabula of the pelvis. The body’s center of
gravity is in the area of the pelvis. If the center of gravity were not to
remain fixed, standing up would be difficult as well. Therefore, what the leg
muscles lack in range of motion and versatility, they make up for in size and
power, facilitating the body’s stabilization, posture, and movement.
Gluteal Region Muscles That Move the Femur
Most muscles that insert on the femur (the thigh bone) and move it,
originate on the pelvic girdle. The psoas major and iliacus make up the
iliopsoas group. Some of the largest and most powerful muscles in the
body are the gluteal muscles or gluteal group. The gluteus maximus is the
largest; deep to the gluteus maximus is the gluteus medius, and deep to the
gluteus medius is the gluteus minimus, the smallest of the trio ((link] and
[link]).
Hip and Thigh Muscles
Quadratus
lumborum
Psoas major
lliacus ;
Pectineus
ihn Sacrum
Tensor Adductor longus
fascia latae Gracilis
Adductor
Rectus magnus
femoris Sartorius
Vastus Vastus medialis
lateralis eer tendon
(or patellar tendon)
i Patellar ligament
Superficial pelvic and Bee muscles
of right leg (anterior view)
Crest of ilium Gluteus
medius (cut)
i Sacrum
lliac ; Gluteus
crest Pectineus roe minimus
: urator
o : Pubis interns Piriformis
Cbturator Glluteus Superior
externus rasanlis gemellus
Inferior
gemellus
Obturator
externus
Adductor (cut)
brevis
Adductor Adductor
longus group
Adductor Gracilis
magnus
Quadratus
‘ femoris
Semimembranosus
Biceps
Semitendinosus femoris
Deep pelvic and thigh muscles Pelvic and thigh muscles of
of right leg (anterior view) right leg (posterior view)
The large and powerful muscles of the hip that
move the femur generally originate on the pelvic
girdle and insert into the femur. The muscles that
move the lower leg typically originate on the
femur and insert into the bones of the knee joint.
The anterior muscles of the femur extend the lower
leg but also aid in flexing the thigh. The posterior
muscles of the femur flex the lower leg but also
aid in extending the thigh. A combination of
gluteal and thigh muscles also adduct, abduct, and
rotate the thigh and lower leg.
Gluteal Region Muscles That Move the Femur
Target motion A 7
| __ Movement | Tarot | T™gescgne” | Primemover | origin | serton |
lliopsoas group
Raises knee at hip, as if
performing a knee attack;
assists lateral rotators in
twisting thigh (and lower leg)
outward; assists with bending
over, maintaining posture
Raises knee at hip, as if
performing a knee attack;
assists lateral rotators in
twisting thigh (and lower leg)
outward; assists with bending
over, maintaining posture
Gluteal group
Lowers knee and moves
thigh back, as when getting
ready to kick a ball
Opens thighs, as when
doing a split
Brings the thighs back
together
Assists with raising knee at
hip and opening thighs;
maintains posture by
stabilizing the iliotibial track,
which connects to the knee
Lateral rotators
Twists thigh (and lower leg)
outward; maintains posture
by stabilizing hip joint
Twists thigh (and lower leg)
outward; maintains posture
by stabilizing hip joint
Twists thigh (and lower leg)
outward; maintains posture
by stabilizing hip joint
Twists thigh (and lower leg)
outward; maintains posture
by stabilizing hip joint
Twists thigh (and lower
leg) outward; maintains
posture by stabilizing
hip joint
Twists thigh (and lower leg)
outward; maintains posture
by stabilizing hip joint
Adductors
Brings the thighs back
together; assists with
raising the knee
Brings the thighs back
together; assists with
raising the knee
Brings the thighs back
together; assists with raising
the knee and moving the
thigh back
Opens thighs; assists with
raising the knee and turning
the thigh (and lower leg)
inward
Lumbar vertebrae
(L1-L5);
thoracic vertebra (T12)
Psoas major
lliacus
Gluteus maximus
Thigh: flexion and
lateral rotation;
torso: flexion
lliac fossa; iliac crest;
lateral sacrum
Thigh: flexion and
lateral rotation;
torso: flexion
Extension
Dorsal ilium; sacrum;
coccyx
Femur
Femur
Gluteus
minimus
Femur
Flexion;
abduction
Tensor fascia
lata
Obutrator
internus
Femur Anterior aspect of
iliac crest; anterior
superior iliac spine
Anterolateral surface
of sacrum
Lateral rotation
Inner surface of
obturator membrane;
greater sciatic notch;
margins of obturator
foramen
Lateral rotation
Outer surfaces of
obturator membrane,
pubic, and ischium;
margins of obturator
foramen
Obturator
externus
Lateral rotation
Lateral rotation Superior
gemellus
Ischial spine
Inferior
gemellus
Lateral rotation Ischial tuberosity
Quadratus
femoris
Lateral rotation Ischial tuberosity
Adduction;
flexion
Adductor longus | Pubis near pubic
symphysis
Adduction; Adductor brevis
flexion
Body of pubis; inferior
ramus of pubis
Adductor
magnus
Adduction;
flexion;
extension
Ischial rami; pubic rami;
ischial tuberosity
Adduction; flexion; | Pectineus Pectineal line of pubis
medial rotation
Lesser trochanter
of femur
Lesser trochanter
of femur
Gluteal tuberosity
of femur; iliotibial
tract
Abduction Gluteus Lateral surface of illum | Greater trochanter
medius of femur
External surface of Greater trochanter
ilium of femur
lliotibial tract
Greater trochanter
of femur
Greater trochanter
in front of piriformis
Trochanteric fossa
of posterior femur
Greater trochanter
of femur
Greater trochanter
of femur
Trochanteric
crest of femur
Linea aspera
Linea aspera
above adductor
longus
Linea aspera;
adductor tubercle
of femur
Lesser trochanter
to linea aspera of
posterior aspect of
femur
The tensor fascia latae is a thick, squarish muscle in the superior aspect of
the lateral thigh. It acts as a synergist of the gluteus medius and iliopsoas in
flexing and abducting the thigh. It also helps stabilize the lateral aspect of
the knee by pulling on the iliotibial tract (band), making it taut. Deep to
the gluteus maximus, the piriformis, obturator internus, obturator
externus, superior gemellus, inferior gemellus, and quadratus femoris
laterally rotate the femur at the hip.
The adductor longus, adductor brevis, and adductor magnus can both
medially and laterally rotate the thigh depending on the placement of the
foot. The adductor longus flexes the thigh, whereas the adductor magnus
extends it. The pectineus adducts and flexes the femur at the hip as well.
The pectineus is located in the femoral triangle, which is formed at the
junction between the hip and the leg and also includes the femoral nerve,
the femoral artery, the femoral vein, and the deep inguinal lymph nodes.
Thigh Muscles That Move the Femur, Tibia, and Fibula
Deep fascia in the thigh separates it into medial, anterior, and posterior
compartments (see [link] and [link]). The muscles in the medial
compartment of the thigh are responsible for adducting the femur at the
hip. Along with the adductor longus, adductor brevis, adductor magnus, and
pectineus, the strap-like gracilis adducts the thigh in addition to flexing the
leg at the knee.
Thigh Muscles That Move the Femur, Tibia, and Fibula
Target Target metion Prime mover
direction
Medial compartment of thigh
Moves back of lower
legs up toward
buttocks, as when
kneeling; assists in
opening thighs
Femur;
tibia/fibula
Tibia/fibula:
flexion; thigh:
adduction
Gracilis
Anterior compartment of thigh: Quadriceps femoris group
Moves lower leg out
in front of body, as
when kicking; assists
in raising the knee
Moves lower leg out
in front of body, as
when kicking
Moves lower leg out
in front of body, as
when kicking
Moves lower leg out
in front of body, as
when kicking
Moves back of lower
legs up and back
toward the buttocks,
as when kneeling;
assists in moving
thigh diagonally
upward and outward
as when mounting
a bike
Posterior compartment of thigh: Hamstring group
Moves back of lower
legs up and back
oward the buttocks,
as when kneeling;
moves thigh down
and back; twists the
high (and lower leg)
outward
Moves back of lower
legs up toward
buttocks, as when
kneeling; moves
high down and back;
twists the thigh
(and lower leg)
inward
Moves back of lower
legs up and back
toward the buttocks
as when kneeling;
moves thigh down
and back; twists the
thigh (and lower leg)
inward
Femur;
tibia/fibula
Tibia/fibula
Tibia/fibula
Tibia/fibula
Femur;
tibia/fibula
Femur;
tibia/fibula
Femur;
tibia/fibula
Femur;
tibia/fibula
Tibia/fibula:
extension;
thigh: flexion
Extension
Extension
Extension
Tibia: flexion;
thigh: flexion,
abduction,
lateral
rotation
Tibia/fibula:
flexion; thigh:
extension,
lateral
rotation
Tibia/fibula:
flexion; thigh:
extension,
medial
rotation
Tibia/fibula:
flexion; thigh:
extension,
medial
rotation
Rectus
femoris
Vastus
lateralis
Vastus
medialis
Vastus
intermedius
Sartorius
Biceps
femoris
Semitendinosus
Semi-
membranosus
Inferior ramus;
body of pubis;
ischial ramus
Anterior inferior
iliac spine; superior
margin of
acetabulum
Greater trochanter;
intertrochanteric
line; linea aspera
Linea aspera;
intertrochanteric
line
Proximal femur
shaft
Anterior superior
iliac spine
Ischial tuberosity;
linea aspera;
distal femur
Ischial tuberosity
Ischial tuberosity
Medial surface
of tibia
Patella; tibial
tuberosity
Patella; tibial
tuberosity
Patella; tibial
tuberosity
Patella; tibial
tuberosity
Medial aspect
of proximal
tibia
Head of fibula;
lateral condyle
of tibia
Upper tibial
shaft
Medial condyle
of tibia; lateral
condyle of
femur
The muscles of the anterior compartment of the thigh flex the thigh and
extend the leg. This compartment contains the quadriceps femoris group,
which actually comprises four muscles that extend and stabilize the knee.
The rectus femoris is on the anterior aspect of the thigh, the vastus
lateralis is on the lateral aspect of the thigh, the vastus medialis is on the
medial aspect of the thigh, and the vastus intermedius is between the
vastus lateralis and vastus medialis and deep to the rectus femoris. The
tendon common to all four is the quadriceps tendon (patellar tendon),
which inserts into the patella and continues below it as the patellar
ligament. The patellar ligament attaches to the tibial tuberosity. In addition
to the quadriceps femoris, the sartorius is a band-like muscle that extends
from the anterior superior iliac spine to the medial side of the proximal
tibia. This versatile muscle flexes the leg at the knee and flexes, abducts,
and laterally rotates the leg at the hip. This muscle allows us to sit cross-
legged.
The posterior compartment of the thigh includes muscles that flex the leg
and extend the thigh. The three long muscles on the back of the knee are the
hamstring group, which flexes the knee. These are the biceps femoris,
semitendinosus, and semimembranosus. The tendons of these muscles
form the popliteal fossa, the diamond-shaped space at the back of the knee.
Muscles That Move the Feet and Toes
Similar to the thigh muscles, the muscles of the leg are divided by deep
fascia into compartments, although the leg has three: anterior, lateral, and
posterior ([link] and [link]).
Muscles of the i Leg
Gastrocnemius
(lateral head)
Superior extensor"
retinaculum
Inferior edeneare——
retinaculum
Superficial muscles of the right
lower leg (anterior view)
Tibialis anterior
Fibularis longus
Extensor digitorum
longus
Fibularis brevis
Extensor hallucis
longus
Fibularis tertius
Gastrocnemius
(medial head)
Plantaris
Soleus
Calcaneal (Achilles) |
tendon } |
>
Superficial muscles of the right
lower leg (posterior view)
Calcaneus (heel)
Popliteus
Soleus (cut)
Fibularis longus
Tibialis posterior
Flexor digitorum
longus
Flexor hallucis
longus
Fibularis brevis
Deep muscles of the right
lower leg (posterior view)
The muscles of the anterior compartment of the lower leg are
generally responsible for dorsiflexion, and the muscles of the
posterior compartment of the lower leg are generally
responsible for plantar flexion. The lateral and medial
muscles in both compartments invert, evert, and rotate the
foot.
Muscles That Move the Feet and Toes
Target motion ‘
Movement | Target | Taegeamenon | Primemover | origin] serton |
Anterior compartment of leg
Raises the sole of the foot off the
ground, as when preparing to
foot-tap; bends the inside of the
foot upwards, as when catching
your balance while falling laterally
toward the opposite side as the
balancing foot
Raises the sole of the foot off
the ground, as when preparing
to foot-tap; extends the big toe
Raises the sole of the foot off
the ground, as when preparing
to foot-tap; extends toes
Lateral compartment of leg
Lowers the sole of the foot to
the ground, as when foot-tapping
or jumping; bends the inside of
the foot downwards, as when
catching your balance while falling
laterally toward the same side
as the balancing foot
Lowers the sole of the foot to
the ground, as when foot-tapping
or jumping; bends the inside of
the foot downward, as when
catching your balance while
falling laterally toward the same
side as the balancing foot
Foot;
big toe
Foot;
toes
2-5
Dorsiflexion;
inversion
Foot: dorsiflexion;
big toe: extension
Foot: dorsiflexion;
toes: extension
Plantar flexion
and eversion
Plantar flexion
and eversion
Posterior compartment of leg: Superficial muscles
Lowers the sole of the foot to the
ground, as when foot-tapping or
jumping; assists in moving the
back of the lower legs up and
back toward the buttocks
Lowers the sole of the foot to the
ground, as when foot-tapping or
jumping; maintains posture while
walking
Lowers the sole of the foot to the
ground, as when foot-tapping or
jumping; assists in moving the
back of the lower legs up and back
toward the buttocks
Lowers the sole of the foot to the
ground, as when foot-tapping or
jumping
Posterior compartment of leg: Deep muscles
Moves the back of the lower legs
up and back toward the buttocks;
assists in rotation of the leg at the
knee and thigh
Lowers the sole of the foot to the
ground, as when foot-tapping or
jumping; bends the inside of the
foot upward and flexes toes
Flexes the big toe
Foot;
tibia/
fibula
Foot;
tibia/
fibula
Tibia/
fibula
Foot;
toes 2-5
Big toe;
foot
Foot: plantar
flexion;
tibia/fibula: flexion
Plantar
flexion
Foot: plantar
flexion;
tibia/fibula:
flexion
Plantar flexion
Tibialis anterior
Extensor hallucis
longus
Extensor
digitorum longus
Fibularis longus
Fibularis
(peroneus) brevis
Gastrocnemius
Plantaris
Tibialis posterior
Tibia/fibula: flexion} Popliteus
thigh and lower
leg: medial and
lateral rotation
Foot: plantar
flexion and
inversion
toes: flexion
Big toe:
flexion foot:
plantar flexion
Flexor digitorum
longus
Flexor hallucis
longus
Lateral condyle
and upper tibial
shaft; interosseous
membrane
Anteromedial fibula
shaft; interosseous
membrane
Lateral condyle of
tibia; proximal portion
of fibula; interosseous
membrane
Upper portion of
lateral fibula
Distal fibula shaft
Medial and lateral
condyles of femur
Superior tibia;
fibula; interosseous
membrane
Posterior femur
above lateral
condyle
Superior tibia and
fibula; interosseous
membrane
Lateral condyle of
femur; lateral
meniscus
Posterior tibia
Midshaft of fibula;
interosseous
membrane
Interior surface
of medial
cuneiform;
First metatarsal
bone
Distal phalanx
of big toe
Middle and distal
phalanges of
toes 2-5
First metatarsal;
medial cuneiform
Proximal end
of fifth
metatarsal
Posterior
calcaneus
Posterior
calcaneus
Calcaneus or
calcaneus
tendon
Several tarsals
and metatarsals
2-4
Proximal tibia
Distal phalanges
of toes 2-5
Distal phalanx
of big toe
The muscles in the anterior compartment of the leg: the tibialis anterior,
a long and thick muscle on the lateral surface of the tibia, the extensor
hallucis longus, deep under it, and the extensor digitorum longus, lateral
to it, all contribute to raising the front of the foot when they contract. The
fibularis tertius, a small muscle that originates on the anterior surface of
the fibula, is associated with the extensor digitorum longus and sometimes
fused to it, but is not present in all people. Thick bands of connective tissue
called the superior extensor retinaculum (transverse ligament of the
ankle) and the inferior extensor retinaculum, hold the tendons of these
muscles in place during dorsiflexion.
The lateral compartment of the leg includes two muscles: the fibularis
longus (peroneus longus) and the fibularis brevis (peroneus brevis). The
superficial muscles in the posterior compartment of the leg all insert onto
the calcaneal tendon (Achilles tendon), a strong tendon that inserts into the
calcaneal bone of the ankle. The muscles in this compartment are large and
strong and keep humans upright. The most superficial and visible muscle of
the calf is the gastrocnemius. Deep to the gastrocnemius is the wide, flat
soleus. The plantaris runs obliquely between the two; some people may
have two of these muscles, whereas no plantaris is observed in about seven
percent of other cadaver dissections. The plantaris tendon is a desirable
substitute for the fascia lata in hernia repair, tendon transplants, and repair
of ligaments. There are four deep muscles in the posterior compartment of
the leg as well: the popliteus, flexor digitorum longus, flexor hallucis
longus, and tibialis posterior.
The foot also has intrinsic muscles, which originate and insert within it
(similar to the intrinsic muscles of the hand). These muscles primarily
provide support for the foot and its arch, and contribute to movements of
the toes ([link] and [link]). The principal support for the longitudinal arch of
the foot is a deep fascia called plantar aponeurosis, which runs from the
calcaneus bone to the toes (inflammation of this tissue is the cause of
“plantar fasciitis,” which can affect runners. The intrinsic muscles of the
foot consist of two groups. The dorsal group includes only one muscle, the
extensor digitorum brevis. The second group is the plantar group, which
consists of four layers, starting with the most superficial.
Intrinsic Muscles of the Foot
Tendocalcaneus
Fibularis longus Extensor digitorum brevis
——_____ Tibialis anterior
Extensor digitorum longus
-. Extensor hallucis longus
Y
Fibularis brevis
Fibularis tertius
(a) Dorsal superficial muscles of
the right foot (lateral view)
Abductor
digiti
minimi
Plantar
aponeurosis
Quadratus Flexor digiti
a plantae minimi brevis
allucis
Flexor Flexor
digitorum hallucis
brevis ae brevis
y @ ti) i (Vy . ;
Cl} Wh \\\/ \ Wi Lumbricals
v dH W
(b) Superficial muscles of the (c) Intermediate muscles of (d) Deep muscles of the
left sole (plantar view) the left sole (plantar view) left sole (plantar view)
The muscles along the dorsal side of the foot (a) generally
extend the toes while the muscles of the plantar side of the
foot (b, c, d) generally flex the toes. The plantar muscles
exist in three layers, providing the foot the strength to
counterbalance the weight of the body. In this diagram,
these three layers are shown from a plantar view beginning
with the bottom-most layer just under the plantar skin of
the foot (b) and ending with the top-most layer (d) located
just inferior to the foot and toe bones.
Intrinsic Muscles in the Foot
Target motion Prime
Dorsal group
Extends toes 2-5 Toes 2-5 | Extension Extensor Calcaneus; Base of proximal
digitorum extensor phalanx of big toe;
brevis retinaculum extensor expansions
on toes 2-5
Plantar group (layer 1)
Abducts and flexes Adduction; Abductor Calcaneal Proximal phalanx of
big toe flexion hallucis tuberosity; flexor big toe
retinaculum
Flexes toes 2-4 Middle Flexion Flexor Calcaneal Middle phalanx of
toes digitorum tuberosity toes 2-4
brevis
Abducts and flexes Toe 5 Abduction; Abductor Calcaneal tuberosity | Proximal phalanx
small toe flexion digiti minimi of little toe
Plantar group (layer 2)
Assists in flexing Toes 2-5 | Flexion Quadratus Medial and lateral Tendon of flexor
toes 2-5 plantae sides of calcaneus digitorum longus
Extends toes 2-5 at | Toes 2-5 | Extension; Lumbricals Tendons of flexor Medial side of
the interphalangeal flexion digitorum longus proximal phalanx of
joints; flexes the toes 2-5
small toes at the
metatarsophalangeal
joints
Plantar group (layer 3)
Flexes big toe Big toe Flexion Flexor Lateral cuneiform; Base of proximal
hallucis cuboid bones phalanx of big toe
brevis
Adducts and flexes Adduction; Adductor Bases of Base of proximal
big toe flexion hallucis metatarsals 2—4; phalanx of big toe
fibularis longus
tendon sheath;
ligament across
metatarsophalangeal
joints
Flexes small toe Little toe | Flexion Flexor digiti Base of metatarsal 5; | Base of proximal
minimi brevis | tendon sheath of phalanx of little toe
fibularis longus
Plantar group (layer 4)
Abducts and flexes Middle Abduction; Dorsal Sides of metatarsals | Both sides of toe 2;
middle toes at toes flexion; interossei for each other toe,
metatarsophalangeal extension extensor expansion
joints; extends over first phalanx on
middle toes at side opposite toe 2
interphalangeal
joints
Abducts toes 3-5; Small Abduction; Plantar Side of each Extensor expansion
flexes proximal toes flexion; interossei metatarsal that faces | on first phalanx of
phalanges and extension metatarsal 2 (absent | each toe (except to
extends distal from metatarsal 2) 2) on side facing
toe 2
phalanges
Chapter Review
The pelvic girdle attaches the legs to the axial skeleton. The hip joint is
where the pelvic girdle and the leg come together. The hip is joined to the
pelvic girdle by many muscles. In the gluteal region, the psoas major and
iliacus form the iliopsoas. The large and strong gluteus maximus, gluteus
medius, and gluteus minimus extend and abduct the femur. Along with the
gluteus maximus, the tensor fascia lata muscle forms the iliotibial tract. The
lateral rotators of the femur at the hip are the piriformis, obturator internus,
obturator externus, superior gemellus, inferior gemellus, and quadratus
femoris. On the medial part of the thigh, the adductor longus, adductor
brevis, and adductor magnus adduct the thigh and medially rotate it. The
pectineus muscle adducts and flexes the femur at the hip.
The thigh muscles that move the femur, tibia, and fibula are divided into
medial, anterior, and posterior compartments. The medial compartment
includes the adductors, pectineus, and the gracilis. The anterior
compartment comprises the quadriceps femoris, quadriceps tendon, patellar
ligament, and the sartorius. The quadriceps femoris is made of four
muscles: the rectus femoris, the vastus lateralis, the vastus medius, and the
vastus intermedius, which together extend the knee. The posterior
compartment of the thigh includes the hamstrings: the biceps femoris,
semitendinosus, and the semimembranosus, which all flex the knee.
The muscles of the leg that move the foot and toes are divided into anterior,
lateral, superficial- and deep-posterior compartments. The anterior
compartment includes the tibialis anterior, the extensor hallucis longus, the
extensor digitorum longus, and the fibularis (peroneus) tertius. The lateral
compartment houses the fibularis (peroneus) longus and the fibularis
(peroneus) brevis. The superficial posterior compartment has the
gastrocnemius, soleus, and plantaris; and the deep posterior compartment
has the popliteus, tibialis posterior, flexor digitorum longus, and flexor
hallucis longus.
Review Questions
Exercise:
Problem:
The large muscle group that attaches the leg to the pelvic girdle and
produces extension of the hip joint is the group.
a. gluteal
b. obturator
c. adductor
d. abductor
Solution:
A
Exercise:
Problem:
Which muscle produces movement that allows you to cross your legs?
a. the gluteus maximus
b. the piriformis
c. the gracilis
d. the sartorius
Solution:
D
Exercise:
Problem: What is the largest muscle in the lower leg?
a. soleus
b. gastrocnemius
c. tibialis anterior
d. tibialis posterior
Solution:
B
Exercise:
Problem:
The vastus intermedius muscle is deep to which of the following
muscles?
a. biceps femoris
b. rectus femoris
c. vastus medialis
d. vastus lateralis
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Which muscles form the hamstrings? How do they function together?
Solution:
The biceps femoris, semimembranosus, and semitendinosus form the
hamstrings. The hamstrings flex the leg at the knee joint.
Exercise:
Problem:
Which muscles form the quadriceps? How do they function together?
Solution:
The rectus femoris, vastus medialis, vastus lateralis, and vastus
intermedius form the quadriceps. The quadriceps muscles extend the
leg at the knee joint.
Glossary
adductor brevis
muscle that adducts and medially rotates the thigh
adductor longus
muscle that adducts, medially rotates, and flexes the thigh
adductor magnus
muscle with an anterior fascicle that adducts, medially rotates and
flexes the thigh, and a posterior fascicle that assists in thigh extension
anterior compartment of the leg
region that includes muscles that dorsiflex the foot
anterior compartment of the thigh
region that includes muscles that flex the thigh and extend the leg
biceps femoris
hamstring muscle
calcaneal tendon
(also, Achilles tendon) strong tendon that inserts into the calcaneal
bone of the ankle
dorsal group
region that includes the extensor digitorum brevis
extensor digitorum brevis
muscle that extends the toes
extensor digitorum longus
muscle that is lateral to the tibialis anterior
extensor hallucis longus
muscle that is partly deep to the tibialis anterior and extensor
digitorum longus
femoral triangle
region formed at the junction between the hip and the leg and includes
the pectineus, femoral nerve, femoral artery, femoral vein, and deep
inguinal lymph nodes
fibularis brevis
(also, peroneus brevis) muscle that plantar flexes the foot at the ankle
and everts it at the intertarsal joints
fibularis longus
(also, peroneus longus) muscle that plantar flexes the foot at the ankle
and everts it at the intertarsal joints
fibularis tertius
small muscle that is associated with the extensor digitorum longus
flexor digitorum longus
muscle that flexes the four small toes
flexor hallucis longus
muscle that flexes the big toe
gastrocnemius
most superficial muscle of the calf
gluteal group
muscle group that extends, flexes, rotates, adducts, and abducts the
femur
gluteus maximus
largest of the gluteus muscles that extends the femur
gluteus medius
muscle deep to the gluteus maximus that abducts the femur at the hip
gluteus minimus
smallest of the gluteal muscles and deep to the gluteus medius
gracilis
muscle that adducts the thigh and flexes the leg at the knee
hamstring group
three long muscles on the back of the leg
iliacus
muscle that, along with the psoas major, makes up the iliopsoas
iliopsoas group
muscle group consisting of iliacus and psoas major muscles, that flexes
the thigh at the hip, rotates it laterally, and flexes the trunk of the body
onto the hip
iliotibial tract
muscle that inserts onto the tibia; made up of the gluteus maximus and
connective tissues of the tensor fasciae latae
inferior extensor retinaculum
cruciate ligament of the ankle
inferior gemellus
muscle deep to the gluteus maximus on the lateral surface of the thigh
that laterally rotates the femur at the hip
lateral compartment of the leg
region that includes the fibularis (peroneus) longus and the fibularis
(peroneus) brevis and their associated blood vessels and nerves
medial compartment of the thigh
a region that includes the adductor longus, adductor brevis, adductor
magnus, pectineus, gracilis, and their associated blood vessels and
nerves
obturator externus
muscle deep to the gluteus maximus on the lateral surface of the thigh
that laterally rotates the femur at the hip
obturator internus
muscle deep to the gluteus maximus on the lateral surface of the thigh
that laterally rotates the femur at the hip
patellar ligament
extension of the quadriceps tendon below the patella
pectineus
muscle that abducts and flexes the femur at the hip
pelvic girdle
hips, a foundation for the lower limb
piriformis
muscle deep to the gluteus maximus on the lateral surface of the thigh
that laterally rotates the femur at the hip
plantar aponeurosis
muscle that supports the longitudinal arch of the foot
plantar group
four-layered group of intrinsic foot muscles
plantaris
muscle that runs obliquely between the gastrocnemius and the soleus
popliteal fossa
diamond-shaped space at the back of the knee
popliteus
muscle that flexes the leg at the knee and creates the floor of the
popliteal fossa
posterior compartment of the leg
region that includes the superficial gastrocnemius, soleus, and
plantaris, and the deep popliteus, flexor digitorum longus, flexor
hallucis longus, and tibialis posterior
posterior compartment of the thigh
region that includes muscles that flex the leg and extend the thigh
psoas major
muscle that, along with the iliacus, makes up the iliopsoas
quadratus femoris
muscle deep to the gluteus maximus on the lateral surface of the thigh
that laterally rotates the femur at the hip
quadriceps femoris group
four muscles, that extend and stabilize the knee
quadriceps tendon
(also, patellar tendon) tendon common to all four quadriceps muscles,
inserts into the patella
rectus femoris
quadricep muscle on the anterior aspect of the thigh
Sartorius
band-like muscle that flexes, abducts, and laterally rotates the leg at
the hip
semimembranosus
hamstring muscle
semitendinosus
hamstring muscle
soleus
wide, flat muscle deep to the gastrocnemius
superior extensor retinaculum
transverse ligament of the ankle
superior gemellus
muscle deep to the gluteus maximus on the lateral surface of the thigh
that laterally rotates the femur at the hip
tensor fascia lata
muscle that flexes and abducts the thigh
tibialis anterior
muscle located on the lateral surface of the tibia
tibialis posterior
muscle that plantar flexes and inverts the foot
vastus intermedius
quadricep muscle that is between the vastus lateralis and vastus
medialis and is deep to the rectus femoris
vastus lateralis
quadricep muscle on the lateral aspect of the thigh
vastus medialis
quadricep muscle on the medial aspect of the thigh
Heart Anatomy
By the end of this section, you will be able to:
e Describe the location and position of the heart within the body cavity
e Describe the internal and external anatomy of the heart
e Identify the tissue layers of the heart
e Relate the structure of the heart to its function as a pump
e Compare systemic circulation to pulmonary circulation
e Identify the veins and arteries of the coronary circulation system
e Trace the pathway of oxygenated and deoxygenated blood thorough
the chambers of the heart
The vital importance of the heart is obvious. If one assumes an average rate
of contraction of 75 contractions per minute, a human heart would contract
approximately 108,000 times in one day, more than 39 million times in one
year, and nearly 3 billion times during a 75-year lifespan. Each of the major
pumping chambers of the heart ejects approximately 70 mL blood per
contraction in a resting adult. This would be equal to 5.25 liters of fluid per
minute and approximately 14,000 liters per day. Over one year, that would
equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly
60,000 miles of vessels. In order to understand how that happens, it is
necessary to understand the anatomy and physiology of the heart.
Location of the Heart
The human heart is located within the thoracic cavity, medially between the
lungs in the space known as the mediastinum. [link] shows the position of
the heart within the thoracic cavity. Within the mediastinum, the heart is
separated from the other mediastinal structures by a tough membrane
known as the pericardium, or pericardial sac, and sits in its own space
called the pericardial cavity. The dorsal surface of the heart lies near the
bodies of the vertebrae, and its anterior surface sits deep to the sternum and
costal cartilages. The great veins, the superior and inferior venae cavae, and
the great arteries, the aorta and pulmonary trunk, are attached to the
superior surface of the heart, called the base. The base of the heart is
located at the level of the third costal cartilage, as seen in [link]. The
inferior tip of the heart, the apex, lies just to the left of the sternum between
the junction of the fourth and fifth ribs near their articulation with the costal
cartilages. The right side of the heart is deflected anteriorly, and the left side
is deflected posteriorly. It is important to remember the position and
orientation of the heart when placing a stethoscope on the chest of a patient
and listening for heart sounds, and also when looking at images taken from
a midsagittal perspective. The slight deviation of the apex to the left is
reflected in a depression in the medial surface of the inferior lobe of the left
lung, called the cardiac notch.
Position of the Heart in the Thorax
Thoracic
aorta
Sagittal view
Mediastinum
; Arch of aorta
Superior vena cava
Right lung Pulmonary trunk
Right auricle Left auricle
Right atrium Left lung
Right ventricle Left ventricle
Ribs (cut) Pericardial cavity
Ditiimwiesien
Apex of heart
Edge of parietal Edge of parietal
pleura (cut) pericardium (cut)
The heart is located within the thoracic cavity, medially
between the lungs in the mediastinum. It is about the size
of a fist, is broad at the top, and tapers toward the base.
Note:
Everyday Connection
CPR
The position of the heart in the torso between the vertebrae and sternum
(see [link] for the position of the heart within the thorax) allows for
individuals to apply an emergency technique known as cardiopulmonary
resuscitation (CPR) if the heart of a patient should stop. By applying
pressure with the flat portion of one hand on the sternum in the area
between the line at T4 and T9 ((link]), it is possible to manually compress
the blood within the heart enough to push some of the blood within it into
the pulmonary and systemic circuits. This is particularly critical for the
brain, as irreversible damage and death of neurons occur within minutes of
loss of blood flow. Current standards call for compression of the chest at
least 5 cm deep and at a rate of 100 compressions per minute, a rate equal
to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you
are unfamiliar with this song, a version is available on www.youtube.com.
At this stage, the emphasis is on performing high-quality chest
compressions, rather than providing artificial respiration. CPR is generally
performed until the patient regains spontaneous contraction or is declared
dead by an experienced healthcare professional.
When performed by untrained or overzealous individuals, CPR can result
in broken ribs or a broken sternum, and can inflict additional severe
damage on the patient. It is also possible, if the hands are placed too low
on the sternum, to manually drive the xiphoid process into the liver, a
consequence that may prove fatal for the patient. Proper training is
essential. This proven life-sustaining technique is so valuable that virtually
all medical personnel as well as concerned members of the public should
be certified and routinely recertified in its application. CPR courses are
offered at a variety of locations, including colleges, hospitals, the
American Red Cross, and some commercial companies. They normally
include practice of the compression technique on a mannequin.
CPR Technique
If the heart should stop, CPR can maintain
the flow of blood until the heart resumes
beating. By applying pressure to the
sternum, the blood within the heart will be
squeezed out of the heart and into the
circulation. Proper positioning of the
hands on the sternum to perform CPR
would be between the lines at T4 and T9.
Note:
meee OPENStAX COLLEGE
it ‘s
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Visit the American Heart Association website to help locate a course near
your home in the United States. There are also many other national and
regional heart associations that offer the same service, depending upon the
location.
Shape and Size of the Heart
The shape of the heart is similar to a pinecone, rather broad at the superior
surface and tapering to the apex (see [link]). A typical heart is
approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in)
wide, and 6 cm (2.5 in) in thickness. Given the size difference between
most members of the sexes, the weight of a female heart is approximately
250-300 grams (9 to 11 ounces), and the weight of a male heart is
approximately 300-350 grams (11 to 12 ounces). The heart of a well-
trained athlete, especially one specializing in aerobic sports, can be
considerably larger than this. Cardiac muscle responds to exercise in a
manner similar to that of skeletal muscle. That is, exercise results in the
addition of protein myofilaments that increase the size of the individual
cells without increasing their numbers, a concept called hypertrophy. Hearts
of athletes can pump blood more effectively at lower rates than those of
nonathletes. Enlarged hearts are not always a result of exercise; they can
result from pathologies, such as hypertrophic cardiomyopathy. The cause
of an abnormally enlarged heart muscle is unknown, but the condition is
often undiagnosed and can cause sudden death in apparently otherwise
healthy young people.
Chambers and Circulation through the Heart
The human heart consists of four chambers: The left side and the right side
each have one atrium and one ventricle. Each of the upper chambers, the
right atrium (plural = atria) and the left atrium, acts as a receiving chamber
and contracts to push blood into the lower chambers, the right ventricle and
the left ventricle. The ventricles serve as the primary pumping chambers of
the heart, propelling blood to the lungs or to the rest of the body.
There are two distinct but linked circuits in the human circulation called the
pulmonary and systemic circuits. Although both circuits transport blood and
everything it carries, we can initially view the circuits from the point of
view of gases. The pulmonary circuit transports blood to and from the
lungs, where it picks up oxygen and delivers carbon dioxide for exhalation.
The systemic circuit transports oxygenated blood to virtually all of the
tissues of the body and returns relatively deoxygenated blood and carbon
dioxide to the heart to be sent back to the pulmonary circulation.
The right ventricle pumps deoxygenated blood into the pulmonary trunk,
which leads toward the lungs and bifurcates into the left and right
pulmonary arteries. These vessels in turn branch many times before
reaching the pulmonary capillaries, where gas exchange occurs: Carbon
dioxide exits the blood and oxygen enters. The pulmonary trunk arteries
and their branches are the only arteries in the post-natal body that carry
relatively deoxygenated blood. Highly oxygenated blood returning from the
pulmonary capillaries in the lungs passes through a series of vessels that
join together to form the pulmonary veins—the only post-natal veins in the
body that carry highly oxygenated blood. The pulmonary veins conduct
blood into the left atrium, which pumps the blood into the left ventricle,
which in turn pumps oxygenated blood into the aorta and on to the many
branches of the systemic circuit. Eventually, these vessels will lead to the
systemic capillaries, where exchange with the tissue fluid and cells of the
body occurs. In this case, oxygen and nutrients exit the systemic capillaries
to be used by the cells in their metabolic processes, and carbon dioxide and
waste products will enter the blood.
The blood exiting the systemic capillaries is lower in oxygen concentration
than when it entered. The capillaries will ultimately unite to form venules,
joining to form ever-larger veins, eventually flowing into the two major
systemic veins, the superior vena cava and the inferior vena cava, which
return blood to the right atrium. The blood in the superior and inferior
venae cavae flows into the right atrium, which pumps blood into the right
ventricle. This process of blood circulation continues as long as the
individual remains alive. Understanding the flow of blood through the
pulmonary and systemic circuits is critical to all health professions ({link]).
Dual System of the Human Blood Circulation
Aorta
Left pulmonary
Right pulmonary arteries
arteries
Pulmonary trunk
Left atrium
Right pulmonary
veins Left pulmonary
veins
Pulmonary
semilunar
valve
Aortic semilunar
valve
Mitral valve
Right atrium
Tricuspid valve Left ventricle
Right ventricle
Systemic Systemic
veins from capillaries
upper body of upper
body
Systemic
arteries to
Pulmona
es upper body
capillaries
in lungs
Pulmonary
Right atrium trunk
Left atrium
Right
ventricle Left
\ ventricle
Systemic
veins from
Systemic
lower body arteries to
lower body
Systemic
capillaries
of lower body
Blood flows from the right atrium to the right ventricle,
where it is pumped into the pulmonary circuit. The blood in
the pulmonary artery branches is low in oxygen but
relatively high in carbon dioxide. Gas exchange occurs in
the pulmonary capillaries (oxygen into the blood, carbon
dioxide out), and blood high in oxygen and low in carbon
dioxide is returned to the left atrium. From here, blood
enters the left ventricle, which pumps it into the systemic
circuit. Following exchange in the systemic capillaries
(oxygen and nutrients out of the capillaries and carbon
dioxide and wastes in), blood returns to the right atrium and
the cycle is repeated.
Membranes, Surface Features, and Layers
Our exploration of more in-depth heart structures begins by examining the
membrane that surrounds the heart, the prominent surface features of the
heart, and the layers that form the wall of the heart. Each of these
components plays its own unique role in terms of function.
Membranes
The membrane that directly surrounds the heart and defines the pericardial
cavity is called the pericardium or pericardial sac. It also surrounds the
“roots” of the major vessels, or the areas of closest proximity to the heart.
The pericardium, which literally translates as “around the heart,” consists of
two distinct sublayers: the sturdy outer fibrous pericardium and the inner
serous pericardium. The fibrous pericardium is made of tough, dense
connective tissue that protects the heart and maintains its position in the
thorax. The more delicate serous pericardium consists of two layers: the
parietal pericardium, which is fused to the fibrous pericardium, and an inner
visceral pericardium, or epicardium, which is fused to the heart and is part
of the heart wall. The pericardial cavity, filled with lubricating serous fluid,
lies between the epicardium and the pericardium.
In most organs within the body, visceral serous membranes such as the
epicardium are microscopic. However, in the case of the heart, it is not a
microscopic layer but rather a macroscopic layer, consisting of a simple
squamous epithelium called a mesothelium, reinforced with loose,
irregular, or areolar connective tissue that attaches to the pericardium. This
mesothelium secretes the lubricating serous fluid that fills the pericardial
cavity and reduces friction as the heart contracts. [link] illustrates the
pericardial membrane and the layers of the heart.
Pericardial Membranes and Layers of the Heart Wall
Pericardial cavity
Endocardium
Fibrous pericardium
Myocardium
Parietal layer of serous
pericardium
Epicardium (viceral layer
of serous pericardium)
The pericardial membrane that surrounds the heart
consists of three layers and the pericardial cavity. The
heart wall also consists of three layers. The pericardial
membrane and the heart wall share the epicardium.
Note:
Disorders of the...
Heart: Cardiac Tamponade
If excess fluid builds within the pericardial space, it can lead to a condition
called cardiac tamponade, or pericardial tamponade. With each contraction
of the heart, more fluid—in most instances, blood—accumulates within the
pericardial cavity. In order to fill with blood for the next contraction, the
heart must relax. However, the excess fluid in the pericardial cavity puts
pressure on the heart and prevents full relaxation, so the chambers within
the heart contain slightly less blood as they begin each heart cycle. Over
time, less and less blood is ejected from the heart. If the fluid builds up
slowly, as in hypothyroidism, the pericardial cavity may be able to expand
gradually to accommodate this extra volume. Some cases of fluid in excess
of one liter within the pericardial cavity have been reported. Rapid
accumulation of as little as 100 mL of fluid following trauma may trigger
cardiac tamponade. Other common causes include myocardial rupture,
pericarditis, cancer, or even cardiac surgery. Removal of this excess fluid
requires insertion of drainage tubes into the pericardial cavity. Premature
removal of these drainage tubes, for example, following cardiac surgery, or
clot formation within these tubes are causes of this condition. Untreated,
cardiac tamponade can lead to death.
Surface Features of the Heart
Inside the pericardium, the surface features of the heart are visible,
including the four chambers. There is a superficial leaf-like extension of the
atria near the superior surface of the heart, one on each side, called an
auricle—a name that means “ear like”—because its shape resembles the
external ear of a human ([link]). Auricles are relatively thin-walled
structures that can fill with blood and empty into the atria or upper
chambers of the heart. You may also hear them referred to as atrial
appendages. Also prominent is a series of fat-filled grooves, each of which
is known as a sulcus (plural = sulci), along the superior surfaces of the
heart. Major coronary blood vessels are located in these sulci. The deep
coronary sulcus is located between the atria and ventricles. Located
between the left and right ventricles are two additional sulci that are not as
deep as the coronary sulcus. The anterior interventricular sulcus is
visible on the anterior surface of the heart, whereas the posterior
interventricular sulcus is visible on the posterior surface of the heart.
[link] illustrates anterior and posterior views of the surface of the heart.
External Anatomy of the Heart
Brachiocephalic trunk Left common carotid artery
Left subclavian artery
Aortic arch
Ligamentum arteriosum
Left pulmonary artery
Superior vena cava
Right pulmonary
artery
Ascending aorta
Pulmonary trunk
F
Right pulmonary << é
veins
Left pulmonary veins
Auricle of left atrium
Anterior view
Right atrium Circumflex artery
Right coronary artery Left coronary artery
Anterior cardiac vein
: ; Left ventricle
Right ventricle
Great cardiac vein
Anterior interventricular artery
Right marginal artery
Small cardiac vein
Inferior vena cava
Apex
Superior vena cava
Right pul t
Left pulmonary artery ignt pulmonary artery
. Right pulmonary veins
Left pulmonary veins
Auricle of left atrium Right atrium
Left atrium Inferior vena cava
Circumflex branch Coronary sinus
Posterior view of left coronary artery Small cardiac vein
Great cardiac vein Right coronary artery
Posterior vein of
left ventricle Posterior interventricular artery
Middle cardiac vein
Left ventricle
Right ventricle
Inside the pericardium, the surface features of the heart are
visible.
Layers
The wall of the heart is composed of three layers of unequal thickness.
From superficial to deep, these are the epicardium, the myocardium, and the
endocardium (see [link]). The outermost layer of the wall of the heart is
also the innermost layer of the pericardium, the epicardium, or the visceral
pericardium discussed earlier.
The middle and thickest layer is the myocardium, made largely of cardiac
muscle cells. It is built upon a framework of collagenous fibers, plus the
blood vessels that supply the myocardium and the nerve fibers that help
regulate the heart. It is the contraction of the myocardium that pumps blood
through the heart and into the major arteries. The muscle pattern is elegant
and complex, as the muscle cells swirl and spiral around the chambers of
the heart. They form a figure 8 pattern around the atria and around the bases
of the great vessels. Deeper ventricular muscles also form a figure 8 around
the two ventricles and proceed toward the apex. More superficial layers of
ventricular muscle wrap around both ventricles. This complex swirling
pattern allows the heart to pump blood more effectively than a simple linear
pattern would. [link] illustrates the arrangement of muscle cells.
Heart Musculature
Atrial
musculature
Ventricular
musculature
The swirling pattern of
cardiac muscle tissue
contributes significantly to
the heart’s ability to pump
blood effectively.
Although the ventricles on the right and left sides pump the same amount of
blood per contraction, the muscle of the left ventricle is much thicker and
better developed than that of the right ventricle. In order to overcome the
high resistance required to pump blood into the long systemic circuit, the
left ventricle must generate a great amount of pressure. The right ventricle
does not need to generate as much pressure, since the pulmonary circuit is
shorter and provides less resistance. [link] illustrates the differences in
muscular thickness needed for each of the ventricles.
Differences in Ventricular Muscle Thickness
Left ventricle
Right ventricle
Relaxed Contracted
The myocardium in the left ventricle is significantly
thicker than that of the right ventricle. Both ventricles
pump the same amount of blood, but the left ventricle
must generate a much greater pressure to overcome
greater resistance in the systemic circuit. The
ventricles are shown in both relaxed and contracting
states. Note the differences in the relative size of the
lumens, the region inside each ventricle where the
blood is contained.
The innermost layer of the heart wall, the endocardium, is joined to the
myocardium with a thin layer of connective tissue. The endocardium lines
the chambers where the blood circulates and covers the heart valves. It is
made of simple squamous epithelium called endothelium, which is
continuous with the endothelial lining of the blood vessels (see [link]).
Once regarded as a simple lining layer, recent evidence indicates that the
endothelium of the endocardium and the coronary capillaries may play
active roles in regulating the contraction of the muscle within the
myocardium. The endothelium may also regulate the growth patterns of the
cardiac muscle cells throughout life, and the endothelins it secretes create
an environment in the surrounding tissue fluids that regulates ionic
concentrations and states of contractility. Endothelins are potent
vasoconstrictors and, in a normal individual, establish a homeostatic
balance with other vasoconstrictors and vasodilators.
Internal Structure of the Heart
Recall that the heart’s contraction cycle follows a dual pattern of circulation
—the pulmonary and systemic circuits—because of the pairs of chambers
that pump blood into the circulation. In order to develop a more precise
understanding of cardiac function, it is first necessary to explore the internal
anatomical structures in more detail.
Septa of the Heart
The word septum is derived from the Latin for “something that encloses;”
in this case, a septum (plural = septa) refers to a wall or partition that
divides the heart into chambers. The septa are physical extensions of the
myocardium lined with endocardium. Located between the two atria is the
interatrial septum. Normally in an adult heart, the interatrial septum bears
an oval-shaped depression known as the fossa ovalis, a remnant of an
opening in the fetal heart known as the foramen ovale. The foramen ovale
allowed blood in the fetal heart to pass directly from the right atrium to the
left atrium, allowing some blood to bypass the pulmonary circuit. Within
seconds after birth, a flap of tissue known as the septum primum that
previously acted as a valve closes the foramen ovale and establishes the
typical cardiac circulation pattern.
Between the two ventricles is a second septum known as the
interventricular septum. Unlike the interatrial septum, the interventricular
septum is normally intact after its formation during fetal development. It is
substantially thicker than the interatrial septum, since the ventricles
generate far greater pressure when they contract.
The septum between the atria and ventricles is known as the
atrioventricular septum. It is marked by the presence of four openings
that allow blood to move from the atria into the ventricles and from the
ventricles into the pulmonary trunk and aorta. Located in each of these
openings between the atria and ventricles is a valve, a specialized structure
that ensures one-way flow of blood. The valves between the atria and
ventricles are known generically as atrioventricular valves. The valves at
the openings that lead to the pulmonary trunk and aorta are known
generically as semilunar valves. The interventricular septum is visible in
[link]. In this figure, the atrioventricular septum has been removed to better
show the bicupid and tricuspid valves; the interatrial septum is not visible,
since its location is covered by the aorta and pulmonary trunk. Since these
openings and valves structurally weaken the atrioventricular septum, the
remaining tissue is heavily reinforced with dense connective tissue called
the cardiac skeleton, or skeleton of the heart. It includes four rings that
surround the openings between the atria and ventricles, and the openings to
the pulmonary trunk and aorta, and serve as the point of attachment for the
heart valves. The cardiac skeleton also provides an important boundary in
the heart electrical conduction system.
Internal Structures of the Heart
Superior vena cava Left pulmonary artery
Right pulmonary artery peiaidum
Pulmonary trunk Left pulmonary veins
Right pulmonary
veins Mitral (bicuspid) valve
Right atrium
Aortic valve
Fossa ovalis
Tricuspid valve Pulmonary valve
Right ventricle Left ventricle
Chordae tendineae Papillary muscle
Interventricular septum
Epicardium
Myocardium
Endocardium
Trabeculae carneae
Moderator band
Inferior vena cava
Anterior view
This anterior view of the heart shows the four
chambers, the major vessels and their early branches,
as well as the valves. The presence of the pulmonary
trunk and aorta covers the interatrial septum, and the
atrioventricular septum is cut away to show the
atrioventricular valves.
Note:
Disorders of the...
Heart: Heart Defects
One very common form of interatrial septum pathology is patent foramen
ovale, which occurs when the septum primum does not close at birth, and
the fossa ovalis is unable to fuse. The word patent is from the Latin root
patens for “open.” It may be benign or asymptomatic, perhaps never being
diagnosed, or in extreme cases, it may require surgical repair to close the
opening permanently. As much as 20—25 percent of the general population
may have a patent foramen ovale, but fortunately, most have the benign,
asymptomatic version. Patent foramen ovale is normally detected by
auscultation of a heart murmur (an abnormal heart sound) and confirmed
by imaging with an echocardiogram. Despite its prevalence in the general
population, the causes of patent ovale are unknown, and there are no
known risk factors. In nonlife-threatening cases, it is better to monitor the
condition than to risk heart surgery to repair and seal the opening.
Coarctation of the aorta is a congenital abnormal narrowing of the aorta
that is normally located at the insertion of the ligamentum arteriosum, the
remnant of the fetal shunt called the ductus arteriosus. If severe, this
condition drastically restricts blood flow through the primary systemic
artery, which is life threatening. In some individuals, the condition may be
fairly benign and not detected until later in life. Detectable symptoms in an
infant include difficulty breathing, poor appetite, trouble feeding, or failure
to thrive. In older individuals, symptoms include dizziness, fainting,
shortness of breath, chest pain, fatigue, headache, and nosebleeds.
Treatment involves surgery to resect (remove) the affected region or
angioplasty to open the abnormally narrow passageway. Studies have
shown that the earlier the surgery is performed, the better the chance of
survival.
A patent ductus arteriosus is a congenital condition in which the ductus
arteriosus fails to close. The condition may range from severe to benign.
Failure of the ductus arteriosus to close results in blood flowing from the
higher pressure aorta into the lower pressure pulmonary trunk. This
additional fluid moving toward the lungs increases pulmonary pressure and
makes respiration difficult. Symptoms include shortness of breath
(dyspnea), tachycardia, enlarged heart, a widened pulse pressure, and poor
weight gain in infants. Treatments include surgical closure (ligation),
manual closure using platinum coils or specialized mesh inserted via the
femoral artery or vein, or nonsteroidal anti-inflammatory drugs to block
the synthesis of prostaglandin E2, which maintains the vessel in an open
position. If untreated, the condition can result in congestive heart failure.
Septal defects are not uncommon in individuals and may be congenital or
caused by various disease processes. Tetralogy of Fallot is a congenital
condition that may also occur from exposure to unknown environmental
factors; it occurs when there is an opening in the interventricular septum
caused by blockage of the pulmonary trunk, normally at the pulmonary
semilunar valve. This allows blood that is relatively low in oxygen from
the right ventricle to flow into the left ventricle and mix with the blood that
is relatively high in oxygen. Symptoms include a distinct heart murmur,
low blood oxygen percent saturation, dyspnea or difficulty in breathing,
polycythemia, broadening (clubbing) of the fingers and toes, and in
children, difficulty in feeding or failure to grow and develop. It is the most
common cause of cyanosis following birth. The term “tetralogy” is derived
from the four components of the condition, although only three may be
present in an individual patient: pulmonary infundibular stenosis (rigidity
of the pulmonary valve), overriding aorta (the aorta is shifted above both
ventricles), ventricular septal defect (opening), and right ventricular
hypertrophy (enlargement of the right ventricle). Other heart defects may
also accompany this condition, which is typically confirmed by
echocardiography imaging. Tetralogy of Fallot occurs in approximately
AO0 out of one million live births. Normal treatment involves extensive
surgical repair, including the use of stents to redirect blood flow and
replacement of valves and patches to repair the septal defect, but the
condition has a relatively high mortality. Survival rates are currently 75
percent during the first year of life; 60 percent by 4 years of age; 30
percent by 10 years; and 5 percent by 40 years.
In the case of severe septal defects, including both tetralogy of Fallot and
patent foramen ovale, failure of the heart to develop properly can lead to a
condition commonly known as a “blue baby.” Regardless of normal skin
pigmentation, individuals with this condition have an insufficient supply of
oxygenated blood, which leads to cyanosis, a blue or purple coloration of
the skin, especially when active.
Septal defects are commonly first detected through auscultation, listening
to the chest using a stethoscope. In this case, instead of hearing normal
heart sounds attributed to the flow of blood and closing of heart valves,
unusual heart sounds may be detected. This is often followed by medical
imaging to confirm or rule out a diagnosis. In many cases, treatment may
not be needed. Some common congenital heart defects are illustrated in
[link].
Congenital Heart Defects
Narrow segment
Ha of aorta
Foramen ovale
fails to close
(a) Patent foramen ovale (b) Coarctation of the aorta
Ductus arteriosus
remains open
Stenosed
pulmonary
semilunar valve
(c) Patent ductus arteriosus (d) Tetralogy of Fallot
Aorta emerges
from both
ventricles
Interventricular
septal defect
Enlarged right
ventricle
(a) A patent foramen ovale defect is an abnormal opening
in the interatrial septum, or more commonly, a failure of the
foramen ovale to close. (b) Coarctation of the aorta is an
abnormal narrowing of the aorta. (c) A patent ductus
arteriosus is the failure of the ductus arteriosus to close. (d)
Tetralogy of Fallot includes an abnormal opening in the
interventricular septum.
Right Atrium
The right atrium serves as the receiving chamber for blood returning to the
heart from the systemic circulation. The two major systemic veins, the
superior and inferior venae cavae, and the large coronary vein called the
coronary sinus that drains the heart myocardium empty into the right
atrium. The superior vena cava drains blood from regions superior to the
diaphragm: the head, neck, upper limbs, and the thoracic region. It empties
into the superior and posterior portions of the right atrium. The inferior
vena cava drains blood from areas inferior to the diaphragm: the lower
limbs and abdominopelvic region of the body. It, too, empties into the
posterior portion of the atria, but inferior to the opening of the superior vena
cava. Immediately superior and slightly medial to the opening of the
inferior vena cava on the posterior surface of the atrium is the opening of
the coronary sinus. This thin-walled vessel drains most of the coronary
veins that return systemic blood from the heart. The majority of the internal
heart structures discussed in this and subsequent sections are illustrated in
[link].
While the bulk of the internal surface of the right atrium is smooth, the
depression of the fossa ovalis is medial, and the anterior surface
demonstrates prominent ridges of muscle called the pectinate muscles. The
right auricle also has pectinate muscles. The left atrium does not have
pectinate muscles except in the auricle.
The atria receive venous blood on a nearly continuous basis, preventing
venous flow from stopping while the ventricles are contracting. While most
ventricular filling occurs while the atria are relaxed, they do demonstrate a
contractile phase and actively pump blood into the ventricles just prior to
ventricular contraction. The opening between the atrium and ventricle is
guarded by the tricuspid valve.
Right Ventricle
The right ventricle receives blood from the right atrium through the
tricuspid valve. Each flap of the valve is attached to strong strands of
connective tissue, the chordae tendineae, literally “tendinous cords,” or
sometimes more poetically referred to as “heart strings.” There are several
chordae tendineae associated with each of the flaps. They are composed of
approximately 80 percent collagenous fibers with the remainder consisting
of elastic fibers and endothelium. They connect each of the flaps to a
papillary muscle that extends from the inferior ventricular surface. There
are three papillary muscles in the right ventricle, called the anterior,
posterior, and septal muscles, which correspond to the three sections of the
valves.
When the myocardium of the ventricle contracts, pressure within the
ventricular chamber rises. Blood, like any fluid, flows from higher pressure
to lower pressure areas, in this case, toward the pulmonary trunk and the
atrium. To prevent any potential backflow, the papillary muscles also
contract, generating tension on the chordae tendineae. This prevents the
flaps of the valves from being forced into the atria and regurgitation of the
blood back into the atria during ventricular contraction. [link] shows
papillary muscles and chordae tendineae attached to the tricuspid valve.
Chordae Tendineae and Papillary Muscles
Chordae tendineae
Papillary muscles
Trabeculae carneae
In this frontal section, you can see papillary
muscles attached to the tricuspid valve on
the right as well as the mitral valve on the
left via chordae tendineae. (credit:
modification of work by “PV
KS”/flickr.com)
The walls of the ventricle are lined with trabeculae carneae, ridges of
cardiac muscle covered by endocardium. In addition to these muscular
ridges, a band of cardiac muscle, also covered by endocardium, known as
the moderator band (see [link]) reinforces the thin walls of the right
ventricle and plays a crucial role in cardiac conduction. It arises from the
inferior portion of the interventricular septum and crosses the interior space
of the right ventricle to connect with the inferior papillary muscle.
When the right ventricle contracts, it ejects blood into the pulmonary trunk,
which branches into the left and right pulmonary arteries that carry it to
each lung. The superior surface of the right ventricle begins to taper as it
approaches the pulmonary trunk. At the base of the pulmonary trunk is the
pulmonary semilunar valve that prevents backflow from the pulmonary
trunk.
Left Atrium
After exchange of gases in the pulmonary capillaries, blood returns to the
left atrium high in oxygen via one of the four pulmonary veins. While the
left atrium does not contain pectinate muscles, it does have an auricle that
includes these pectinate ridges. Blood flows nearly continuously from the
pulmonary veins back into the atrium, which acts as the receiving chamber,
and from here through an opening into the left ventricle. Most blood flows
passively into the heart while both the atria and ventricles are relaxed, but
toward the end of the ventricular relaxation period, the left atrium will
contract, pumping blood into the ventricle. This atrial contraction accounts
for approximately 20 percent of ventricular filling. The opening between
the left atrium and ventricle is guarded by the mitral valve.
Left Ventricle
Recall that, although both sides of the heart will pump the same amount of
blood, the muscular layer is much thicker in the left ventricle compared to
the right (see [link]). Like the right ventricle, the left also has trabeculae
carneae, but there is no moderator band. The mitral valve is connected to
papillary muscles via chordae tendineae. There are two papillary muscles
on the left—the anterior and posterior—as opposed to three on the right.
The left ventricle is the major pumping chamber for the systemic circuit; it
ejects blood into the aorta through the aortic semilunar valve.
Heart Valve Structure and Function
A transverse section through the heart slightly above the level of the
atrioventricular septum reveals all four heart valves along the same plane
({link]). The valves ensure unidirectional blood flow through the heart.
Between the right atrium and the right ventricle is the right
atrioventricular valve, or tricuspid valve. It typically consists of three
flaps, or leaflets, made of endocardium reinforced with additional
connective tissue. The flaps are connected by chordae tendineae to the
papillary muscles, which control the opening and closing of the valves.
Heart Valves
Posterior
Tricuspid valve Bicuspid (mitral)
Aortic valve Pulmonary valve
Anterior
With the atria and major vessels removed, all
four valves are clearly visible, although it is
difficult to distinguish the three separate cusps
of the tricuspid valve.
Emerging from the right ventricle at the base of the pulmonary trunk is the
pulmonary semilunar valve, or the pulmonary valve; it is also known as
the pulmonic valve or the right semilunar valve. The pulmonary valve is
comprised of three small flaps of endothelium reinforced with connective
tissue. When the ventricle relaxes, the pressure differential causes blood to
flow back into the ventricle from the pulmonary trunk. This flow of blood
fills the pocket-like flaps of the pulmonary valve, causing the valve to close
and producing an audible sound. Unlike the atrioventricular valves, there
are no papillary muscles or chordae tendineae associated with the
pulmonary valve.
Located at the opening between the left atrium and left ventricle is the
mitral valve, also called the bicuspid valve or the left atrioventricular
valve. Structurally, this valve consists of two cusps, known as the anterior
medial cusp and the posterior medial cusp, compared to the three cusps of
the tricuspid valve. In a clinical setting, the valve is referred to as the mitral
valve, rather than the bicuspid valve. The two cusps of the mitral valve are
attached by chordae tendineae to two papillary muscles that project from
the wall of the ventricle.
At the base of the aorta is the aortic semilunar valve, or the aortic valve,
which prevents backflow from the aorta. It normally is composed of three
flaps. When the ventricle relaxes and blood attempts to flow back into the
ventricle from the aorta, blood will fill the cusps of the valve, causing it to
close and producing an audible sound.
In [link]a, the two atrioventricular valves are open and the two semilunar
valves are closed. This occurs when both atria and ventricles are relaxed
and when the atria contract to pump blood into the ventricles. [link]b shows
a frontal view. Although only the left side of the heart is illustrated, the
process is virtually identical on the right.
Blood Flow from the Left Atrium to the Left Ventricle
Posterior
Tricuspid Bicuspid (mitral) valve
valve
\ Left
side of WA . < \ side of
heart
valve
(closed)
Anterior (closed)
(a) Mitral
valve
(open)
Aortic valve
(Closed)
Chordae
tendineae
(loose)
Papillary
muscle
(relaxed)
(b)
(a) A transverse section through the heart illustrates
the four heart valves. The two atrioventricular valves
are open; the two semilunar valves are closed. The
atria and vessels have been removed. (b) A frontal
section through the heart illustrates blood flow
through the mitral valve. When the mitral valve is
open, it allows blood to move from the left atrium to
the left ventricle. The aortic semilunar valve is closed
to prevent backflow of blood from the aorta to the left
ventricle.
[link]a shows the atrioventricular valves closed while the two semilunar
valves are open. This occurs when the ventricles contract to eject blood into
the pulmonary trunk and aorta. Closure of the two atrioventricular valves
prevents blood from being forced back into the atria. This stage can be seen
from a frontal view in [link |b.
Blood Flow from the Left Ventricle into the Great Vessels
Posterior
Tricuspid Bicuspid (mitral) valve
valve - 5 ——— Jf (closed)
\.. {~
Aortic
valve
Pulmonary
(open) valve
Anterior open)
(a) Mitral
valve
(closed)
Aortic valve
(open)
Chordae
tendineae
(tight)
Papillary
muscle
(contracted)
(b)
(a) A transverse section through the heart illustrates
the four heart valves during ventricular contraction.
The two atrioventricular valves are closed, but the two
semilunar valves are open. The atria and vessels have
been removed. (b) A frontal view shows the closed
mitral (bicuspid) valve that prevents backflow of
blood into the left atrium. The aortic semilunar valve
is open to allow blood to be ejected into the aorta.
When the ventricles begin to contract, pressure within the ventricles rises
and blood flows toward the area of lowest pressure, which is initially in the
atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid)
valves to close. These valves are tied down to the papillary muscles by
chordae tendineae. During the relaxation phase of the cardiac cycle, the
papillary muscles are also relaxed and the tension on the chordae tendineae
is slight (see [link]b). However, as the myocardium of the ventricle
contracts, so do the papillary muscles. This creates tension on the chordae
tendineae (see [link]b), helping to hold the cusps of the atrioventricular
valves in place and preventing them from being blown back into the atria.
The aortic and pulmonary semilunar valves lack the chordae tendineae and
papillary muscles associated with the atrioventricular valves. Instead, they
consist of pocket-like folds of endocardium reinforced with additional
connective tissue. When the ventricles relax and the change in pressure
forces the blood toward the ventricles, the blood presses against these cusps
and seals the openings.
Note:
—
meee OPENSTAX COLLEGE
F
Bear es
Visit this site to observe an echocardiogram of actual heart valves opening
and closing. Although much of the heart has been “removed” from this gif
loop so the chordae tendineae are not visible, why is their presence more
critical for the atrioventricular valves (tricuspid and mitral) than the
semilunar (aortic and pulmonary) valves?
Note:
Disorders of the...
Heart Valves
When heart valves do not function properly, they are often described as
incompetent and result in valvular heart disease, which can range from
benign to lethal. Some of these conditions are congenital, that is, the
individual was born with the defect, whereas others may be attributed to
disease processes or trauma. Some malfunctions are treated with
medications, others require surgery, and still others may be mild enough
that the condition is merely monitored since treatment might trigger more
serious Consequences.
Valvular disorders are often caused by carditis, or inflammation of the
heart. One common trigger for this inflammation is rheumatic fever, or
scarlet fever, an autoimmune response to the presence of a bacterium,
Streptococcus pyogenes, normally a disease of childhood.
While any of the heart valves may be involved in valve disorders, mitral
regurgitation is the most common, detected in approximately 2 percent of
the population, and the pulmonary semilunar valve is the least frequently
involved. When a valve malfunctions, the flow of blood to a region will
often be disrupted. The resulting inadequate flow of blood to this region
will be described in general terms as an insufficiency. The specific type of
insufficiency is named for the valve involved: aortic insufficiency, mitral
insufficiency, tricuspid insufficiency, or pulmonary insufficiency.
If one of the cusps of the valve is forced backward by the force of the
blood, the condition is referred to as a prolapsed valve. Prolapse may occur
if the chordae tendineae are damaged or broken, causing the closure
mechanism to fail. The failure of the valve to close properly disrupts the
normal one-way flow of blood and results in regurgitation, when the blood
flows backward from its normal path. Using a stethoscope, the disruption
to the normal flow of blood produces a heart murmur.
Stenosis is a condition in which the heart valves become rigid and may
calcify over time. The loss of flexibility of the valve interferes with normal
function and may cause the heart to work harder to propel blood through
the valve, which eventually weakens the heart. Aortic stenosis affects
approximately 2 percent of the population over 65 years of age, and the
percentage increases to approximately 4 percent in individuals over 85
years. Occasionally, one or more of the chordae tendineae will tear or the
papillary muscle itself may die as a component of a myocardial infarction
(heart attack). In this case, the patient’s condition will deteriorate
dramatically and rapidly, and immediate surgical intervention may be
required.
Auscultation, or listening to a patient’s heart sounds, is one of the most
useful diagnostic tools, since it is proven, safe, and inexpensive. The term
auscultation is derived from the Latin for “to listen,” and the technique has
been used for diagnostic purposes as far back as the ancient Egyptians.
Valve and septal disorders will trigger abnormal heart sounds. If a valvular
disorder is detected or suspected, a test called an echocardiogram, or
simply an “echo,” may be ordered. Echocardiograms are sonograms of the
heart and can help in the diagnosis of valve disorders as well as a wide
variety of heart pathologies.
Note:
[ml aC
Visit this site for a free download, including excellent animations and
audio of heart sounds.
Note:
Career Connection
Cardiologist
Cardiologists are medical doctors that specialize in the diagnosis and
treatment of diseases of the heart. After completing 4 years of medical
school, cardiologists complete a three-year residency in internal medicine
followed by an additional three or more years in cardiology. Following this
10-year period of medical training and clinical experience, they qualify for
a rigorous two-day examination administered by the Board of Internal
Medicine that tests their academic training and clinical abilities, including
diagnostics and treatment. After successful completion of this examination,
a physician becomes a board-certified cardiologist. Some board-certified
cardiologists may be invited to become a Fellow of the American College
of Cardiology (FACC). This professional recognition is awarded to
outstanding physicians based upon merit, including outstanding
credentials, achievements, and community contributions to cardiovascular
medicine.
Note:
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Visit this site to learn more about cardiologists.
Note:
Career Connection
Cardiovascular Technologist/Technician
Cardiovascular technologists/technicians are trained professionals who
perform a variety of imaging techniques, such as sonograms or
echocardiograms, used by physicians to diagnose and treat diseases of the
heart. Nearly all of these positions require an associate degree, and these
technicians earn a median salary of $49,410 as of May 2010, according to
the U.S. Bureau of Labor Statistics. Growth within the field is fast,
projected at 29 percent from 2010 to 2020.
There is a considerable overlap and complementary skills between cardiac
technicians and vascular technicians, and so the term cardiovascular
technician is often used. Special certifications within the field require
documenting appropriate experience and completing additional and often
expensive certification examinations. These subspecialties include
Certified Rhythm Analysis Technician (CRAT), Certified Cardiographic
Technician (CCT), Registered Congenital Cardiac Sonographer (RCCS),
Registered Cardiac Electrophysiology Specialist (RCES), Registered
Cardiovascular Invasive Specialist (RCIS), Registered Cardiac
Sonographer (RCS), Registered Vascular Specialist (RVS), and Registered
Phlebology Sonographer (RPhS).
Note:
[= [=]
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Visit this site for more information on cardiovascular
technologists/technicians.
Coronary Circulation
You will recall that the heart is a remarkable pump composed largely of
cardiac muscle cells that are incredibly active throughout life. Like all other
cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients,
and a way to remove wastes, so it needs a dedicated, complex, and
extensive coronary circulation. And because of the critical and nearly
ceaseless activity of the heart throughout life, this need for a blood supply is
even greater than for a typical cell. However, coronary circulation is not
continuous; rather, it cycles, reaching a peak when the heart muscle is
relaxed and nearly ceasing while it is contracting.
Coronary Arteries
Coronary arteries supply blood to the myocardium and other components
of the heart. The first portion of the aorta after it arises from the left
ventricle gives rise to the coronary arteries. There are three dilations in the
wall of the aorta just superior to the aortic semilunar valve. Two of these,
the left posterior aortic sinus and anterior aortic sinus, give rise to the left
and right coronary arteries, respectively. The third sinus, the right posterior
aortic sinus, typically does not give rise to a vessel. Coronary vessel
branches that remain on the surface of the artery and follow the sulci are
called epicardial coronary arteries.
The left coronary artery distributes blood to the left side of the heart, the left
atrium and ventricle, and the interventricular septum. The circumflex
artery arises from the left coronary artery and follows the coronary sulcus
to the left. Eventually, it will fuse with the small branches of the right
coronary artery. The larger anterior interventricular artery, also known
as the left anterior descending artery (LAD), is the second major branch
arising from the left coronary artery. It follows the anterior interventricular
sulcus around the pulmonary trunk. Along the way it gives rise to numerous
smaller branches that interconnect with the branches of the posterior
interventricular artery, forming anastomoses. An anastomosis is an area
where vessels unite to form interconnections that normally allow blood to
circulate to a region even if there may be partial blockage in another
branch. The anastomoses in the heart are very small. Therefore, this ability
is somewhat restricted in the heart so a coronary artery blockage often
results in death of the cells (myocardial infarction) supplied by the
particular vessel.
The right coronary artery proceeds along the coronary sulcus and distributes
blood to the right atrium, portions of both ventricles, and the heart
conduction system. Normally, one or more marginal arteries arise from the
right coronary artery inferior to the right atrium. The marginal arteries
supply blood to the superficial portions of the right ventricle. On the
posterior surface of the heart, the right coronary artery gives rise to the
posterior interventricular artery, also known as the posterior descending
artery. It runs along the posterior portion of the interventricular sulcus
toward the apex of the heart, giving rise to branches that supply the
interventricular septum and portions of both ventricles. [link] presents
views of the coronary circulation from both the anterior and posterior
views.
Coronary Circulation
Aortic arch
Left coronary
z t
Ascending aorta ately
Pulmonary trunk
Right coronary
Circumflex artery
artery
Right atrium
, ; Anterior
Atrial arteries interventricular
artery
Anterior
cardiac veins :
Great cardiac
: vein
Small cardiac
vein
Marginal artery . .
Coronary sinus Small cardiac
vein
Anterior view
Circumflex artery
Great cardiac
vein
Marginal
artery
Posterior
interventricular
artery
Posterior
cardiac
vein
Right
coronary
artery
Middle cardiac Marginal
vein artery
Posterior view
The anterior view of the heart shows the prominent coronary
surface vessels. The posterior view of the heart shows the
prominent coronary surface vessels.
Note:
Diseases of the...
Heart: Myocardial Infarction
Myocardial infarction (MI) is the formal term for what is commonly
referred to as a heart attack. It normally results from a lack of blood flow
(ischemia) and oxygen (hypoxia) to a region of the heart, resulting in death
of the cardiac muscle cells. An MI often occurs when a coronary artery is
blocked by the buildup of atherosclerotic plaque consisting of lipids,
cholesterol and fatty acids, and white blood cells, primarily macrophages.
It can also occur when a portion of an unstable atherosclerotic plaque
travels through the coronary arterial system and lodges in one of the
smaller vessels. The resulting blockage restricts the flow of blood and
oxygen to the myocardium and causes death of the tissue. MIs may be
triggered by excessive exercise, in which the partially occluded artery is no
longer able to pump sufficient quantities of blood, or severe stress, which
may induce spasm of the smooth muscle in the walls of the vessel.
In the case of acute MI, there is often sudden pain beneath the sternum
(retrosternal pain) called angina pectoris, often radiating down the left arm
in males but not in female patients. Until this anomaly between the sexes
was discovered, many female patients suffering MIs were misdiagnosed
and sent home. In addition, patients typically present with difficulty
breathing and shortness of breath (dyspnea), irregular heartbeat
(palpations), nausea and vomiting, sweating (diaphoresis), anxiety, and
fainting (syncope), although not all of these symptoms may be present.
Many of the symptoms are shared with other medical conditions, including
anxiety attacks and simple indigestion, so differential diagnosis is critical.
It is estimated that between 22 and 64 percent of MIs present without any
symptoms.
An MI can be confirmed by examining the patient’s ECG, which
frequently reveals alterations in the ST and Q components. Some
classification schemes of MI are referred to as ST-elevated MI (STEMI)
and non-elevated MI (non-STEMTI). In addition, echocardiography or
cardiac magnetic resonance imaging may be employed. Common blood
tests indicating an MI include elevated levels of creatine kinase MB (an
enzyme that catalyzes the conversion of creatine to phosphocreatine,
consuming ATP) and cardiac troponin (the regulatory protein for muscle
contraction), both of which are released by damaged cardiac muscle cells.
Immediate treatments for MI are essential and include administering
supplemental oxygen, aspirin that helps to break up clots, and
nitroglycerine administered sublingually (under the tongue) to facilitate its
absorption. Despite its unquestioned success in treatments and use since
the 1880s, the mechanism of nitroglycerine is still incompletely understood
but is believed to involve the release of nitric oxide, a known vasodilator,
and endothelium-derived releasing factor, which also relaxes the smooth
muscle in the tunica media of coronary vessels. Longer-term treatments
include injections of thrombolytic agents such as streptokinase that
dissolve the clot, the anticoagulant heparin, balloon angioplasty and stents
to open blocked vessels, and bypass surgery to allow blood to pass around
the site of blockage. If the damage is extensive, coronary replacement with
a donor heart or coronary assist device, a sophisticated mechanical device
that supplements the pumping activity of the heart, may be employed.
Despite the attention, development of artificial hearts to augment the
severely limited supply of heart donors has proven less than satisfactory
but will likely improve in the future.
MIs may trigger cardiac arrest, but the two are not synonymous. Important
risk factors for MI include cardiovascular disease, age, smoking, high
blood levels of the low-density lipoprotein (LDL, often referred to as “bad”
cholesterol), low levels of high-density lipoprotein (HDL, or “good”
cholesterol), hypertension, diabetes mellitus, obesity, lack of physical
exercise, chronic kidney disease, excessive alcohol consumption, and use
of illegal drugs.
Coronary Veins
Coronary veins drain the heart and generally parallel the large surface
arteries (see [link]). The great cardiac vein can be seen initially on the
surface of the heart following the interventricular sulcus, but it eventually
flows along the coronary sulcus into the coronary sinus on the posterior
surface. The great cardiac vein initially parallels the anterior
interventricular artery and drains the areas supplied by this vessel. It
receives several major branches, including the posterior cardiac vein, the
middle cardiac vein, and the small cardiac vein. The posterior cardiac vein
parallels and drains the areas supplied by the marginal artery branch of the
circumflex artery. The middle cardiac vein parallels and drains the areas
supplied by the posterior interventricular artery. The small cardiac vein
parallels the right coronary artery and drains the blood from the posterior
surfaces of the right atrium and ventricle. The coronary sinus is a large,
thin-walled vein on the posterior surface of the heart lying within the
atrioventricular sulcus and emptying directly into the right atrium. The
anterior cardiac veins parallel the small cardiac arteries and drain the
anterior surface of the right ventricle. Unlike these other cardiac veins, it
bypasses the coronary sinus and drains directly into the right atrium.
Note:
Diseases of the...
Heart: Coronary Artery Disease
Coronary artery disease is the leading cause of death worldwide. It occurs
when the buildup of plaque—a fatty material including cholesterol,
connective tissue, white blood cells, and some smooth muscle cells—
within the walls of the arteries obstructs the flow of blood and decreases
the flexibility or compliance of the vessels. This condition is called
atherosclerosis, a hardening of the arteries that involves the accumulation
of plaque. As the coronary blood vessels become occluded, the flow of
blood to the tissues will be restricted, a condition called ischemia that
causes the cells to receive insufficient amounts of oxygen, called hypoxia.
[link] shows the blockage of coronary arteries highlighted by the injection
of dye. Some individuals with coronary artery disease report pain radiating
from the chest called angina pectoris, but others remain asymptomatic. If
untreated, coronary artery disease can lead to MI or a heart attack.
Atherosclerotic Coronary Arteries
ik ol =
Blockage of common trunk wy
of left coronary artery
In this coronary angiogram (X-ray), the dye
makes visible two occluded coronary
arteries. Such blockages can lead to
decreased blood flow (ischemia) and
insufficient oxygen (hypoxia) delivered to
the cardiac tissues. If uncorrected, this can
lead to cardiac muscle death (myocardial
infarction).
The disease progresses slowly and often begins in children and can be seen
as fatty “streaks” in the vessels. It then gradually progresses throughout
life. Well-documented risk factors include smoking, family history,
hypertension, obesity, diabetes, high alcohol consumption, lack of exercise,
stress, and hyperlipidemia or high circulating levels of lipids in the blood.
Treatments may include medication, changes to diet and exercise,
angioplasty with a balloon catheter, insertion of a stent, or coronary bypass
procedure.
Angioplasty is a procedure in which the occlusion is mechanically widened
with a balloon. A specialized catheter with an expandable tip is inserted
into a superficial vessel, normally in the leg, and then directed to the site of
the occlusion. At this point, the balloon is inflated to compress the plaque
material and to open the vessel to increase blood flow. Then, the balloon is
deflated and retracted. A stent consisting of a specialized mesh is typically
inserted at the site of occlusion to reinforce the weakened and damaged
walls. Stent insertions have been routine in cardiology for more than 40
years.
Coronary bypass surgery may also be performed. This surgical procedure
grafts a replacement vessel obtained from another, less vital portion of the
body to bypass the occluded area. This procedure is clearly effective in
treating patients experiencing a MI, but overall does not increase longevity.
Nor does it seem advisable in patients with stable although diminished
cardiac capacity since frequently loss of mental acuity occurs following the
procedure. Long-term changes to behavior, emphasizing diet and exercise
plus a medicine regime tailored to lower blood pressure, lower cholesterol
and lipids, and reduce clotting are equally as effective.
Chapter Review
The heart resides within the pericardial sac and is located in the mediastinal
space within the thoracic cavity. The pericardial sac consists of two fused
layers: an outer fibrous capsule and an inner parietal pericardium lined with
a serous membrane. Between the pericardial sac and the heart is the
pericardial cavity, which is filled with lubricating serous fluid. The walls of
the heart are composed of an outer epicardium, a thick myocardium, and an
inner lining layer of endocardium. The human heart consists of a pair of
atria, which receive blood and pump it into a pair of ventricles, which pump
blood into the vessels. The right atrium receives systemic blood relatively
low in oxygen and pumps it into the right ventricle, which pumps it into the
pulmonary circuit. Exchange of oxygen and carbon dioxide occurs in the
lungs, and blood high in oxygen returns to the left atrium, which pumps
blood into the left ventricle, which in turn pumps blood into the aorta and
the remainder of the systemic circuit. The septa are the partitions that
separate the chambers of the heart. They include the interatrial septum, the
interventricular septum, and the atrioventricular septum. Two of these
openings are guarded by the atrioventricular valves, the right tricuspid valve
and the left mitral valve, which prevent the backflow of blood. Each is
attached to chordae tendineae that extend to the papillary muscles, which
are extensions of the myocardium, to prevent the valves from being blown
back into the atria. The pulmonary valve is located at the base of the
pulmonary trunk, and the left semilunar valve is located at the base of the
aorta. The right and left coronary arteries are the first to branch off the aorta
and arise from two of the three sinuses located near the base of the aorta
and are generally located in the sulci. Cardiac veins parallel the small
cardiac arteries and generally drain into the coronary sinus.
Interactive Link Questions
Exercise:
Problem:
Visit this site to observe an echocardiogram of actual heart valves
opening and closing. Although much of the heart has been “removed”
from this gif loop so the chordae tendineae are not visible, why is their
presence more critical for the atrioventricular valves (tricuspid and
mitral) than the semilunar (aortic and pulmonary) valves?
Solution:
The pressure gradient between the atria and the ventricles is much
greater than that between the ventricles and the pulmonary trunk and
aorta. Without the presence of the chordae tendineae and papillary
muscles, the valves would be blown back (prolapsed) into the atria and
blood would regurgitate.
Review Questions
Exercise:
Problem:
Which of the following is not important in preventing backflow of
blood?
a. chordae tendineae
b. papillary muscles
c. AV valves
d. endocardium
Solution:
D
Exercise:
Problem: Which valve separates the left atrium from the left ventricle?
a. mitral
b. tricuspid
c. pulmonary
d. aortic
Solution:
A
Exercise:
Problem:
Which of the following lists the valves in the order through which the
blood flows from the vena cava through the heart?
a. tricuspid, pulmonary semilunar, bicuspid, aortic semilunar
b. mitral, pulmonary semilunar, bicuspid, aortic semilunar
c. aortic semilunar, pulmonary semilunar, tricuspid, bicuspid
d. bicuspid, aortic semilunar, tricuspid, pulmonary semilunar
Solution:
A
Exercise:
Problem:
Which chamber initially receives blood from the systemic circuit?
a. left atrium
b. left ventricle
c. right atrium
d. right ventricle
Solution:
CG
Exercise:
Problem:
The layer secretes chemicals that help to regulate ionic
environments and strength of contraction and serve as powerful
vasoconstrictors.
a. pericardial sac
b. endocardium
c. myocardium
d. epicardium
Solution:
B
Exercise:
Problem:The myocardium would be the thickest in the
a. left atrium
b. left ventricle
c. right atrium
d. right ventricle
Solution:
B
Exercise:
Problem:In which septum is it normal to find openings in the adult?
a. interatrial septum
b. interventricular septum
c. atrioventricular septum
d. all of the above
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Describe how the valves keep the blood moving in one direction.
Solution:
When the ventricles contract and pressure begins to rise in the
ventricles, there is an initial tendency for blood to flow back
(regurgitate) to the atria. However, the papillary muscles also contract,
placing tension on the chordae tendineae and holding the
atrioventricular valves (tricuspid and mitral) in place to prevent the
valves from prolapsing and being forced back into the atria. The
semilunar valves (pulmonary and aortic) lack chordae tendineae and
papillary muscles, but do not face the same pressure gradients as do
the atrioventricular valves. As the ventricles relax and pressure drops
within the ventricles, there is a tendency for the blood to flow
backward. However, the valves, consisting of reinforced endothelium
and connective tissue, fill with blood and seal off the opening
preventing the return of blood.
Exercise:
Problem:
Why is the pressure in the pulmonary circulation lower than in the
systemic circulation?
Solution:
The pulmonary circuit consists of blood flowing to and from the lungs,
whereas the systemic circuit carries blood to and from the entire body.
The systemic circuit is far more extensive, consisting of far more
vessels and offers much greater resistance to the flow of blood, so the
heart must generate a higher pressure to overcome this resistance. This
can be seen in the thickness of the myocardium in the ventricles.
Glossary
anastomosis
(plural = anastomoses) area where vessels unite to allow blood to
circulate even if there may be partial blockage in another branch
anterior cardiac veins
vessels that parallel the small cardiac arteries and drain the anterior
surface of the right ventricle; bypass the coronary sinus and drain
directly into the right atrium
anterior interventricular artery
(also, left anterior descending artery or LAD) major branch of the left
coronary artery that follows the anterior interventricular sulcus
anterior interventricular sulcus
sulcus located between the left and right ventricles on the anterior
surface of the heart
aortic valve
(also, aortic semilunar valve) valve located at the base of the aorta
atrioventricular septum
cardiac septum located between the atria and ventricles;
atrioventricular valves are located here
atrioventricular valves
one-way valves located between the atria and ventricles; the valve on
the right is called the tricuspid valve, and the one on the left is the
mitral or bicuspid valve
atrium
(plural = atria) upper or receiving chamber of the heart that pumps
blood into the lower chambers just prior to their contraction; the right
atrium receives blood from the systemic circuit that flows into the right
ventricle; the left atrium receives blood from the pulmonary circuit
that flows into the left ventricle
auricle
extension of an atrium visible on the superior surface of the heart
bicuspid valve
(also, mitral valve or left atrioventricular valve) valve located between
the left atrium and ventricle; consists of two flaps of tissue
cardiac notch
depression in the medial surface of the inferior lobe of the left lung
where the apex of the heart is located
cardiac skeleton
(also, skeleton of the heart) reinforced connective tissue located within
the atrioventricular septum; includes four rings that surround the
openings between the atria and ventricles, and the openings to the
pulmonary trunk and aorta; the point of attachment for the heart valves
cardiomyocyte
muscle cell of the heart
chordae tendineae
string-like extensions of tough connective tissue that extend from the
flaps of the atrioventricular valves to the papillary muscles
circumflex artery
branch of the left coronary artery that follows coronary sulcus
coronary arteries
branches of the ascending aorta that supply blood to the heart; the left
coronary artery feeds the left side of the heart, the left atrium and
ventricle, and the interventricular septum; the right coronary artery
feeds the right atrium, portions of both ventricles, and the heart
conduction system
coronary sinus
large, thin-walled vein on the posterior surface of the heart that lies
within the atrioventricular sulcus and drains the heart myocardium
directly into the right atrium
coronary sulcus
sulcus that marks the boundary between the atria and ventricles
coronary veins
vessels that drain the heart and generally parallel the large surface
arteries
endocardium
innermost layer of the heart lining the heart chambers and heart valves;
composed of endothelium reinforced with a thin layer of connective
tissue that binds to the myocardium
endothelium
layer of smooth, simple squamous epithelium that lines the
endocardium and blood vessels
epicardial coronary arteries
surface arteries of the heart that generally follow the sulci
epicardium
innermost layer of the serous pericardium and the outermost layer of
the heart wall
foramen ovale
opening in the fetal heart that allows blood to flow directly from the
right atrium to the left atrium, bypassing the fetal pulmonary circuit
fossa ovalis
oval-shaped depression in the interatrial septum that marks the former
location of the foramen ovale
great cardiac vein
vessel that follows the interventricular sulcus on the anterior surface of
the heart and flows along the coronary sulcus into the coronary sinus
on the posterior surface; parallels the anterior interventricular artery
and drains the areas supplied by this vessel
hypertrophic cardiomyopathy
pathological enlargement of the heart, generally for no known reason
inferior vena cava
large systemic vein that returns blood to the heart from the inferior
portion of the body
interatrial septum
cardiac septum located between the two atria; contains the fossa ovalis
after birth
interventricular septum
cardiac septum located between the two ventricles
left atrioventricular valve
(also, mitral valve or bicuspid valve) valve located between the left
atrium and ventricle; consists of two flaps of tissue
marginal arteries
branches of the right coronary artery that supply blood to the
superficial portions of the right ventricle
mesothelium
simple squamous epithelial portion of serous membranes, such as the
superficial portion of the epicardium (the visceral pericardium) and the
deepest portion of the pericardium (the parietal pericardium)
middle cardiac vein
vessel that parallels and drains the areas supplied by the posterior
interventricular artery; drains into the great cardiac vein
mitral valve
(also, left atrioventricular valve or bicuspid valve) valve located
between the left atrium and ventricle; consists of two flaps of tissue
moderator band
band of myocardium covered by endocardium that arises from the
inferior portion of the interventricular septum in the right ventricle and
crosses to the anterior papillary muscle; contains conductile fibers that
carry electrical signals followed by contraction of the heart
myocardium
thickest layer of the heart composed of cardiac muscle cells built upon
a framework of primarily collagenous fibers and blood vessels that
supply it and the nervous fibers that help to regulate it
papillary muscle
extension of the myocardium in the ventricles to which the chordae
tendineae attach
pectinate muscles
muscular ridges seen on the anterior surface of the right atrium
pericardial cavity
cavity surrounding the heart filled with a lubricating serous fluid that
reduces friction as the heart contracts
pericardial sac
(also, pericardium) membrane that separates the heart from other
mediastinal structures; consists of two distinct, fused sublayers: the
fibrous pericardium and the parietal pericardium
pericardium
(also, pericardial sac) membrane that separates the heart from other
mediastinal structures; consists of two distinct, fused sublayers: the
fibrous pericardium and the parietal pericardium
posterior cardiac vein
vessel that parallels and drains the areas supplied by the marginal
artery branch of the circumflex artery; drains into the great cardiac
vein
posterior interventricular artery
(also, posterior descending artery) branch of the right coronary artery
that runs along the posterior portion of the interventricular sulcus
toward the apex of the heart and gives rise to branches that supply the
interventricular septum and portions of both ventricles
posterior interventricular sulcus
sulcus located between the left and right ventricles on the anterior
surface of the heart
pulmonary arteries
left and right branches of the pulmonary trunk that carry deoxygenated
blood from the heart to each of the lungs
pulmonary capillaries
capillaries surrounding the alveoli of the lungs where gas exchange
occurs: carbon dioxide exits the blood and oxygen enters
pulmonary circuit
blood flow to and from the lungs
pulmonary trunk
large arterial vessel that carries blood ejected from the right ventricle;
divides into the left and right pulmonary arteries
pulmonary valve
(also, pulmonary semilunar valve, the pulmonic valve, or the right
semilunar valve) valve at the base of the pulmonary trunk that prevents
backflow of blood into the right ventricle; consists of three flaps
pulmonary veins
veins that carry highly oxygenated blood into the left atrium, which
pumps the blood into the left ventricle, which in turn pumps
oxygenated blood into the aorta and to the many branches of the
systemic circuit
right atrioventricular valve
(also, tricuspid valve) valve located between the right atrium and
ventricle; consists of three flaps of tissue
semilunar valves
valves located at the base of the pulmonary trunk and at the base of the
aorta
septum
(plural = septa) walls or partitions that divide the heart into chambers
septum primum
flap of tissue in the fetus that covers the foramen ovale within a few
seconds after birth
small cardiac vein
parallels the right coronary artery and drains blood from the posterior
surfaces of the right atrium and ventricle; drains into the great cardiac
vein
sulcus
(plural = sulci) fat-filled groove visible on the surface of the heart;
coronary vessels are also located in these areas
superior vena cava
large systemic vein that returns blood to the heart from the superior
portion of the body
systemic circuit
blood flow to and from virtually all of the tissues of the body
trabeculae carneae
ridges of muscle covered by endocardium located in the ventricles
tricuspid valve
term used most often in clinical settings for the right atrioventricular
valve
valve
in the cardiovascular system, a specialized structure located within the
heart or vessels that ensures one-way flow of blood
ventricle
one of the primary pumping chambers of the heart located in the lower
portion of the heart; the left ventricle is the major pumping chamber on
the lower left side of the heart that ejects blood into the systemic
circuit via the aorta and receives blood from the left atrium; the right
ventricle is the major pumping chamber on the lower right side of the
heart that ejects blood into the pulmonary circuit via the pulmonary
trunk and receives blood from the right atrium
Cardiac Muscle and Electrical Activity
By the end of this section, you will be able to:
e Describe the structure of cardiac muscle
e Identify and describe the components of the conducting system that
distributes electrical impulses through the heart
¢ Compare the effect of ion movement on membrane potential of cardiac
conductive and contractile cells
e Relate characteristics of an electrocardiogram to events in the cardiac
cycle
e Identify blocks that can interrupt the cardiac cycle
Recall that cardiac muscle shares a few characteristics with both skeletal
muscle and smooth muscle, but it has some unique properties of its own.
Not the least of these exceptional properties is its ability to initiate an
electrical potential at a fixed rate that spreads rapidly from cell to cell to
trigger the contractile mechanism. This property is known as
autorhythmicity. Neither smooth nor skeletal muscle can do this. Even
though cardiac muscle has autorhythmicity, heart rate is modulated by the
endocrine and nervous systems.
There are two major types of cardiac muscle cells: myocardial contractile
cells and myocardial conducting cells. The myocardial contractile cells
constitute the bulk (99 percent) of the cells in the atria and ventricles.
Contractile cells conduct impulses and are responsible for contractions that
pump blood through the body. The myocardial conducting cells (1 percent
of the cells) form the conduction system of the heart. Except for Purkinje
cells, they are generally much smaller than the contractile cells and have
few of the myofibrils or filaments needed for contraction. Their function is
similar in many respects to neurons, although they are specialized muscle
cells. Myocardial conduction cells initiate and propagate the action
potential (the electrical impulse) that travels throughout the heart and
triggers the contractions that propel the blood.
Structure of Cardiac Muscle
Compared to the giant cylinders of skeletal muscle, cardiac muscle cells, or
cardiomyocytes, are considerably shorter with much smaller diameters.
Cardiac muscle also demonstrates striations, the alternating pattern of dark
A bands and light I bands attributed to the precise arrangement of the
myofilaments and fibrils that are organized in sarcomeres along the length
of the cell ({link]a). These contractile elements are virtually identical to
skeletal muscle. T (transverse) tubules penetrate from the surface plasma
membrane, the sarcolemma, to the interior of the cell, allowing the
electrical impulse to reach the interior. The T tubules are only found at the
Z, discs, whereas in skeletal muscle, they are found at the junction of the A
and I bands. Therefore, there are one-half as many T tubules in cardiac
muscle as in skeletal muscle. In addition, the sarcoplasmic reticulum stores
few calcium ions, so most of the calcium ions must come from outside the
cells. The result is a slower onset of contraction. Mitochondria are plentiful,
providing energy for the contractions of the heart. Typically,
cardiomyocytes have a single, central nucleus, but two or more nuclei may
be found in some cells.
Cardiac muscle cells branch freely. A junction between two adjoining cells
is marked by a critical structure called an intercalated disc, which helps
support the synchronized contraction of the muscle ([link]b). The
sarcolemmas from adjacent cells bind together at the intercalated discs.
They consist of desmosomes, specialized linking proteoglycans, tight
junctions, and large numbers of gap junctions that allow the passage of ions
between the cells and help to synchronize the contraction ([link]c).
Intercellular connective tissue also helps to bind the cells together. The
importance of strongly binding these cells together is necessitated by the
forces exerted by contraction.
Cardiac Muscle
Intercalated discs
Intercalated discs
Mitochondria Intercalated discs
Gap junction
Cardiac
muscle fiber
(a)
Desmosome
—
A band | band
(c)
(a) Cardiac muscle cells have myofibrils composed of
myofilaments arranged in sarcomeres, T tubules to transmit
the impulse from the sarcolemma to the interior of the cell,
numerous mitochondria for energy, and intercalated discs
that are found at the junction of different cardiac muscle
cells. (b) A photomicrograph of cardiac muscle cells shows
the nuclei and intercalated discs. (c) An intercalated disc
connects cardiac muscle cells and consists of desmosomes
and gap junctions. LM x 1600. (Micrograph provided by
the Regents of the University of Michigan Medical School
© 2012)
Cardiac muscle undergoes aerobic respiration patterns, primarily
metabolizing lipids and carbohydrates. Myoglobin, lipids, and glycogen are
all stored within the cytoplasm. Cardiac muscle cells undergo twitch-type
contractions with long refractory periods followed by brief relaxation
periods. The relaxation is essential so the heart can fill with blood for the
next cycle. The refractory period is very long to prevent the possibility of
tetany, a condition in which muscle remains involuntarily contracted. In the
heart, tetany is not compatible with life, since it would prevent the heart
from pumping blood.
Note:
Everyday Connection
Repair and Replacement
Damaged cardiac muscle cells have extremely limited abilities to repair
themselves or to replace dead cells via mitosis. Recent evidence indicates
that at least some stem cells remain within the heart that continue to divide
and at least potentially replace these dead cells. However, newly formed or
repaired cells are rarely as functional as the original cells, and cardiac
function is reduced. In the event of a heart attack or MI, dead cells are
often replaced by patches of scar tissue. Autopsies performed on
individuals who had successfully received heart transplants show some
proliferation of original cells. If researchers can unlock the mechanism that
generates new cells and restore full mitotic capabilities to heart muscle, the
prognosis for heart attack survivors will be greatly enhanced. To date,
myocardial cells produced within the patient (in situ) by cardiac stem cells
seem to be nonfunctional, although those grown in Petri dishes (in vitro) do
beat. Perhaps soon this mystery will be solved, and new advances in
treatment will be commonplace.
Conduction System of the Heart
If embryonic heart cells are separated into a Petri dish and kept alive, each
is capable of generating its own electrical impulse followed by contraction.
When two independently beating embryonic cardiac muscle cells are placed
together, the cell with the higher inherent rate sets the pace, and the impulse
spreads from the faster to the slower cell to trigger a contraction. As more
cells are joined together, the fastest cell continues to assume control of the
rate. A fully developed adult heart maintains the capability of generating its
own electrical impulse, triggered by the fastest cells, as part of the cardiac
conduction system. The components of the cardiac conduction system
include the sinoatrial node, the atrioventricular node, the atrioventricular
bundle, the atrioventricular bundle branches, and the Purkinje cells ([link]).
Conduction System of the Heart
——™
Yd ~
~.
Frontal plane = 2
|
through heart | Arch of aorta
= ~~
(SS ———— Bachman’s bundle
<=
Sinoatrial LL Pe |
(SA) node
Anterior internodal
Atrioventricular
(AV) node
Middle internodal
Posterior internodal
Left atrium
Atrioventricular (AV)
bundle (bundle of His)
Left ventricle
Right and left bundle
branches
Right atrium
Right ventricle
Purkinje fibers
Anterior view of frontal section
Specialized conducting components of the heart include
the sinoatrial node, the internodal pathways, the
atrioventricular node, the atrioventricular bundle, the
right and left bundle branches, and the Purkinje fibers.
Sinoatrial (SA) Node
Normal cardiac rhythm is established by the sinoatrial (SA) node, a
specialized clump of myocardial conducting cells located in the superior
and posterior walls of the right atrium in close proximity to the orifice of
the superior vena cava. The SA node has the highest inherent rate of
depolarization and is known as the pacemaker of the heart. It initiates the
sinus rhythm, or normal electrical pattern followed by contraction of the
heart.
This impulse spreads from its initiation in the SA node throughout the atria
through specialized internodal pathways, to the atrial myocardial
contractile cells and the atrioventricular node. The internodal pathways
consist of three bands (anterior, middle, and posterior) that lead directly
from the SA node to the next node in the conduction system, the
atrioventricular node (see [link]). The impulse takes approximately 50 ms
(milliseconds) to travel between these two nodes. The relative importance
of this pathway has been debated since the impulse would reach the
atrioventricular node simply following the cell-by-cell pathway through the
contractile cells of the myocardium in the atria. In addition, there is a
specialized pathway called Bachmann’s bundle or the interatrial band
that conducts the impulse directly from the right atrium to the left atrium.
Regardless of the pathway, as the impulse reaches the atrioventricular
septum, the connective tissue of the cardiac skeleton prevents the impulse
from spreading into the myocardial cells in the ventricles except at the
atrioventricular node. [link] illustrates the initiation of the impulse in the
SA node that then spreads the impulse throughout the atria to the
atrioventricular node.
Cardiac Conduction
ACS
=o BN ae
“aN
(. |.
© \
7
(1) The sinoatrial (SA) node and the
remainder of the conduction system are at
rest. (2) The SA node initiates the action
potential, which sweeps across the atria. (3)
After reaching the atrioventricular node,
there is a delay of approximately 100 ms
that allows the atria to complete pumping
blood before the impulse is transmitted to
the atrioventricular bundle. (4) Following
the delay, the impulse travels through the
atrioventricular bundle and bundle branches
to the Purkinje fibers, and also reaches the
right papillary muscle via the moderator
band. (5) The impulse spreads to the
contractile fibers of the ventricle. (6)
Ventricular contraction begins.
The electrical event, the wave of depolarization, is the trigger for muscular
contraction. The wave of depolarization begins in the right atrium, and the
impulse spreads across the superior portions of both atria and then down
through the contractile cells. The contractile cells then begin contraction
from the superior to the inferior portions of the atria, efficiently pumping
blood into the ventricles.
Atrioventricular (AV) Node
The atrioventricular (AV) node is a second clump of specialized
myocardial conductive cells, located in the inferior portion of the right
atrium within the atrioventricular septum. The septum prevents the impulse
from spreading directly to the ventricles without passing through the AV
node. There is a critical pause before the AV node depolarizes and transmits
the impulse to the atrioventricular bundle (see [link], step 3). This delay in
transmission is partially attributable to the small diameter of the cells of the
node, which slow the impulse. Also, conduction between nodal cells is less
efficient than between conducting cells. These factors mean that it takes the
impulse approximately 100 ms to pass through the node. This pause is
critical to heart function, as it allows the atrial cardiomyocytes to complete
their contraction that pumps blood into the ventricles before the impulse is
transmitted to the cells of the ventricle itself. With extreme stimulation by
the SA node, the AV node can transmit impulses maximally at 220 per
minute. This establishes the typical maximum heart rate in a healthy young
individual. Damaged hearts or those stimulated by drugs can contract at
higher rates, but at these rates, the heart can no longer effectively pump
blood.
Atrioventricular Bundle (Bundle of His), Bundle Branches, and
Purkinje Fibers
Arising from the AV node, the atrioventricular bundle, or bundle of His,
proceeds through the interventricular septum before dividing into two
atrioventricular bundle branches, commonly called the left and right
bundle branches. The left bundle branch has two fascicles. The left bundle
branch supplies the left ventricle, and the right bundle branch the right
ventricle. Since the left ventricle is much larger than the right, the left
bundle branch is also considerably larger than the right. Portions of the right
bundle branch are found in the moderator band and supply the right
papillary muscles. Because of this connection, each papillary muscle
receives the impulse at approximately the same time, so they begin to
contract simultaneously just prior to the remainder of the myocardial
contractile cells of the ventricles. This is believed to allow tension to
develop on the chordae tendineae prior to right ventricular contraction.
There is no corresponding moderator band on the left. Both bundle
branches descend and reach the apex of the heart where they connect with
the Purkinje fibers (see [link], step 4). This passage takes approximately 25
ms.
The Purkinje fibers are additional myocardial conductive fibers that spread
the impulse to the myocardial contractile cells in the ventricles. They
extend throughout the myocardium from the apex of the heart toward the
atrioventricular septum and the base of the heart. The Purkinje fibers have a
fast inherent conduction rate, and the electrical impulse reaches all of the
ventricular muscle cells in about 75 ms (see [link], step 5). Since the
electrical stimulus begins at the apex, the contraction also begins at the apex
and travels toward the base of the heart, similar to squeezing a tube of
toothpaste from the bottom. This allows the blood to be pumped out of the
ventricles and into the aorta and pulmonary trunk. The total time elapsed
from the initiation of the impulse in the SA node until depolarization of the
ventricles is approximately 225 ms.
Membrane Potentials and Ion Movement in Cardiac Conductive Cells
Action potentials are considerably different between cardiac conductive
cells and cardiac contractive cells. While Na* and K* play essential roles,
Ca** is also critical for both types of cells. Unlike skeletal muscles and
neurons, cardiac conductive cells do not have a stable resting potential.
Conductive cells contain a series of sodium ion channels that allow a
normal and slow influx of sodium ions that causes the membrane potential
to rise slowly from an initial value of -60 mV up to about —40 mV. The
resulting movement of sodium ions creates spontaneous depolarization
(or prepotential depolarization). At this point, calcium ion channels open
and Ca** enters the cell, further depolarizing it at a more rapid rate until it
reaches a value of approximately +5 mV. At this point, the calcium ion
channels close and K* channels open, allowing outflux of K* and resulting
in repolarization. When the membrane potential reaches approximately —60
mV, the K* channels close and Na™ channels open, and the prepotential
phase begins again. This phenomenon explains the autorhythmicity
properties of cardiac muscle ({link]).
Action Potential at the SA Node
lies Rapid influx of Ca2*
ae Outflux of Kt
Depolarization
fe) Repolarization
Slow influx of Nat
Prepotential
potential
my) 40 \ Threshold
-60
Membrane _90
—80
0.8 1.6
Time (s)
The prepotential is due to a slow influx of
sodium ions until the threshold is reached
followed by a rapid depolarization and
repolarization. The prepotential accounts for the
membrane reaching threshold and initiates the
spontaneous depolarization and contraction of
the cell. Note the lack of a resting potential.
Membrane Potentials and Ion Movement in Cardiac Contractile Cells
There is a distinctly different electrical pattern involving the contractile
cells. In this case, there is a rapid depolarization, followed by a plateau
phase and then repolarization. This phenomenon accounts for the long
refractory periods required for the cardiac muscle cells to pump blood
effectively before they are capable of firing for a second time. These
cardiac myocytes normally do not initiate their own electrical potential but
rather wait for an impulse to reach them.
Contractile cells demonstrate a much more stable resting phase than
conductive cells at approximately —80 mV for cells in the atria and -90 mV
for cells in the ventricles. Despite this initial difference, the other
components of their action potentials are virtually identical. In both cases,
when stimulated by an action potential, voltage-gated channels rapidly
open, beginning the positive-feedback mechanism of depolarization. This
rapid influx of positively charged ions raises the membrane potential to
approximately +30 mV, at which point the sodium channels close. The rapid
depolarization period typically lasts 3-5 ms. Depolarization is followed by
the plateau phase, in which membrane potential declines relatively slowly.
This is due in large part to the opening of the slow Ca** channels, allowing
Ca** to enter the cell while few K* channels are open, allowing K* to exit
the cell. The relatively long plateau phase lasts approximately 175 ms. Once
the membrane potential reaches approximately zero, the Ca** channels
close and K* channels open, allowing K* to exit the cell. The repolarization
lasts approximately 75 ms. At this point, membrane potential drops until it
reaches resting levels once more and the cycle repeats. The entire event
lasts between 250 and 300 ms ([link]).
The absolute refractory period for cardiac contractile muscle lasts
approximately 200 ms, and the relative refractory period lasts
approximately 50 ms, for a total of 250 ms. This extended period is critical,
since the heart muscle must contract to pump blood effectively and the
contraction must follow the electrical events. Without extended refractory
periods, premature contractions would occur in the heart and would not be
compatible with life.
Action Potential in Cardiac Contractile Cells
Na* channels
close
Slow Ca?* channels open
oe Slow Ca2* channels close
Repolarization
The plateau
Rapid depolarization
K* channels close
mV
Voltage-gated
Refractory period
ion channels we
open Absolute Relative
Influx of Na*
Time (ms)
(a)
Skeletal muscle Cardiac muscle
Action potential Action potential
Contraction Contraction
Tension Tension
Time (ms) Time (ms)
(b)
(a) Note the long plateau phase due to the influx of
calcium ions. The extended refractory period allows
the cell to fully contract before another electrical event
can occur. (b) The action potential for heart muscle is
compared to that of skeletal muscle.
Calcium Ions
Calcium ions play two critical roles in the physiology of cardiac muscle.
Their influx through slow calcium channels accounts for the prolonged
plateau phase and absolute refractory period that enable cardiac muscle to
function properly. Calcium ions also combine with the regulatory protein
troponin in the troponin-tropomyosin complex; this complex removes the
inhibition that prevents the heads of the myosin molecules from forming
cross bridges with the active sites on actin that provide the power stroke of
contraction. This mechanism is virtually identical to that of skeletal muscle.
Approximately 20 percent of the calcium required for contraction is
supplied by the influx of Ca?* during the plateau phase. The remaining Ca?*
for contraction is released from storage in the sarcoplasmic reticulum.
Comparative Rates of Conduction System Firing
The pattern of prepotential or spontaneous depolarization, followed by
rapid depolarization and repolarization just described, are seen in the SA
node and a few other conductive cells in the heart. Since the SA node is the
pacemaker, it reaches threshold faster than any other component of the
conduction system. It will initiate the impulses spreading to the other
conducting cells. The SA node, without nervous or endocrine control,
would initiate a heart impulse approximately 80—100 times per minute.
Although each component of the conduction system is capable of
generating its own impulse, the rate progressively slows as you proceed
from the SA node to the Purkinje fibers. Without the SA node, the AV node
would generate a heart rate of 40-60 beats per minute. If the AV node were
blocked, the atrioventricular bundle would fire at a rate of approximately
30—40 impulses per minute. The bundle branches would have an inherent
rate of 20—30 impulses per minute, and the Purkinje fibers would fire at 15—
20 impulses per minute. While a few exceptionally trained aerobic athletes
demonstrate resting heart rates in the range of 30—40 beats per minute (the
lowest recorded figure is 28 beats per minute for Miguel Indurain, a
cyclist), for most individuals, rates lower than 50 beats per minute would
indicate a condition called bradycardia. Depending upon the specific
individual, as rates fall much below this level, the heart would be unable to
maintain adequate flow of blood to vital tissues, initially resulting in
decreasing loss of function across the systems, unconsciousness, and
ultimately death.
Electrocardiogram
By careful placement of surface electrodes on the body, it is possible to
record the complex, compound electrical signal of the heart. This tracing of
the electrical signal is the electrocardiogram (ECG), also commonly
abbreviated EKG (K coming kardiology, from the German term for
cardiology). Careful analysis of the ECG reveals a detailed picture of both
normal and abnormal heart function, and is an indispensable clinical
diagnostic tool. The standard electrocardiograph (the instrument that
generates an ECG) uses 3, 5, or 12 leads. The greater the number of leads
an electrocardiograph uses, the more information the ECG provides. The
term “lead” may be used to refer to the cable from the electrode to the
electrical recorder, but it typically describes the voltage difference between
two of the electrodes. The 12-lead electrocardiograph uses 10 electrodes
placed in standard locations on the patient’s skin ([link]). In continuous
ambulatory electrocardiographs, the patient wears a small, portable, battery-
operated device known as a Holter monitor, or simply a Holter, that
continuously monitors heart electrical activity, typically for a period of 24
hours during the patient’s normal routine.
Standard Placement of ECG Leads
In a 12-lead ECG, six electrodes are
placed on the chest, and four
electrodes are placed on the limbs.
A normal ECG tracing is presented in [link]. Each component, segment,
and interval is labeled and corresponds to important electrical events,
demonstrating the relationship between these events and contraction in the
heart.
There are five prominent points on the ECG: the P wave, the QRS complex,
and the T wave. The small P wave represents the depolarization of the atria.
The atria begin contracting approximately 25 ms after the start of the P
wave. The large QRS complex represents the depolarization of the
ventricles, which requires a much stronger electrical signal because of the
larger size of the ventricular cardiac muscle. The ventricles begin to
contract as the QRS reaches the peak of the R wave. Lastly, the T wave
represents the repolarization of the ventricles. The repolarization of the atria
occurs during the QRS complex, which masks it on an ECG.
The major segments and intervals of an ECG tracing are indicated in [link].
Segments are defined as the regions between two waves. Intervals include
one segment plus one or more waves. For example, the PR segment begins
at the end of the P wave and ends at the beginning of the QRS complex.
The PR interval starts at the beginning of the P wave and ends with the
beginning of the QRS complex. The PR interval is more clinically relevant,
as it measures the duration from the beginning of atrial depolarization (the
P wave) to the initiation of the QRS complex. Since the Q wave may be
difficult to view in some tracings, the measurement is often extended to the
R that is more easily visible. Should there be a delay in passage of the
impulse from the SA node to the AV node, it would be visible in the PR
interval. [link] correlates events of heart contraction to the corresponding
segments and intervals of an ECG.
Note:
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—
meee, <OPENStAX COLLEGE
Visit this site for a more detailed analysis of ECGs.
Electrocardiogram
5mm
P-R S-T
segment segment
Millivolts
QRS complex
PR interval QT interval
A normal tracing shows the P wave, QRS
complex, and T wave. Also indicated are the
PR, QT, QRS, and ST intervals, plus the P-R
and S-T segments.
ECG Tracing Correlated to the Cardiac Cycle
0)
This diagram correlates an ECG tracing with the
electrical and mechanical events of a heart contraction.
Each segment of an ECG tracing corresponds to one
event in the cardiac cycle.
Note:
Everyday Connection
ECG Abnormalities
Occassionally, an area of the heart other than the SA node will initiate an
impulse that will be followed by a premature contraction. Such an area,
which may actually be a component of the conduction system or some
other contractile cells, is known as an ectopic focus or ectopic pacemaker.
An ectopic focus may be stimulated by localized ischemia; exposure to
certain drugs, including caffeine, digitalis, or acetylcholine; elevated
stimulation by both sympathetic or parasympathetic divisions of the
autonomic nervous system; or a number of disease or pathological
conditions. Occasional occurances are generally transitory and nonlife
threatening, but if the condition becomes chronic, it may lead to either an
arrhythmia, a deviation from the normal pattern of impulse conduction and
contraction, or to fibrillation, an uncoordinated beating of the heart.
While interpretation of an ECG is possible and extremely valuable after
some training, a full understanding of the complexities and intricacies
generally requires several years of experience. In general, the size of the
electrical variations, the duration of the events, and detailed vector analysis
provide the most comprehensive picture of cardiac function. For example,
an amplified P wave may indicate enlargement of the atria, an enlarged Q
wave may indicate a MI, and an enlarged suppressed or inverted Q wave
often indicates enlarged ventricles. T waves often appear flatter when
insufficient oxygen is being delivered to the myocardium. An elevation of
the ST segment above baseline is often seen in patients with an acute MI,
and may appear depressed below the baseline when hypoxia is occurring.
As useful as analyzing these electrical recordings may be, there are
limitations. For example, not all areas suffering a MI may be obvious on
the ECG. Additionally, it will not reveal the effectiveness of the pumping,
which requires further testing, such as an ultrasound test called an
echocardiogram or nuclear medicine imaging. It is also possible for there
to be pulseless electrical activity, which will show up on an ECG tracing,
although there is no corresponding pumping action. Common
abnormalities that may be detected by the ECGs are shown in [link].
Common ECG Abnormalities
Note how half of the P waves
are not followed by the QRS
complex and T waves while the
other half are.
Question: What would you
expect to happen to heart rate
(pulse)?
|
|
(a) Second-degree (partial) block
| | Note the abnormal electrical
} | pattern prior to the QRS
| } complexes. Also note how the
frequency between the QRS
complexes has increased.
~ Question: What would you
_ expect to happen to heart rate
(pulse)?
(b) Atrial fibrillation
Note the unusual shape of the
QRS complex, focusing on the
“S” component.
Question: What would you
expect to happen to heart rate
(pulse)?
(c
Ventricular tachycardia
Note the total lack of normal
electrical activity.
Question: What would you
_ expect to happen to heart
| | rate (pulse)?
| } |
(d) Ventricular fibrillation
Note that in a third-degree block
some of the impulses initiated by
the SA node do not reach the
AV node while others do. Also note
that the P waves are not followed
by the QRS complex.
| | | / Question: What would you expect
(e) Third-degree block to happen to heart rate (pulse)?
(a) In a second-degree or partial block, one-half of the P
waves are not followed by the QRS complex and T waves
while the other half are. (b) In atrial fibrillation, the
electrical pattern is abnormal prior to the QRS complex,
and the frequency between the QRS complexes has
increased. (c) In ventricular tachycardia, the shape of the
QRS complex is abnormal. (d) In ventricular fibrillation,
there is no normal electrical activity. (e) In a third-degree
block, there is no correlation between atrial activity (the P
wave) and ventricular activity (the QRS complex).
Visit this site for a more complete library of abnormal ECGs.
Note:
Everyday Connection
External Automated Defibrillators
In the event that the electrical activity of the heart is severely disrupted,
cessation of electrical activity or fibrillation may occur. In fibrillation, the
heart beats in a wild, uncontrolled manner, which prevents it from being
able to pump effectively. Atrial fibrillation (see [link]b) is a serious
condition, but as long as the ventricles continue to pump blood, the
patient’s life may not be in immediate danger. Ventricular fibrillation (see
[link ]d) is a medical emergency that requires life support, because the
ventricles are not effectively pumping blood. In a hospital setting, it is
often described as “code blue.” If untreated for as little as a few minutes,
ventricular fibrillation may lead to brain death. The most common
treatment is defibrillation, which uses special paddles to apply a charge to
the heart from an external electrical source in an attempt to establish a
normal sinus rhythm ([link]). A defibrillator effectively stops the heart so
that the SA node can trigger a normal conduction cycle. Because of their
effectiveness in reestablishing a normal sinus rhythm, external automated
defibrillators (EADs) are being placed in areas frequented by large
numbers of people, such as schools, restaurants, and airports. These
devices contain simple and direct verbal instructions that can be followed
by nonmedical personnel in an attempt to save a life.
Defibrillators
N
\
(a) An external automatic defibrillator can be used by
nonmedical personnel to reestablish a normal sinus rhythm
in a person with fibrillation. (b) Defibrillator paddles are
more commonly used in hospital settings. (credit b:
“widerider107”/flickr.com)
A heart block refers to an interruption in the normal conduction pathway.
The nomenclature for these is very straightforward. SA nodal blocks occur
within the SA node. AV nodal blocks occur within the AV node. Infra-
Hisian blocks involve the bundle of His. Bundle branch blocks occur within
either the left or right atrioventricular bundle branches. Hemiblocks are
partial and occur within one or more fascicles of the atrioventricular bundle
branch. Clinically, the most common types are the AV nodal and infra-
Hisian blocks.
AV blocks are often described by degrees. A first-degree or partial block
indicates a delay in conduction between the SA and AV nodes. This can be
recognized on the ECG as an abnormally long PR interval. A second-degree
or incomplete block occurs when some impulses from the SA node reach
the AV node and continue, while others do not. In this instance, the ECG
would reveal some P waves not followed by a QRS complex, while others
would appear normal. In the third-degree or complete block, there is no
correlation between atrial activity (the P wave) and ventricular activity (the
QRS complex). Even in the event of a total SA block, the AV node will
assume the role of pacemaker and continue initiating contractions at 40-60
contractions per minute, which is adequate to maintain consciousness.
Second- and third-degree blocks are demonstrated on the ECG presented in
[link].
When arrhythmias become a chronic problem, the heart maintains a
junctional rhythm, which originates in the AV node. In order to speed up the
heart rate and restore full sinus rhythm, a cardiologist can implant an
artificial pacemaker, which delivers electrical impulses to the heart
muscle to ensure that the heart continues to contract and pump blood
effectively. These artificial pacemakers are programmable by the
cardiologists and can either provide stimulation temporarily upon demand
or on a continuous basis. Some devices also contain built-in defibrillators.
Cardiac Muscle Metabolism
Normally, cardiac muscle metabolism is entirely aerobic. Oxygen from the
lungs is brought to the heart, and every other organ, attached to the
hemoglobin molecules within the erythrocytes. Heart cells also store
appreciable amounts of oxygen in myoglobin. Normally, these two
mechanisms, circulating oxygen and oxygen attached to myoglobin, can
supply sufficient oxygen to the heart, even during peak performance.
Fatty acids and glucose from the circulation are broken down within the
mitochondria to release energy in the form of ATP. Both fatty acid droplets
and glycogen are stored within the sarcoplasm and provide additional
nutrient supply. (Seek additional content for more detail about metabolism.)
Chapter Review
The heart is regulated by both neural and endocrine control, yet it is capable
of initiating its own action potential followed by muscular contraction. The
conductive cells within the heart establish the heart rate and transmit it
through the myocardium. The contractile cells contract and propel the
blood. The normal path of transmission for the conductive cells is the
sinoatrial (SA) node, internodal pathways, atrioventricular (AV) node,
atrioventricular (AV) bundle of His, bundle branches, and Purkinje fibers.
The action potential for the conductive cells consists of a prepotential phase
with a slow influx of Na* followed by a rapid influx of Ca** and outflux of
K*. Contractile cells have an action potential with an extended plateau
phase that results in an extended refractory period to allow complete
contraction for the heart to pump blood effectively. Recognizable points on
the ECG include the P wave that corresponds to atrial depolarization, the
QRS complex that corresponds to ventricular depolarization, and the T
wave that corresponds to ventricular repolarization.
Review Questions
Exercise:
Problem: Which of the following is unique to cardiac muscle cells?
a. Only cardiac muscle contains a sarcoplasmic reticulum.
b. Only cardiac muscle has gap junctions.
c. Only cardiac muscle is capable of autorhythmicity
d. Only cardiac muscle has a high concentration of mitochondria.
Solution:
c
Exercise:
Problem:The influx of which ion accounts for the plateau phase?
a. sodium
b. potassium
c. chloride
d. calcium
Solution:
D
Exercise:
Problem:
Which portion of the ECG corresponds to repolarization of the atria?
a. P wave
b. QRS complex
c. T wave
d. none of the above: atrial repolarization is masked by ventricular
depolarization
Solution:
D
Exercise:
Problem:
Which component of the heart conduction system would have the
slowest rate of firing?
a. atrioventricular node
b. atrioventricular bundle
c. bundle branches
d. Purkinje fibers
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Why is the plateau phase so critical to cardiac muscle function?
Solution:
It prevents additional impulses from spreading through the heart
prematurely, thereby allowing the muscle sufficient time to contract
and pump blood effectively.
Exercise:
Problem:
How does the delay of the impulse at the atrioventricular node
contribute to cardiac function?
Solution:
It ensures sufficient time for the atrial muscle to contract and pump
blood into the ventricles prior to the impulse being conducted into the
lower chambers.
Exercise:
Problem:
How do gap junctions and intercalated disks aid contraction of the
heart?
Solution:
Gap junctions within the intercalated disks allow impulses to spread
from one cardiac muscle cell to another, allowing sodium, potassium,
and calcium ions to flow between adjacent cells, propagating the
action potential, and ensuring coordinated contractions.
Exercise:
Problem:
Why do the cardiac muscles cells demonstrate autorhythmicity?
Solution:
Without a true resting potential, there is a slow influx of sodium ions
through slow channels that produces a prepotential that gradually
reaches threshold.
Glossary
artificial pacemaker
medical device that transmits electrical signals to the heart to ensure
that it contracts and pumps blood to the body
atrioventricular bundle
(also, bundle of His) group of specialized myocardial conductile cells
that transmit the impulse from the AV node through the
interventricular septum; form the left and right atrioventricular bundle
branches
atrioventricular bundle branches
(also, left or right bundle branches) specialized myocardial conductile
cells that arise from the bifurcation of the atrioventricular bundle and
pass through the interventricular septum; lead to the Purkinje fibers
and also to the right papillary muscle via the moderator band
atrioventricular (AV) node
clump of myocardial cells located in the inferior portion of the right
atrium within the atrioventricular septum; receives the impulse from
the SA node, pauses, and then transmits it into specialized conducting
cells within the interventricular septum
autorhythmicity
ability of cardiac muscle to initiate its own electrical impulse that
triggers the mechanical contraction that pumps blood at a fixed pace
without nervous or endocrine control
Bachmann’s bundle
(also, interatrial band) group of specialized conducting cells that
transmit the impulse directly from the SA node in the right atrium to
the left atrium
bundle of His
(also, atrioventricular bundle) group of specialized myocardial
conductile cells that transmit the impulse from the AV node through
the interventricular septum; form the left and right atrioventricular
bundle branches
electrocardiogram (ECG)
surface recording of the electrical activity of the heart that can be used
for diagnosis of irregular heart function; also abbreviated as EKG
heart block
interruption in the normal conduction pathway
interatrial band
(also, Bachmann’s bundle) group of specialized conducting cells that
transmit the impulse directly from the SA node in the right atrium to
the left atrium
intercalated disc
physical junction between adjacent cardiac muscle cells; consisting of
desmosomes, specialized linking proteoglycans, and gap junctions that
allow passage of ions between the two cells
internodal pathways
specialized conductile cells within the atria that transmit the impulse
from the SA node throughout the myocardial cells of the atrium and to
the AV node
myocardial conducting cells
specialized cells that transmit electrical impulses throughout the heart
and trigger contraction by the myocardial contractile cells
myocardial contractile cells
bulk of the cardiac muscle cells in the atria and ventricles that conduct
impulses and contract to propel blood
P wave
component of the electrocardiogram that represents the depolarization
of the atria
pacemaker
cluster of specialized myocardial cells known as the SA node that
initiates the sinus rhythm
prepotential depolarization
(also, spontaneous depolarization) mechanism that accounts for the
autorhythmic property of cardiac muscle; the membrane potential
increases as sodium ions diffuse through the always-open sodium ion
channels and causes the electrical potential to rise
Purkinje fibers
specialized myocardial conduction fibers that arise from the bundle
branches and spread the impulse to the myocardial contraction fibers
of the ventricles
QRS complex
component of the electrocardiogram that represents the depolarization
of the ventricles and includes, as a component, the repolarization of the
atria
sinoatrial (SA) node
known as the pacemaker, a specialized clump of myocardial
conducting cells located in the superior portion of the right atrium that
has the highest inherent rate of depolarization that then spreads
throughout the heart
sinus rhythm
normal contractile pattern of the heart
spontaneous depolarization
(also, prepotential depolarization) the mechanism that accounts for the
autorhythmic property of cardiac muscle; the membrane potential
increases as sodium ions diffuse through the always-open sodium ion
channels and causes the electrical potential to rise
T wave
component of the electrocardiogram that represents the repolarization
of the ventricles
Cardiac Cycle
By the end of this section, you will be able to:
e Describe the relationship between blood pressure and blood flow
e Summarize the events of the cardiac cycle
e¢ Compare atrial and ventricular systole and diastole
e Relate heart sounds detected by auscultation to action of heart’s valves
The period of time that begins with contraction of the atria and ends with
ventricular relaxation is known as the cardiac cycle ([link]). The period of
contraction that the heart undergoes while it pumps blood into circulation is
called systole. The period of relaxation that occurs as the chambers fill with
blood is called diastole. Both the atria and ventricles undergo systole and
diastole, and it is essential that these components be carefully regulated and
coordinated to ensure blood is pumped efficiently to the body.
Overview of the Cardiac Cycle
|
The cardiac cycle begins with atrial systole and
progresses to ventricular systole, atrial
diastole, and ventricular diastole, when the
cycle begins again. Correlations to the ECG
are highlighted.
Pressures and Flow
Fluids, whether gases or liquids, are materials that flow according to
pressure gradients—that is, they move from regions that are higher in
pressure to regions that are lower in pressure. Accordingly, when the heart
chambers are relaxed (diastole), blood will flow into the atria from the
veins, which are higher in pressure. As blood flows into the atria, the
pressure will rise, so the blood will initially move passively from the atria
into the ventricles. When the action potential triggers the muscles in the
atria to contract (atrial systole), the pressure within the atria rises further,
pumping blood into the ventricles. During ventricular systole, pressure rises
in the ventricles, pumping blood into the pulmonary trunk from the right
ventricle and into the aorta from the left ventricle. Again, as you consider
this flow and relate it to the conduction pathway, the elegance of the system
should become apparent.
Phases of the Cardiac Cycle
At the beginning of the cardiac cycle, both the atria and ventricles are
relaxed (diastole). Blood is flowing into the right atrium from the superior
and inferior venae cavae and the coronary sinus. Blood flows into the left
atrium from the four pulmonary veins. The two atrioventricular valves, the
tricuspid and mitral valves, are both open, so blood flows unimpeded from
the atria and into the ventricles. Approximately 70—80 percent of ventricular
filling occurs by this method. The two semilunar valves, the pulmonary and
aortic valves, are closed, preventing backflow of blood into the right and
left ventricles from the pulmonary trunk on the right and the aorta on the
left.
Atrial Systole and Diastole
Contraction of the atria follows depolarization, represented by the P wave
of the ECG. As the atrial muscles contract from the superior portion of the
atria toward the atrioventricular septum, pressure rises within the atria and
blood is pumped into the ventricles through the open atrioventricular
(tricuspid, and mitral or bicuspid) valves. At the start of atrial systole, the
ventricles are normally filled with approximately 70-80 percent of their
capacity due to inflow during diastole. Atrial contraction, also referred to as
the “atrial kick,” contributes the remaining 20—30 percent of filling (see
[link]). Atrial systole lasts approximately 100 ms and ends prior to
ventricular systole, as the atrial muscle returns to diastole.
Ventricular Systole
Ventricular systole (see [link]) follows the depolarization of the ventricles
and is represented by the QRS complex in the ECG. It may be conveniently
divided into two phases, lasting a total of 270 ms. At the end of atrial
systole and just prior to atrial contraction, the ventricles contain
approximately 130 mL blood in a resting adult in a standing position. This
volume is known as the end diastolic volume (EDV) or preload.
Initially, as the muscles in the ventricle contract, the pressure of the blood
within the chamber rises, but it is not yet high enough to open the semilunar
(pulmonary and aortic) valves and be ejected from the heart. However,
blood pressure quickly rises above that of the atria that are now relaxed and
in diastole. This increase in pressure causes blood to flow back toward the
atria, closing the tricuspid and mitral valves. Since blood is not being
ejected from the ventricles at this early stage, the volume of blood within
the chamber remains constant. Consequently, this initial phase of
ventricular systole is known as isovolumic contraction, also called
isovolumetric contraction (see [link]).
In the second phase of ventricular systole, the ventricular ejection phase,
the contraction of the ventricular muscle has raised the pressure within the
ventricle to the point that it is greater than the pressures in the pulmonary
trunk and the aorta. Blood is pumped from the heart, pushing open the
pulmonary and aortic semilunar valves. Pressure generated by the left
ventricle will be appreciably greater than the pressure generated by the right
ventricle, since the existing pressure in the aorta will be so much higher.
Nevertheless, both ventricles pump the same amount of blood. This
quantity is referred to as stroke volume. Stroke volume will normally be in
the range of 70-80 mL. Since ventricular systole began with an EDV of
approximately 130 mL of blood, this means that there is still 50-60 mL of
blood remaining in the ventricle following contraction. This volume of
blood is known as the end systolic volume (ESV).
Ventricular Diastole
Ventricular relaxation, or diastole, follows repolarization of the ventricles
and is represented by the T wave of the ECG. It too is divided into two
distinct phases and lasts approximately 430 ms.
During the early phase of ventricular diastole, as the ventricular muscle
relaxes, pressure on the remaining blood within the ventricle begins to fall.
When pressure within the ventricles drops below pressure in both the
pulmonary trunk and aorta, blood flows back toward the heart, producing
the dicrotic notch (small dip) seen in blood pressure tracings. The semilunar
valves close to prevent backflow into the heart. Since the atrioventricular
valves remain closed at this point, there is no change in the volume of blood
in the ventricle, so the early phase of ventricular diastole is called the
isovolumic ventricular relaxation phase, also called isovolumetric
ventricular relaxation phase (see [link]).
In the second phase of ventricular diastole, called late ventricular diastole,
as the ventricular muscle relaxes, pressure on the blood within the
ventricles drops even further. Eventually, it drops below the pressure in the
atria. When this occurs, blood flows from the atria into the ventricles,
pushing open the tricuspid and mitral valves. As pressure drops within the
ventricles, blood flows from the major veins into the relaxed atria and from
there into the ventricles. Both chambers are in diastole, the atrioventricular
valves are open, and the semilunar valves remain closed (see [link]). The
cardiac cycle is complete.
[link] illustrates the relationship between the cardiac cycle and the ECG.
Relationship between the Cardiac Cycle and ECG
R
One cardiac cycle
Initially, both the atria and ventricles are relaxed
(diastole). The P wave represents depolarization of the
atria and is followed by atrial contraction (systole).
Atrial systole extends until the QRS complex, at
which point, the atria relax. The QRS complex
represents depolarization of the ventricles and is
followed by ventricular contraction. The T wave
represents the repolarization of the ventricles and
marks the beginning of ventricular relaxation.
Heart Sounds
One of the simplest, yet effective, diagnostic techniques applied to assess
the state of a patient’s heart is auscultation using a stethoscope.
In a normal, healthy heart, there are only two audible heart sounds: S, and
S>. S; is the sound created by the closing of the atrioventricular valves
during ventricular contraction and is normally described as a “lub,” or first
heart sound. The second heart sound, Sp, is the sound of the closing of the
semilunar valves during ventricular diastole and is described as a “dub”
({link]). In both cases, as the valves close, the openings within the
atrioventricular septum guarded by the valves will become reduced, and
blood flow through the opening will become more turbulent until the valves
are fully closed. There is a third heart sound, S3, but it is rarely heard in
healthy individuals. It may be the sound of blood flowing into the atria, or
blood sloshing back and forth in the ventricle, or even tensing of the
chordae tendineae. S3 may be heard in youth, some athletes, and pregnant
women. If the sound is heard later in life, it may indicate congestive heart
failure, warranting further tests. Some cardiologists refer to the collective
S1, Sz, and S3 sounds as the “Kentucky gallop,” because they mimic those
produced by a galloping horse. The fourth heart sound, Sy, results from the
contraction of the atria pushing blood into a stiff or hypertrophic ventricle,
indicating failure of the left ventricle. S, occurs prior to S; and the
collective sounds S,, S;, and S> are referred to by some cardiologists as the
“Tennessee gallop,” because of their similarity to the sound produced by a
galloping horse with a different gait. A few individuals may have both S3
and Sy, and this combined sound is referred to as Sv.
Heart Sounds and the Cardiac Cycle
Semilunar
120 valves close
Semilunar
100 valves open
aS
Aortic pressure
Ventricular pressure
Pressure (mm Hg)
(op)
ro]
AV valves
open
/
AV valves
close
Atrial pressure
1st 2nd 3rd
Heart sounds |),
“| ub” “Dub”
In this illustration, the x-axis reflects time with a
recording of the heart sounds. The y-axis represents
pressure.
The term murmur is used to describe an unusual sound coming from the
heart that is caused by the turbulent flow of blood. Murmurs are graded on a
scale of 1 to 6, with 1 being the most common, the most difficult sound to
detect, and the least serious. The most severe is a 6. Phonocardiograms or
auscultograms can be used to record both normal and abnormal sounds
using specialized electronic stethoscopes.
During auscultation, it is common practice for the clinician to ask the
patient to breathe deeply. This procedure not only allows for listening to
airflow, but it may also amplify heart murmurs. Inhalation increases blood
flow into the right side of the heart and may increase the amplitude of right-
sided heart murmurs. Expiration partially restricts blood flow into the left
side of the heart and may amplify left-sided heart murmurs. [link] indicates
proper placement of the bell of the stethoscope to facilitate auscultation.
Stethoscope Placement for Auscultation
Aortic valve Pulmonary valve
Tricuspid valve Mitral valve
Proper placement of the bell of the stethoscope
facilitates auscultation. At each of the four locations
on the chest, a different valve can be heard.
Chapter Review
The cardiac cycle comprises a complete relaxation and contraction of both
the atria and ventricles, and lasts approximately 0.8 seconds. Beginning
with all chambers in diastole, blood flows passively from the veins into the
atria and past the atrioventricular valves into the ventricles. The atria begin
to contract (atrial systole), following depolarization of the atria, and pump
blood into the ventricles. The ventricles begin to contract (ventricular
systole), raising pressure within the ventricles. When ventricular pressure
rises above the pressure in the atria, blood flows toward the atria, producing
the first heart sound, S, or lub. As pressure in the ventricles rises above two
major arteries, blood pushes open the two semilunar valves and moves into
the pulmonary trunk and aorta in the ventricular ejection phase. Following
ventricular repolarization, the ventricles begin to relax (ventricular
diastole), and pressure within the ventricles drops. As ventricular pressure
drops, there is a tendency for blood to flow back into the atria from the
major arteries, producing the dicrotic notch in the ECG and closing the two
semilunar valves. The second heart sound, S» or dub, occurs when the
semilunar valves close. When the pressure falls below that of the atria,
blood moves from the atria into the ventricles, opening the atrioventricular
valves and marking one complete heart cycle. The valves prevent backflow
of blood. Failure of the valves to operate properly produces turbulent blood
flow within the heart; the resulting heart murmur can often be heard with a
stethoscope.
Review Questions
Exercise:
Problem:
The cardiac cycle consists of a distinct relaxation and contraction
phase. Which term is typically used to refer ventricular contraction
while no blood is being ejected?
a. systole
b. diastole
c. quiescent
d. isovolumic contraction
Solution:
D
Exercise:
Problem: Most blood enters the ventricle during
a. atrial systole
b. atrial diastole
c. ventricular systole
d. isovolumic contraction
Solution:
B
Exercise:
Problem:
The first heart sound represents which portion of the cardiac cycle?
a. atrial systole
b. ventricular systole
c. closing of the atrioventricular valves
d. closing of the semilunar valves
Solution:
‘s
Exercise:
Problem: Ventricular relaxation immediately follows
a. atrial depolarization
b. ventricular repolarization
c. ventricular depolarization
d. atrial repolarization
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Describe one cardiac cycle, beginning with both atria and ventricles
relaxed.
Solution:
The cardiac cycle comprises a complete relaxation and contraction of
both the atria and ventricles, and lasts approximately 0.8 seconds.
Beginning with all chambers in diastole, blood flows passively from
the veins into the atria and past the atrioventricular valves into the
ventricles. The atria begin to contract following depolarization of the
atria and pump blood into the ventricles. The ventricles begin to
contract, raising pressure within the ventricles. When ventricular
pressure rises above the pressure in the two major arteries, blood
pushes open the two semilunar valves and moves into the pulmonary
trunk and aorta in the ventricular ejection phase. Following ventricular
repolarization, the ventricles begin to relax, and pressure within the
ventricles drops. When the pressure falls below that of the atria, blood
moves from the atria into the ventricles, opening the atrioventricular
valves and marking one complete heart cycle.
Glossary
cardiac cycle
period of time between the onset of atrial contraction (atrial systole)
and ventricular relaxation (ventricular diastole)
diastole
period of time when the heart muscle is relaxed and the chambers fill
with blood
end diastolic volume (EDV)
(also, preload) the amount of blood in the ventricles at the end of atrial
systole just prior to ventricular contraction
end systolic volume (ESV)
amount of blood remaining in each ventricle following systole
heart sounds
sounds heard via auscultation with a stethoscope of the closing of the
atrioventricular valves (“lub”) and semilunar valves (“dub”’)
isovolumic contraction
(also, isovolumetric contraction) initial phase of ventricular
contraction in which tension and pressure in the ventricle increase, but
no blood is pumped or ejected from the heart
isovolumic ventricular relaxation phase
initial phase of the ventricular diastole when pressure in the ventricles
drops below pressure in the two major arteries, the pulmonary trunk,
and the aorta, and blood attempts to flow back into the ventricles,
producing the dicrotic notch of the ECG and closing the two semilunar
valves
murmur
unusual heart sound detected by auscultation; typically related to septal
or valve defects
preload
(also, end diastolic volume) amount of blood in the ventricles at the
end of atrial systole just prior to ventricular contraction
systole
period of time when the heart muscle is contracting
ventricular ejection phase
second phase of ventricular systole during which blood is pumped
from the ventricle
Structure and Function of Blood Vessels
By the end of this section, you will be able to:
e Compare and contrast the three tunics that make up the walls of most
blood vessels
e Distinguish between elastic arteries, muscular arteries, and arterioles
on the basis of structure, location, and function
e Describe the basic structure of a capillary bed, from the supplying
metarteriole to the venule into which it drains
e Explain the structure and function of venous valves in the large veins
of the extremities
Blood is carried through the body via blood vessels. An artery is a blood
vessel that carries blood away from the heart, where it branches into ever-
smaller vessels. Eventually, the smallest arteries, vessels called arterioles,
further branch into tiny capillaries, where nutrients and wastes are
exchanged, and then combine with other vessels that exit capillaries to form
venules, small blood vessels that carry blood to a vein, a larger blood vessel
that returns blood to the heart.
Arteries and veins transport blood in two distinct circuits: the systemic
circuit and the pulmonary circuit ([link]). Systemic arteries provide blood
rich in oxygen to the body’s tissues. The blood returned to the heart through
systemic veins has less oxygen, since much of the oxygen carried by the
arteries has been delivered to the cells. In contrast, in the pulmonary circuit,
arteries carry blood low in oxygen exclusively to the lungs for gas
exchange. Pulmonary veins then return freshly oxygenated blood from the
lungs to the heart to be pumped back out into systemic circulation.
Although arteries and veins differ structurally and functionally, they share
certain features.
Cardiovascular Circulation
Lungs
2c
GO :
5 & Pulmonary Pulmonary vein
=)
§3 artery
as
Vena cava Aorta
Upper body
Liver
Hepatic vein Hepatic artery
Hepatic portal vein
Systemic
circulation
Stomach,
intestines Hi Vessels transporting
oxygenated blood
Renal artery BH Vessels transporting
Renal vein
deoxygenated blood
Midneye Bi Vessels involved in
gas excange
Lower body
The pulmonary circuit moves blood from the right side of
the heart to the lungs and back to the heart. The systemic
circuit moves blood from the left side of the heart to the
head and body and returns it to the right side of the heart to
repeat the cycle. The arrows indicate the direction of blood
flow, and the colors show the relative levels of oxygen
concentration.
Shared Structures
Different types of blood vessels vary slightly in their structures, but they
share the same general features. Arteries and arterioles have thicker walls
than veins and venules because they are closer to the heart and receive
blood that is surging at a far greater pressure ({link]). Each type of vessel
has a lumen—a hollow passageway through which blood flows. Arteries
have smaller lumens than veins, a characteristic that helps to maintain the
pressure of blood moving through the system. Together, their thicker walls
and smaller diameters give arterial lumens a more rounded appearance in
cross section than the lumens of veins.
Structure of Blood Vessels
Artery Vein
= + Tunica externa
LD i Tunica externa
Ae
Tunica media
S _ Tunica intima
Smooth muscle
Internal elastic
membrane
Vasa vasorum
External elastic
membrane
Nervi vasorum
Endothelium
Elastic fiber
Endothelium
(a) Arteries and (b) veins share the same
general features, but the walls of arteries are
much thicker because of the higher pressure of
the blood that flows through them. (c) A
micrograph shows the relative differences in
thickness. LM x 160. (Micrograph provided by
the Regents of the University of Michigan
Medical School © 2012)
By the time blood has passed through capillaries and entered venules, the
pressure initially exerted upon it by heart contractions has diminished. In
other words, in comparison to arteries, venules and veins withstand a much
lower pressure from the blood that flows through them. Their walls are
considerably thinner and their lumens are correspondingly larger in
diameter, allowing more blood to flow with less vessel resistance. In
addition, many veins of the body, particularly those of the limbs, contain
valves that assist the unidirectional flow of blood toward the heart. This is
critical because blood flow becomes sluggish in the extremities, as a result
of the lower pressure and the effects of gravity.
The walls of arteries and veins are largely composed of living cells and
their products (including collagenous and elastic fibers); the cells require
nourishment and produce waste. Since blood passes through the larger
vessels relatively quickly, there is limited opportunity for blood in the
lumen of the vessel to provide nourishment to or remove waste from the
vessel’s cells. Further, the walls of the larger vessels are too thick for
nutrients to diffuse through to all of the cells. Larger arteries and veins
contain small blood vessels within their walls known as the vasa vasorum
—literally “vessels of the vessel”—to provide them with this critical
exchange. Since the pressure within arteries is relatively high, the vasa
vasorum must function in the outer layers of the vessel (see [link]) or the
pressure exerted by the blood passing through the vessel would collapse it,
preventing any exchange from occurring. The lower pressure within veins
allows the vasa vasorum to be located closer to the lumen. The restriction of
the vasa vasorum to the outer layers of arteries is thought to be one reason
that arterial diseases are more common than venous diseases, since its
location makes it more difficult to nourish the cells of the arteries and
remove waste products. There are also minute nerves within the walls of
both types of vessels that control the contraction and dilation of smooth
muscle. These minute nerves are known as the nervi vasorum.
Both arteries and veins have the same three distinct tissue layers, called
tunics (from the Latin term tunica), for the garments first worn by ancient
Romans; the term tunic is also used for some modem garments. From the
most interior layer to the outer, these tunics are the tunica intima, the tunica
media, and the tunica externa (see [link]). [link] compares and contrasts the
tunics of the arteries and veins.
Comparison of Tunics in Arteries and Veins
Arteries
General Thick walls with small lumens
appearance Generally appear rounded
Endothelium usually appears
wavy due to constriction of
smooth muscle
Internal elastic membrane
present in larger vessels
Tunica
intima
Veins
Thin walls
with large
lumens
Generally
appear
flattened
Endothelium
appears
smooth
Internal elastic
membrane
absent
Comparison of Tunics in Arteries and Veins
Tunica
media
Tunica
externa
Arteries
Normally the thickest layer in
arteries
Smooth muscle cells and elastic
fibers predominate (the
proportions of these vary with
distance from the heart)
External elastic membrane
present in larger vessels
Normally thinner than the tunica
media in all but the largest
arteries
Collagenous and elastic fibers
Nervi vasorum and vasa
vasorum present
Veins
Normally
thinner than
the tunica
externa
Smooth
muscle cells
and
collagenous
fibers
predominate
Nervi vasorum
and vasa
vasorum
present
External
elastic
membrane
absent
Normally the
thickest layer
in veins
Collagenous
and smooth
fibers
predominate
Some smooth
muscle fibers
Nervi vasorum
and vasa
vasorum
present
Tunica Intima
The tunica intima (also called the tunica interna) is composed of epithelial
and connective tissue layers. Lining the tunica intima is the specialized
simple squamous epithelium called the endothelium, which is continuous
throughout the entire vascular system, including the lining of the chambers
of the heart. Damage to this endothelial lining and exposure of blood to the
collagenous fibers beneath is one of the primary causes of clot formation.
Until recently, the endothelium was viewed simply as the boundary between
the blood in the lumen and the walls of the vessels. Recent studies,
however, have shown that it is physiologically critical to such activities as
helping to regulate capillary exchange and altering blood flow. The
endothelium releases local chemicals called endothelins that can constrict
the smooth muscle within the walls of the vessel to increase blood pressure.
Uncompensated overproduction of endothelins may contribute to
hypertension (high blood pressure) and cardiovascular disease.
Next to the endothelium is the basement membrane, or basal lamina, that
effectively binds the endothelium to the connective tissue. The basement
membrane provides strength while maintaining flexibility, and it is
permeable, allowing materials to pass through it. The thin outer layer of the
tunica intima contains a small amount of areolar connective tissue that
consists primarily of elastic fibers to provide the vessel with additional
flexibility; it also contains some collagenous fibers to provide additional
strength.
In larger arteries, there is also a thick, distinct layer of elastic fibers known
as the internal elastic membrane (also called the internal elastic lamina) at
the boundary with the tunica media. Like the other components of the
tunica intima, the internal elastic membrane provides structure while
allowing the vessel to stretch. It is permeated with small openings that
allow exchange of materials between the tunics. The internal elastic
membrane is not apparent in veins. In addition, many veins, particularly in
the lower limbs, contain valves formed by sections of thickened
endothelium that are reinforced with connective tissue, extending into the
lumen.
Under the microscope, the lumen and the entire tunica intima of a vein will
appear smooth, whereas those of an artery will normally appear wavy
because of the partial constriction of the smooth muscle in the tunica media,
the next layer of blood vessel walls.
Tunica Media
The tunica media is the substantial middle layer of the vessel wall (see
[link]). It is generally the thickest layer in arteries, and it is much thicker in
arteries than it is in veins. The tunica media consists of layers of smooth
muscle supported by connective tissue that is primarily made up of elastic
fibers, most of which are arranged in circular sheets. Toward the outer
portion of the tunic, there are also layers of longitudinal muscle.
Contraction and relaxation of the circular muscles decrease and increase the
diameter of the vessel lumen, respectively. Specifically in arteries,
vasoconstriction decreases blood flow as the smooth muscle in the walls of
the tunica media contracts, making the lumen narrower and increasing
blood pressure. Similarly, vasodilation increases blood flow as the smooth
muscle relaxes, allowing the lumen to widen and blood pressure to drop.
Both vasoconstriction and vasodilation are regulated in part by small
vascular nerves, known as nervi vasorum, or “nerves of the vessel,” that
run within the walls of blood vessels. These are generally all sympathetic
fibers, although some trigger vasodilation and others induce
vasoconstriction, depending upon the nature of the neurotransmitter and
receptors located on the target cell. Parasympathetic stimulation does
trigger vasodilation as well as erection during sexual arousal in the external
genitalia of both sexes. Nervous control over vessels tends to be more
generalized than the specific targeting of individual blood vessels. Local
controls, discussed later, account for this phenomenon. (Seek additional
content for more information on these dynamic aspects of the autonomic
nervous system.) Hormones and local chemicals also control blood vessels.
Together, these neural and chemical mechanisms reduce or increase blood
flow in response to changing body conditions, from exercise to hydration.
Regulation of both blood flow and blood pressure is discussed in detail later
in this chapter.
The smooth muscle layers of the tunica media are supported by a
framework of collagenous fibers that also binds the tunica media to the
inner and outer tunics. Along with the collagenous fibers are large numbers
of elastic fibers that appear as wavy lines in prepared slides. Separating the
tunica media from the outer tunica externa in larger arteries is the external
elastic membrane (also called the external elastic lamina), which also
appears wavy in Slides. This structure is not usually seen in smaller arteries,
nor is it seen in veins.
Tunica Externa
The outer tunic, the tunica externa (also called the tunica adventitia), is a
substantial sheath of connective tissue composed primarily of collagenous
fibers. Some bands of elastic fibers are found here as well. The tunica
externa in veins also contains groups of smooth muscle fibers. This is
normally the thickest tunic in veins and may be thicker than the tunica
media in some larger arteries. The outer layers of the tunica externa are not
distinct but rather blend with the surrounding connective tissue outside the
vessel, helping to hold the vessel in relative position. If you are able to
palpate some of the superficial veins on your upper limbs and try to move
them, you will find that the tunica externa prevents this. If the tunica
externa did not hold the vessel in place, any movement would likely result
in disruption of blood flow.
Arteries
An artery is a blood vessel that conducts blood away from the heart. All
arteries have relatively thick walls that can withstand the high pressure of
blood ejected from the heart. However, those close to the heart have the
thickest walls, containing a high percentage of elastic fibers in all three of
their tunics. This type of artery is known as an elastic artery ((link)).
Vessels larger than 10 mm in diameter are typically elastic. Their abundant
elastic fibers allow them to expand, as blood pumped from the ventricles
passes through them, and then to recoil after the surge has passed. If artery
walls were rigid and unable to expand and recoil, their resistance to blood
flow would greatly increase and blood pressure would rise to even higher
levels, which would in turn require the heart to pump harder to increase the
volume of blood expelled by each pump (the stroke volume) and maintain
adequate pressure and flow. Artery walls would have to become even
thicker in response to this increased pressure. The elastic recoil of the
vascular wall helps to maintain the pressure gradient that drives the blood
through the arterial system. An elastic artery is also known as a conducting
artery, because the large diameter of the lumen enables it to accept a large
volume of blood from the heart and conduct it to smaller branches.
Types of Arteries and Arterioles
Elastic Tunica Muscular Tunica Arteriole Tunica
artery a— externa artery = = externa = externa
unica Tunica
media
DP Tu nica
“as
intima
Tunica
media
b Tunica
intima
media
5 ie Tunica
intima
Comparison of the walls of an elastic artery, a muscular artery,
and an arteriole is shown. In terms of scale, the diameter of an
arteriole is measured in micrometers compared to millimeters
for elastic and muscular arteries.
Farther from the heart, where the surge of blood has dampened, the
percentage of elastic fibers in an artery’s tunica intima decreases and the
amount of smooth muscle in its tunica media increases. The artery at this
point is described as a muscular artery. The diameter of muscular arteries
typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows
muscular arteries to play a leading role in vasoconstriction. In contrast, their
decreased quantity of elastic fibers limits their ability to expand.
Fortunately, because the blood pressure has eased by the time it reaches
these more distant vessels, elasticity has become less important.
Notice that although the distinctions between elastic and muscular arteries
are important, there is no “line of demarcation” where an elastic artery
suddenly becomes muscular. Rather, there is a gradual transition as the
vascular tree repeatedly branches. In turn, muscular arteries branch to
distribute blood to the vast network of arterioles. For this reason, a
muscular artery is also known as a distributing artery.
Arterioles
An arteriole is a very small artery that leads to a capillary. Arterioles have
the same three tunics as the larger vessels, but the thickness of each is
greatly diminished. The critical endothelial lining of the tunica intima is
intact. The tunica media is restricted to one or two smooth muscle cell
layers in thickness. The tunica externa remains but is very thin (see [link]).
With a lumen averaging 30 micrometers or less in diameter, arterioles are
critical in slowing down—or resisting—blood flow and, thus, causing a
substantial drop in blood pressure. Because of this, you may see them
referred to as resistance vessels. The muscle fibers in arterioles are
normally slightly contracted, causing arterioles to maintain a consistent
muscle tone—in this case referred to as vascular tone—in a similar manner
to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit
vascular tone due to the partial contraction of smooth muscle. The
importance of the arterioles is that they will be the primary site of both
resistance and regulation of blood pressure. The precise diameter of the
lumen of an arteriole at any given moment is determined by neural and
chemical controls, and vasoconstriction and vasodilation in the arterioles
are the primary mechanisms for distribution of blood flow.
Capillaries
A capillary is a microscopic channel that supplies blood to the tissues
themselves, a process called perfusion. Exchange of gases and other
substances occurs in the capillaries between the blood and the surrounding
cells and their tissue fluid (interstitial fluid). The diameter of a capillary
lumen ranges from 5—10 micrometers; the smallest are just barely wide
enough for an erythrocyte to squeeze through. Flow through capillaries is
often described as microcirculation.
The wall of a capillary consists of the endothelial layer surrounded by a
basement membrane with occasional smooth muscle fibers. There is some
variation in wall structure: In a large capillary, several endothelial cells
bordering each other may line the lumen; in a small capillary, there may be
only a single cell layer that wraps around to contact itself.
For capillaries to function, their walls must be leaky, allowing substances to
pass through. There are three major types of capillaries, which differ
according to their degree of “leakiness:” continuous, fenestrated, and
sinusoid capillaries ({Link]).
Continuous Capillaries
The most common type of capillary, the continuous capillary, is found in
almost all vascularized tissues. Continuous capillaries are characterized by
a complete endothelial lining with tight junctions between endothelial cells.
Although a tight junction is usually impermeable and only allows for the
passage of water and ions, they are often incomplete in capillaries, leaving
intercellular clefts that allow for exchange of water and other very small
molecules between the blood plasma and the interstitial fluid. Substances
that can pass between cells include metabolic products, such as glucose,
water, and small hydrophobic molecules like gases and hormones, as well
as various leukocytes. Continuous capillaries not associated with the brain
are rich in transport vesicles, contributing to either endocytosis or
exocytosis. Those in the brain are part of the blood-brain barrier. Here, there
are tight junctions and no intercellular clefts, plus a thick basement
membrane and astrocyte extensions called end feet; these structures
combine to prevent the movement of nearly all substances.
Types of Capillaries
Continuous Fenestrated Sinusoid
Endothelial layer
(tunica intima)
Incomplete
basement
membrane
Intercellular cleft Fenestrations Intercellular gap
The three major types of capillaries: continuous,
fenestrated, and sinusoid.
Fenestrated Capillaries
A fenestrated capillary is one that has pores (or fenestrations) in addition
to tight junctions in the endothelial lining. These make the capillary
permeable to larger molecules. The number of fenestrations and their
degree of permeability vary, however, according to their location.
Fenestrated capillaries are common in the small intestine, which is the
primary site of nutrient absorption, as well as in the kidneys, which filter
the blood. They are also found in the choroid plexus of the brain and many
endocrine structures, including the hypothalamus, pituitary, pineal, and
thyroid glands.
Sinusoid Capillaries
A sinusoid capillary (or sinusoid) is the least common type of capillary.
Sinusoid capillaries are flattened, and they have extensive intercellular gaps
and incomplete basement membranes, in addition to intercellular clefts and
fenestrations. This gives them an appearance not unlike Swiss cheese.
These very large openings allow for the passage of the largest molecules,
including plasma proteins and even cells. Blood flow through sinusoids is
very slow, allowing more time for exchange of gases, nutrients, and wastes.
Sinusoids are found in the liver and spleen, bone marrow, lymph nodes
(where they carry lymph, not blood), and many endocrine glands including
the pituitary and adrenal glands. Without these specialized capillaries, these
organs would not be able to provide their myriad of functions. For example,
when bone marrow forms new blood cells, the cells must enter the blood
supply and can only do so through the large openings of a sinusoid
capillary; they cannot pass through the small openings of continuous or
fenestrated capillaries. The liver also requires extensive specialized sinusoid
capillaries in order to process the materials brought to it by the hepatic
portal vein from both the digestive tract and spleen, and to release plasma
proteins into circulation.
Metarterioles and Capillary Beds
A metarteriole is a type of vessel that has structural characteristics of both
an arteriole and a capillary. Slightly larger than the typical capillary, the
smooth muscle of the tunica media of the metarteriole is not continuous but
forms rings of smooth muscle (sphincters) prior to the entrance to the
capillaries. Each metarteriole arises from a terminal arteriole and branches
to supply blood to a capillary bed that may consist of 10—100 capillaries.
The precapillary sphincters, circular smooth muscle cells that surround
the capillary at its origin with the metarteriole, tightly regulate the flow of
blood from a metarteriole to the capillaries it supplies. Their function is
critical: If all of the capillary beds in the body were to open simultaneously,
they would collectively hold every drop of blood in the body and there
would be none in the arteries, arterioles, venules, veins, or the heart itself.
Normally, the precapillary sphincters are closed. When the surrounding
tissues need oxygen and have excess waste products, the precapillary
sphincters open, allowing blood to flow through and exchange to occur
before closing once more ((link]). If all of the precapillary sphincters in a
capillary bed are closed, blood will flow from the metarteriole directly into
a thoroughfare channel and then into the venous circulation, bypassing the
capillary bed entirely. This creates what is known as a vascular shunt. In
addition, an arteriovenous anastomosis may bypass the capillary bed and
lead directly to the venous system.
Although you might expect blood flow through a capillary bed to be
smooth, in reality, it moves with an irregular, pulsating flow. This pattern is
called vasomotion and is regulated by chemical signals that are triggered in
response to changes in internal conditions, such as oxygen, carbon dioxide,
hydrogen ion, and lactic acid levels. For example, during strenuous exercise
when oxygen levels decrease and carbon dioxide, hydrogen ion, and lactic
acid levels all increase, the capillary beds in skeletal muscle are open, as
they would be in the digestive system when nutrients are present in the
digestive tract. During sleep or rest periods, vessels in both areas are largely
closed; they open only occasionally to allow oxygen and nutrient supplies
to travel to the tissues to maintain basic life processes.
Capillary Bed
Capillary bed
eee Capillary
Arteriole —————————_ Venule
Precapillary ( ZB |
sphincter Se SS —— Thoroughfare
channel
Metarteriole (serves as
vascular shunt when
precapillary sphincters
are closed)
Arteriovenous ————____4. i
anastomosis
In a capillary bed, arterioles give rise to metarterioles.
Precapillary sphincters located at the junction of a
metarteriole with a capillary regulate blood flow. A
thoroughfare channel connects the metarteriole to a
venule. An arteriovenous anastomosis, which directly
connects the arteriole with the venule, is shown at the
bottom.
Venules
A venule is an extremely small vein, generally 8-100 micrometers in
diameter. Postcapillary venules join multiple capillaries exiting from a
capillary bed. Multiple venules join to form veins. The walls of venules
consist of endothelium, a thin middle layer with a few muscle cells and
elastic fibers, plus an outer layer of connective tissue fibers that constitute a
very thin tunica externa ({link]). Venules as well as capillaries are the
primary sites of emigration or diapedesis, in which the white blood cells
adhere to the endothelial lining of the vessels and then squeeze through
adjacent cells to enter the tissue fluid.
Veins
A vein is a blood vessel that conducts blood toward the heart. Compared to
arteries, veins are thin-walled vessels with large and irregular lumens (see
[link]). Because they are low-pressure vessels, larger veins are commonly
equipped with valves that promote the unidirectional flow of blood toward
the heart and prevent backflow toward the capillaries caused by the inherent
low blood pressure in veins as well as the pull of gravity. [link] compares
the features of arteries and veins.
Comparison of Veins and Venules
Large vein
Tunica externa
+—— Tunica media
Garin
— ®
Tunica intima
@ yy Af Smooth muscle cell
in tunica externa
Vasa vasorum
Nervi vasorum
mate [tunica externa
+—— Tunica media
Tunica intima
Valves
(closed)
Tunica externa
Tunica media
Tunica intima
Venule
Many veins have valves to prevent back
flow of blood, whereas venules do not. In
terms of scale, the diameter of a venule is
measured in micrometers compared to
millimeters for veins.
Comparison of Arteries and Veins
Arteries Veins
Direction of Conducts Conducts blood toward the
blood away
blood flow heart
from the heart
General
Rounded Irregular, often collapsed
appearance
Pressure High Low
ye Il Thick Thin
thickness
Higher in
Relative aaa . : :
arteries Lower in systemic veins
oxygen : : : ;
: Lower in Higher in pulmonary veins
concentration
pulmonary
arteries
Present most commonly in
Valves Not present limbs and in veins inferior to
the heart
Note:
Disorders of the...
Cardiovascular System: Edema and Varicose Veins
Despite the presence of valves and the contributions of other anatomical
and physiological adaptations we will cover shortly, over the course of a
day, some blood will inevitably pool, especially in the lower limbs, due to
the pull of gravity. Any blood that accumulates in a vein will increase the
pressure within it, which can then be reflected back into the smaller veins,
venules, and eventually even the capillaries. Increased pressure will
promote the flow of fluids out of the capillaries and into the interstitial
fluid. The presence of excess tissue fluid around the cells leads to a
condition called edema.
Most people experience a daily accumulation of tissue fluid, especially if
they spend much of their work life on their feet (like most health
professionals). However, clinical edema goes beyond normal swelling and
requires medical treatment. Edema has many potential causes, including
hypertension and heart failure, severe protein deficiency, renal failure, and
many others. In order to treat edema, which is a sign rather than a discrete
disorder, the underlying cause must be diagnosed and alleviated.
Varicose Veins
Varicose veins are commonly
found in the lower limbs. (credit:
Thomas Kriese)
Edema may be accompanied by varicose veins, especially in the superficial
veins of the legs ([link]). This disorder arises when defective valves allow
blood to accumulate within the veins, causing them to distend, twist, and
become visible on the surface of the integument. Varicose veins may occur
in both sexes, but are more common in women and are often related to
pregnancy. More than simple cosmetic blemishes, varicose veins are often
painful and sometimes itchy or throbbing. Without treatment, they tend to
grow worse over time. The use of support hose, as well as elevating the
feet and legs whenever possible, may be helpful in alleviating this
condition. Laser surgery and interventional radiologic procedures can
reduce the size and severity of varicose veins. Severe cases may require
conventional surgery to remove the damaged vessels. As there are typically
redundant circulation patterns, that is, anastomoses, for the smaller and
more superficial veins, removal does not typically impair the circulation.
There is evidence that patients with varicose veins suffer a greater risk of
developing a thrombus or clot.
Veins as Blood Reservoirs
In addition to their primary function of returning blood to the heart, veins
may be considered blood reservoirs, since systemic veins contain
approximately 64 percent of the blood volume at any given time ([link]).
Their ability to hold this much blood is due to their high capacitance, that
is, their capacity to distend (expand) readily to store a high volume of
blood, even at a low pressure. The large lumens and relatively thin walls of
veins make them far more distensible than arteries; thus, they are said to be
Capacitance vessels.
Distribution of Blood Flow
Systemic circulation Systemic veins Large veins
84% 64% 18%
Large venous networks (liver, bone
marrow, and integument)
21%
Venules and medium-sized veins
25%
Systemic arteries Arterioles
13% 2%
Muscular arteries
5%
Elastic arteries
4%
Aorta
2%
Systemic capillaries Systemic capillaries
7% 7%
Pulmonary circulation Pulmonary veins
9% 4%
Pulmonary capillaries
2%
se mens a
3%
When blood flow needs to be redistributed to other portions of the body, the
vasomotor center located in the medulla oblongata sends sympathetic
stimulation to the smooth muscles in the walls of the veins, causing
constriction—or in this case, venoconstriction. Less dramatic than the
vasoconstriction seen in smaller arteries and arterioles, venoconstriction
may be likened to a “stiffening” of the vessel wall. This increases pressure
on the blood within the veins, speeding its return to the heart. As you will
note in [link], approximately 21 percent of the venous blood is located in
venous networks within the liver, bone marrow, and integument. This
volume of blood is referred to as venous reserve. Through
venoconstriction, this “reserve” volume of blood can get back to the heart
more quickly for redistribution to other parts of the circulation.
Note:
Career Connection
Vascular Surgeons and Technicians
Vascular surgery is a specialty in which the physician deals primarily with
diseases of the vascular portion of the cardiovascular system. This includes
repair and replacement of diseased or damaged vessels, removal of plaque
from vessels, minimally invasive procedures including the insertion of
venous catheters, and traditional surgery. Following completion of medical
school, the physician generally completes a 5-year surgical residency
followed by an additional 1 to 2 years of vascular specialty training. In the
United States, most vascular surgeons are members of the Society of
Vascular Surgery.
Vascular technicians are specialists in imaging technologies that provide
information on the health of the vascular system. They may also assist
physicians in treating disorders involving the arteries and veins. This
profession often overlaps with cardiovascular technology, which would
also include treatments involving the heart. Although recognized by the
American Medical Association, there are currently no licensing
requirements for vascular technicians, and licensing is voluntary. Vascular
technicians typically have an Associate’s degree or certificate, involving 18
months to 2 years of training. The United States Bureau of Labor projects
this profession to grow by 29 percent from 2010 to 2020.
Note:
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Visit this site to learn more about vascular surgery.
Note:
Visit this site to learn more about vascular technicians.
Chapter Review
Blood pumped by the heart flows through a series of vessels known as
arteries, arterioles, capillaries, venules, and veins before returning to the
heart. Arteries transport blood away from the heart and branch into smaller
vessels, forming arterioles. Arterioles distribute blood to capillary beds, the
sites of exchange with the body tissues. Capillaries lead back to small
vessels known as venules that flow into the larger veins and eventually back
to the heart.
The arterial system is a relatively high-pressure system, so arteries have
thick walls that appear round in cross section. The venous system is a
lower-pressure system, containing veins that have larger lumens and thinner
walls. They often appear flattened. Arteries, arterioles, venules, and veins
are composed of three tunics known as the tunica intima, tunica media, and
tunica externa. Capillaries have only a tunica intima layer. The tunica
intima is a thin layer composed of a simple squamous epithelium known as
endothelium and a small amount of connective tissue. The tunica media is a
thicker area composed of variable amounts of smooth muscle and
connective tissue. It is the thickest layer in all but the largest arteries. The
tunica externa is primarily a layer of connective tissue, although in veins, it
also contains some smooth muscle. Blood flow through vessels can be
dramatically influenced by vasoconstriction and vasodilation in their walls.
Review Questions
Exercise:
Problem:The endothelium is found in the
a. tunica intima
b. tunica media
c. tunica externa
d. lumen
Solution:
A
Exercise:
Problem: Nervi vasorum control
a. vasoconstriction
b. vasodilation
c. capillary permeability
d. both vasoconstriction and vasodilation
Solution:
D
Exercise:
Problem:
Closer to the heart, arteries would be expected to have a higher
percentage of
a. endothelium
b. smooth muscle fibers
c. elastic fibers
d. collagenous fibers
Solution:
@
Exercise:
Problem: Which of the following best describes veins?
a. thick walled, small lumens, low pressure, lack valves
b. thin walled, large lumens, low pressure, have valves
c. thin walled, small lumens, high pressure, have valves
d. thick walled, large lumens, high pressure, lack valves
Solution:
B
Exercise:
Problem:
An especially leaky type of capillary found in the liver and certain
other tissues is called a
a. capillary bed
b. fenestrated capillary
c. sinusoid capillary
d. metarteriole
Solution:
GC
Critical Thinking Questions
Exercise:
Problem: Arterioles are often referred to as resistance vessels. Why?
Solution:
Arterioles receive blood from arteries, which are vessels with a much
larger lumen. As their own lumen averages just 30 micrometers or less,
arterioles are critical in slowing down—or resisting—blood flow. The
arterioles can also constrict or dilate, which varies their resistance, to
help distribute blood flow to the tissues.
Exercise:
Problem:
Cocaine use causes vasoconstriction. Is this likely to increase or
decrease blood pressure, and why?
Solution:
Vasoconstriction causes the lumens of blood vessels to narrow. This
increases the pressure of the blood flowing within the vessel.
Exercise:
Problem:
A blood vessel with a few smooth muscle fibers and connective tissue,
and only a very thin tunica externa conducts blood toward the heart.
What type of vessel is this?
Solution:
This is a venule.
Glossary
arteriole
(also, resistance vessel) very small artery that leads to a capillary
arteriovenous anastomosis
short vessel connecting an arteriole directly to a venule and bypassing
the capillary beds
artery
blood vessel that conducts blood away from the heart; may be a
conducting or distributing vessel
Capacitance
ability of a vein to distend and store blood
Capacitance vessels
veins
capillary
smallest of blood vessels where physical exchange occurs between the
blood and tissue cells surrounded by interstitial fluid
capillary bed
network of 10-100 capillaries connecting arterioles to venules
continuous capillary
most common type of capillary, found in virtually all tissues except
epithelia and cartilage; contains very small gaps in the endothelial
lining that permit exchange
elastic artery
(also, conducting artery) artery with abundant elastic fibers located
closer to the heart, which maintains the pressure gradient and conducts
blood to smaller branches
external elastic membrane
membrane composed of elastic fibers that separates the tunica media
from the tunica externa; seen in larger arteries
fenestrated capillary
type of capillary with pores or fenestrations in the endothelium that
allow for rapid passage of certain small materials
internal elastic membrane
membrane composed of elastic fibers that separates the tunica intima
from the tunica media; seen in larger arteries
lumen
interior of a tubular structure such as a blood vessel or a portion of the
alimentary canal through which blood, chyme, or other substances
travel
metarteriole
short vessel arising from a terminal arteriole that branches to supply a
capillary bed
microcirculation
blood flow through the capillaries
muscular artery
(also, distributing artery) artery with abundant smooth muscle in the
tunica media that branches to distribute blood to the arteriole network
nervi vasorum
small nerve fibers found in arteries and veins that trigger contraction of
the smooth muscle in their walls
perfusion
distribution of blood into the capillaries so the tissues can be supplied
precapillary sphincters
circular rings of smooth muscle that surround the entrance to a
capillary and regulate blood flow into that capillary
sinusoid capillary
rarest type of capillary, which has extremely large intercellular gaps in
the basement membrane in addition to clefts and fenestrations; found
in areas such as the bone marrow and liver where passage of large
molecules occurs
thoroughfare channel
continuation of the metarteriole that enables blood to bypass a
capillary bed and flow directly into a venule, creating a vascular shunt
tunica externa
(also, tunica adventitia) outermost layer or tunic of a vessel (except
capillaries)
tunica intima
(also, tunica interna) innermost lining or tunic of a vessel
tunica media
middle layer or tunic of a vessel (except capillaries)
vasa vasorum
small blood vessels located within the walls or tunics of larger vessels
that supply nourishment to and remove wastes from the cells of the
vessels
vascular shunt
continuation of the metarteriole and thoroughfare channel that allows
blood to bypass the capillary beds to flow directly from the arterial to
the venous circulation
vasoconstriction
constriction of the smooth muscle of a blood vessel, resulting in a
decreased vascular diameter
vasodilation
relaxation of the smooth muscle in the wall of a blood vessel, resulting
in an increased vascular diameter
vasomotion
irregular, pulsating flow of blood through capillaries and related
structures
vein
blood vessel that conducts blood toward the heart
venous reserve
volume of blood contained within systemic veins in the integument,
bone marrow, and liver that can be returned to the heart for circulation,
if needed
venule
small vessel leading from the capillaries to veins
Circulatory Pathways
By the end of this section, you will be able to:
e Identify the vessels through which blood travels within the pulmonary
circuit, beginning from the right ventricle of the heart and ending at the
left atrium
e Create a flow chart showing the major systemic arteries through which
blood travels from the aorta and its major branches, to the most
significant arteries feeding into the right and left upper and lower
limbs
¢ Create a flow chart showing the major systemic veins through which
blood travels from the feet to the right atrium of the heart
Virtually every cell, tissue, organ, and system in the body is impacted by the
circulatory system. This includes the generalized and more specialized
functions of transport of materials, capillary exchange, maintaining health
by transporting white blood cells and various immunoglobulins
(antibodies), hemostasis, regulation of body temperature, and helping to
maintain acid-base balance. In addition to these shared functions, many
systems enjoy a unique relationship with the circulatory system. [link]
summarizes these relationships.
Interaction of the Circulatory System with Other Body Systems
Digestive Absorbs nutrients and water; delivers nutrients (except most lipids) to
liver for processing by hepatic portal vein; provides nutrients essential
for hematopoiesis and building hemoglobin
Delivers hormones: atrial natriuretic hormone (peptide) secreted by
the heart atrial cells to help regulate blood volumes and pressures;
epinephrine, ANH, angiotensin Il, ADH, and thyroxine to help
regulate blood pressure; estrogen to promote vascular health in
women and men
Carries clotting factors, platelets, and white blood cells for
hemostasis, fighting infection, and repairing damage; regulates
temperature by controlling blood flow to the surface, where heat can
be dissipated; provides some coloration of integument; acts as a
blood reservoir
Lymphatic Pes Transports various white blood cells, including those produced by
lymphatic tissue, and immunoglobulins (antibodies) throughout the
body to maintain health; carries excess tissue fluid not able to be
reabsorbed by the vascular capillaries back to the lymphatic system
for processing
Provides nutrients and oxygen for contraction; removes lactic acid
and distributes heat generated by contraction; muscular pumps aid in
venous return; exercise contributes to cardiovascular health and
helps to prevent atherosclerosis
Nervous Produces cerebrospinal fluid (CSF) within choroid plexuses;
contributes to blood-brain barrier; cardiac and vasomotor centers
regulate cardiac output and blood flow through vessels via autonomic
system
Reproductive : Aids in erection of genitalia in both sexes during sexual arousal;
transports gonadotropic hormones that regulate reproductive
functions
Respiratory Provides blood for critical exchange of gases to carry oxygen needed
for metabolic reactions and carbon dioxide generated as byproducts
of these processes
Skeletal a Provides calcium, phosphate, and other minerals critical for bone
s matrix; transports hormones regulating buildup and absorption of
matrix including growth hormone (somatotropin), thyroid hormone,
calcitonins, and parathyroid hormone; erythropoietin stimulates
myeloid cell hematopoiesis; some level of protection for select
vessels by bony structures
Delivers 20% of resting circulation to kidneys for filtering,
reabsorption of useful products, and secretion of excesses; regulates
blood volume and pressure by regulating fluid loss in the form of
urine and by releasing the enzyme renin that is essential in the
renin-angiotensin-aldosterone mechanism
As you learn about the vessels of the systemic and pulmonary circuits,
notice that many arteries and veins share the same names, parallel one
another throughout the body, and are very similar on the right and left sides
of the body. These pairs of vessels will be traced through only one side of
the body. Where differences occur in branching patterns or when vessels are
singular, this will be indicated. For example, you will find a pair of femoral
arteries and a pair of femoral veins, with one vessel on each side of the
body. In contrast, some vessels closer to the midline of the body, such as the
aorta, are unique. Moreover, some superficial veins, such as the great
saphenous vein in the femoral region, have no arterial counterpart. Another
phenomenon that can make the study of vessels challenging is that names of
vessels can change with location. Like a street that changes name as it
passes through an intersection, an artery or vein can change names as it
passes an anatomical landmark. For example, the left subclavian artery
becomes the axillary artery as it passes through the body wall and into the
axillary region, and then becomes the brachial artery as it flows from the
axillary region into the upper arm (or brachium). You will also find
examples of anastomoses where two blood vessels that previously branched
reconnect. Anastomoses are especially common in veins, where they help
maintain blood flow even when one vessel is blocked or narrowed, although
there are some important ones in the arteries supplying the brain.
As you read about circular pathways, notice that there is an occasional, very
large artery referred to as a trunk, a term indicating that the vessel gives
rise to several smaller arteries. For example, the celiac trunk gives rise to
the left gastric, common hepatic, and splenic arteries.
As you study this section, imagine you are on a “Voyage of Discovery”
similar to Lewis and Clark’s expedition in 1804—1806, which followed
rivers and streams through unfamiliar territory, seeking a water route from
the Atlantic to the Pacific Ocean. You might envision being inside a
miniature boat, exploring the various branches of the circulatory system.
This simple approach has proven effective for many students in mastering
these major circulatory patterns. Another approach that works well for
many students is to create simple line drawings similar to the ones
provided, labeling each of the major vessels. It is beyond the scope of this
text to name every vessel in the body. However, we will attempt to discuss
the major pathways for blood and acquaint you with the major named
arteries and veins in the body. Also, please keep in mind that individual
variations in circulation patterns are not uncommon.
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Visit this site for a brief summary of the arteries.
Pulmonary Circulation
Recall that blood returning from the systemic circuit enters the right atrium
({link]) via the superior and inferior venae cavae and the coronary sinus,
which drains the blood supply of the heart muscle. These vessels will be
described more fully later in this section. This blood is relatively low in
oxygen and relatively high in carbon dioxide, since much of the oxygen has
been extracted for use by the tissues and the waste gas carbon dioxide was
picked up to be transported to the lungs for elimination. From the right
atrium, blood moves into the right ventricle, which pumps it to the lungs for
gas exchange. This system of vessels is referred to as the pulmonary
circuit.
The single vessel exiting the right ventricle is the pulmonary trunk. At the
base of the pulmonary trunk is the pulmonary semilunar valve, which
prevents backflow of blood into the right ventricle during ventricular
diastole. As the pulmonary trunk reaches the superior surface of the heart, it
curves posteriorly and rapidly bifurcates (divides) into two branches, a left
and a right pulmonary artery. To prevent confusion between these vessels,
it is important to refer to the vessel exiting the heart as the pulmonary trunk,
rather than also calling it a pulmonary artery. The pulmonary arteries in turn
branch many times within the lung, forming a series of smaller arteries and
arterioles that eventually lead to the pulmonary capillaries. The pulmonary
capillaries surround lung structures known as alveoli that are the sites of
oxygen and carbon dioxide exchange.
Once gas exchange is completed, oxygenated blood flows from the
pulmonary capillaries into a series of pulmonary venules that eventually
lead to a series of larger pulmonary veins. Four pulmonary veins, two on
the left and two on the right, return blood to the left atrium. At this point,
the pulmonary circuit is complete. [link] defines the major arteries and
veins of the pulmonary circuit discussed in the text.
Pulmonary Circuit
Ascending aorta Aortic arch
Superior vena cava Pulmonary trunk
Right lung Left lung
Left pulmonary
arteries
Left pulmonary
Right pulmonary wink
arteries
Right pulmonary
veins
~ Pulmonary
capillaries
Inferior vena
cava Descending
aorta
Blood exiting from the right ventricle flows
into the pulmonary trunk, which bifurcates into
the two pulmonary arteries. These vessels
branch to supply blood to the pulmonary
capillaries, where gas exchange occurs within
the lung alveoli. Blood returns via the
pulmonary veins to the left atrium.
Pulmonary Arteries and Veins
Vessel Description
Pulmonary Arteries and Veins
Vessel Description
Pulmonary Single large vessel exiting the right ventricle that
trunk divides to form the right and left pulmonary arteries
Peionay Left and right vessels that form from the pulmonary
, a trunk and lead to smaller arterioles and eventually to
arteries eae fi
the pulmonary capillaries
Two sets of paired vessels—one pair on each side—
Pulmonary that are formed from the small venules, leading away
veins from the pulmonary capillaries to flow into the left
atrium
Overview of Systemic Arteries
Blood relatively high in oxygen concentration is returned from the
pulmonary circuit to the left atrium via the four pulmonary veins. From the
left atrium, blood moves into the left ventricle, which pumps blood into the
aorta. The aorta and its branches—the systemic arteries—send blood to
virtually every organ of the body ([link]).
Systemic Arteries
Vertebral
es Right common carotid
Left common carotid
Left subclavian
Axillary
il Pulmonary trunk
Right subclavian
Brachiocephalic trunk
Aortic arch
Ascending aorta Br VT . Descending aorta
SN \) Diaphragm
Celiac trunk = <h A\ Renal
= ena
H Se vant \
Brachial P = Superior mesenteric
/ a
Radial / | j j We aN Inferior mesenteric
ii Common iliac
Ulnar Lh:
i} y us ae Internal iliac
External iliac a o
AY A)
A
Palmar AN fea w iS y
arches ead ait y= Deep
EAN femoral
MAW \ ¥ WA
] F Femoral
Descending
\) genicular
Popliteal
Posterior tibial
Anterior tibial
Fibular
i
au
IN
Dorsalis pedis
Plantar arch ae
The major systemic arteries shown here
deliver oxygenated blood throughout the
body.
The Aorta
The aorta is the largest artery in the body ([link]). It arises from the left
ventricle and eventually descends to the abdominal region, where it
bifurcates at the level of the fourth lumbar vertebra into the two common
iliac arteries. The aorta consists of the ascending aorta, the aortic arch, and
the descending aorta, which passes through the diaphragm and a landmark
that divides into the superior thoracic and inferior abdominal components.
Arteries originating from the aorta ultimately distribute blood to virtually
all tissues of the body. At the base of the aorta is the aortic semilunar valve
that prevents backflow of blood into the left ventricle while the heart is
relaxing. After exiting the heart, the ascending aorta moves in a superior
direction for approximately 5 cm and ends at the sternal angle. Following
this ascent, it reverses direction, forming a graceful arc to the left, called the
aortic arch. The aortic arch descends toward the inferior portions of the
body and ends at the level of the intervertebral disk between the fourth and
fifth thoracic vertebrae. Beyond this point, the descending aorta continues
close to the bodies of the vertebrae and passes through an opening in the
diaphragm known as the aortic hiatus. Superior to the diaphragm, the aorta
is called the thoracic aorta, and inferior to the diaphragm, it is called the
abdominal aorta. The abdominal aorta terminates when it bifurcates into
the two common iliac arteries at the level of the fourth lumbar vertebra. See
[link] for an illustration of the ascending aorta, the aortic arch, and the
initial segment of the descending aorta plus major branches; [link]
summarizes the structures of the aorta.
Aorta
Right common Left common
carotid artery carotid artery
Left subclavian
artery
Right subclavian
artery
Brachiocephalic
artery Aortic arch
Ascending aorta Descending aorta
Left coronary
Right coronary artery
artery
Thoracic aorta
Abdominal aorta
|
The aorta has distinct regions, including
the ascending aorta, aortic arch, and the
descending aorta, which includes the
thoracic and abdominal regions.
Components of the Aorta
Vessel Description
Components of the Aorta
Vessel Description
Largest artery in the body, originating from the left
ventricle and descending to the abdominal region,
where it bifurcates into the common iliac arteries at
Aorta .
the level of the fourth lumbar vertebra; arteries
originating from the aorta distribute blood to
virtually all tissues of the body
Ascending Initial portion of the aorta, rising superiorly from the
aorta left ventricle for a distance of approximately 5 cm
Graceful arc to the left that connects the ascending
; aorta to the descending aorta; ends at the
Aortic arch ; ; :
intervertebral disk between the fourth and fifth
thoracic vertebrae
Portion of the aorta that continues inferiorly past the
Descending ; ws ; ;
end of the aortic arch; subdivided into the thoracic
aorta ;
aorta and the abdominal aorta
Thoracic Portion of the descending aorta superior to the aortic
aorta hiatus
Abdominal Portion of the aorta inferior to the aortic hiatus and
aorta superior to the common iliac arteries
Coronary Circulation
The first vessels that branch from the ascending aorta are the paired
coronary arteries (see [link]), which arise from two of the three sinuses in
the ascending aorta just superior to the aortic semilunar valve. These
sinuses contain the aortic baroreceptors and chemoreceptors critical to
maintain cardiac function. The left coronary artery arises from the left
posterior aortic sinus. The right coronary artery arises from the anterior
aortic sinus. Normally, the right posterior aortic sinus does not give rise to a
vessel.
The coronary arteries encircle the heart, forming a ring-like structure that
divides into the next level of branches that supplies blood to the heart
tissues. (Seek additional content for more detail on cardiac circulation.)
Aortic Arch Branches
There are three major branches of the aortic arch: the brachiocephalic
artery, the left common carotid artery, and the left subclavian (literally
“under the clavicle”) artery. As you would expect based upon proximity to
the heart, each of these vessels is classified as an elastic artery.
The brachiocephalic artery is located only on the right side of the body;
there is no corresponding artery on the left. The brachiocephalic artery
branches into the right subclavian artery and the right common carotid
artery. The left subclavian and left common carotid arteries arise
independently from the aortic arch but otherwise follow a similar pattern
and distribution to the corresponding arteries on the right side (see [link]).
Each subclavian artery supplies blood to the arms, chest, shoulders, back,
and central nervous system. It then gives rise to three major branches: the
internal thoracic artery, the vertebral artery, and the thyrocervical artery.
The internal thoracic artery, or mammary artery, supplies blood to the
thymus, the pericardium of the heart, and the anterior chest wall. The
vertebral artery passes through the vertebral foramen in the cervical
vertebrae and then through the foramen magnum into the cranial cavity to
supply blood to the brain and spinal cord. The paired vertebral arteries join
together to form the large basilar artery at the base of the medulla
oblongata. This is an example of an anastomosis. The subclavian artery also
gives rise to the thyrocervical artery that provides blood to the thyroid, the
cervical region of the neck, and the upper back and shoulder.
The common carotid artery divides into internal and external carotid
arteries. The right common carotid artery arises from the brachiocephalic
artery and the left common carotid artery arises directly from the aortic
arch. The external carotid artery supplies blood to numerous structures
within the face, lower jaw, neck, esophagus, and larynx. These branches
include the lingual, facial, occipital, maxillary, and superficial temporal
arteries. The internal carotid artery initially forms an expansion known as
the carotid sinus, containing the carotid baroreceptors and chemoreceptors.
Like their counterparts in the aortic sinuses, the information provided by
these receptors is critical to maintaining cardiovascular homeostasis (see
[link]).
The internal carotid arteries along with the vertebral arteries are the two
primary suppliers of blood to the human brain. Given the central role and
vital importance of the brain to life, it is critical that blood supply to this
organ remains uninterrupted. Recall that blood flow to the brain is
remarkably constant, with approximately 20 percent of blood flow directed
to this organ at any given time. When blood flow is interrupted, even for
just a few seconds, a transient ischemic attack (TIA), or mini-stroke, may
occur, resulting in loss of consciousness or temporary loss of neurological
function. In some cases, the damage may be permanent. Loss of blood flow
for longer periods, typically between 3 and 4 minutes, will likely produce
irreversible brain damage or a stroke, also called a cerebrovascular
accident (CVA). The locations of the arteries in the brain not only provide
blood flow to the brain tissue but also prevent interruption in the flow of
blood. Both the carotid and vertebral arteries branch once they enter the
cranial cavity, and some of these branches form a structure known as the
arterial circle (or circle of Willis), an anastomosis that is remarkably like a
traffic circle that sends off branches (in this case, arterial branches to the
brain). As a rule, branches to the anterior portion of the cerebrum are
normally fed by the internal carotid arteries; the remainder of the brain
receives blood flow from branches associated with the vertebral arteries.
The internal carotid artery continues through the carotid canal of the
temporal bone and enters the base of the brain through the carotid foramen
where it gives rise to several branches ([link] and [link]). One of these
branches is the anterior cerebral artery that supplies blood to the frontal
lobe of the cerebrum. Another branch, the middle cerebral artery, supplies
blood to the temporal and parietal lobes, which are the most common sites
of CVAs. The ophthalmic artery, the third major branch, provides blood to
the eyes.
The right and left anterior cerebral arteries join together to form an
anastomosis called the anterior communicating artery. The initial
segments of the anterior cerebral arteries and the anterior communicating
artery form the anterior portion of the arterial circle. The posterior portion
of the arterial circle is formed by a left and a right posterior
communicating artery that branches from the posterior cerebral artery,
which arises from the basilar artery. It provides blood to the posterior
portion of the cerebrum and brain stem. The basilar artery is an
anastomosis that begins at the junction of the two vertebral arteries and
sends branches to the cerebellum and brain stem. It flows into the posterior
cerebral arteries. [link] summarizes the aortic arch branches, including the
major branches supplying the brain.
Arteries Supplying the Head and Neck
Superficial temporal
Maxillary
Occipital \\
Internal carotid \ Ss
CO) \ Facial
Carotid sinus ,
Lingual
Vertebral
External carotid
The common carotid artery gives rise to the
external and internal carotid arteries. The
external carotid artery remains superficial and
gives rise to many arteries of the head. The
internal carotid artery first forms the carotid
sinus and then reaches the brain via the carotid
canal and carotid foramen, emerging into the
cranium via the foramen lacerum. The
vertebral artery branches from the subclavian
artery and passes through the transverse
foramen in the cervical vertebrae, entering the
base of the skull at the vertebral foramen. The
subclavian artery continues toward the arm as
the axillary artery.
Arteries Serving the Brain
Middle cerebral
Ophthalmic
Internal carotid
Basilar
Vertebral
Anterior communicating
Anterior cerebral
Posterior communicating
Posterior cerebral
This inferior view shows the network of arteries
serving the brain. The structure is referred to as the
arterial circle or circle of Willis.
Aortic Arch Branches and Brain Circulation
Vessel Description
Single vessel located on the right side of the
body; the first vessel branching from the aortic
Brachiocephalic arch; gives rise to the right subclavian artery and
artery the right common carotid artery; supplies blood
to the head, neck, upper limb, and wall of the
thoracic region
Aortic Arch Branches and Brain Circulation
Vessel
Subclavian
artery
Internal
thoracic artery
Vertebral artery
Thyrocervical
artery
Common
carotid artery
External carotid
artery
Description
The right subclavian artery arises from the
brachiocephalic artery while the left subclavian
artery arises from the aortic arch; gives rise to
the internal thoracic, vertebral, and thyrocervical
arteries; supplies blood to the arms, chest,
shoulders, back, and central nervous system
Also called the mammary artery; arises from the
subclavian artery; supplies blood to the thymus,
pericardium of the heart, and anterior chest wall
Arises from the subclavian artery and passes
through the vertebral foramen through the
foramen magnum to the brain; joins with the
internal carotid artery to form the arterial circle;
supplies blood to the brain and spinal cord
Arises from the subclavian artery; supplies
blood to the thyroid, the cervical region, the
upper back, and shoulder
The right common carotid artery arises from the
brachiocephalic artery and the left common
carotid artery arises from the aortic arch; each
gives rise to the extemal and internal carotid
arteries; supplies the respective sides of the head
and neck
Arises from the common carotid artery; supplies
blood to numerous structures within the face,
lower jaw, neck, esophagus, and larynx
Aortic Arch Branches and Brain Circulation
Vessel
Internal carotid
artery
Arterial circle
or circle of
Willis
Anterior
cerebral artery
Middle cerebral
artery
Ophthalmic
artery
Anterior
communicating
artery
Posterior
communicating
artery
Description
Arises from the common carotid artery and
begins with the carotid sinus; goes through the
carotid canal of the temporal bone to the base of
the brain; combines with the branches of the
vertebral artery, forming the arterial circle;
supplies blood to the brain
An anastomosis located at the base of the brain
that ensures continual blood supply; formed
from the branches of the internal carotid and
vertebral arteries; supplies blood to the brain
Arises from the internal carotid artery; supplies
blood to the frontal lobe of the cerebrum
Another branch of the internal carotid artery;
supplies blood to the temporal and parietal lobes
of the cerebrum
Branch of the internal carotid artery; supplies
blood to the eyes
An anastomosis of the right and left internal
carotid arteries; supplies blood to the brain
Branches of the posterior cerebral artery that
form part of the posterior portion of the arterial
circle; supplies blood to the brain
Aortic Arch Branches and Brain Circulation
Vessel Description
Branch of the basilar artery that forms a portion
Posterior of the posterior segment of the arterial circle of
cerebral artery Willis; supplies blood to the posterior portion of
the cerebrum and brain stem
Formed from the fusion of the two vertebral
arteries; sends branches to the cerebellum, brain
stem, and the posterior cerebral arteries; the
main blood supply to the brain stem
Basilar artery
Thoracic Aorta and Major Branches
The thoracic aorta begins at the level of vertebra T5 and continues through
to the diaphragm at the level of T12, initially traveling within the
mediastinum to the left of the vertebral column. As it passes through the
thoracic region, the thoracic aorta gives rise to several branches, which are
collectively referred to as visceral branches and parietal branches ((link]).
Those branches that supply blood primarily to visceral organs are known as
the visceral branches and include the bronchial arteries, pericardial
arteries, esophageal arteries, and the mediastinal arteries, each named after
the tissues it supplies. Each bronchial artery (typically two on the left and
one on the right) supplies systemic blood to the lungs and visceral pleura, in
addition to the blood pumped to the lungs for oxygenation via the
pulmonary circuit. The bronchial arteries follow the same path as the
respiratory branches, beginning with the bronchi and ending with the
bronchioles. There is considerable, but not total, intermingling of the
systemic and pulmonary blood at anastomoses in the smaller branches of
the lungs. This may sound incongruous—that is, the mixing of systemic
arterial blood high in oxygen with the pulmonary arterial blood lower in
oxygen—but the systemic vessels also deliver nutrients to the lung tissue
just as they do elsewhere in the body. The mixed blood drains into typical
pulmonary veins, whereas the bronchial artery branches remain separate
and drain into bronchial veins described later. Each pericardial artery
supplies blood to the pericardium, the esophageal artery provides blood to
the esophagus, and the mediastinal artery provides blood to the
mediastinum. The remaining thoracic aorta branches are collectively
referred to as parietal branches or somatic branches, and include the
intercostal and superior phrenic arteries. Each intercostal artery provides
blood to the muscles of the thoracic cavity and vertebral column. The
superior phrenic artery provides blood to the superior surface of the
diaphragm. [link] lists the arteries of the thoracic region.
Arteries of the Thoracic and Abdominal Regions
ici isn : a
nterna a = |
Aortic arch
Visceral branches of
the thoracic aorta
Parietal (somatic) J fi Bronchial
branches of L \ ; <>) — Esophageal
thoracic aorta L =~ D> L Mediastinal
= - 4 Ba __ > —__ aw A
intercostal a , Sa Saewa es Pericardial
Superior phrenic
af
|
= XN fl Aortic hiatus
Inferior phrenic > = y
Diaoh SS Celiac trunk
iaphragm —__ 7 x a Yh 7 Left gastric
Adrenal me Splenic
; = . aS \ ‘:
Renal i SJ — Common hepatic
Gonadal : ;
Superior mesenteric
Lumbar ————__4#*_———————— SS
/ A Abdominal aorta
Median sacral ————_——______*._ =a
Common iliac [ = Inferior mesenteric
Internal iliac
en iliac
The thoracic aorta gives rise to the arteries of the
visceral and parietal branches.
Arteries of the Thoracic Region
Vessel
Visceral
branches
Bronchial
artery
Pericardial
artery
Esophageal
artery
Mediastinal
artery
Parietal
branches
Intercostal
artery
Superior
phrenic
artery
Description
A group of arterial branches of the thoracic aorta;
supplies blood to the viscera (i.e., organs) of the
thorax
Systemic branch from the aorta that provides
oxygenated blood to the lungs; this blood supply is
in addition to the pulmonary circuit that brings blood
for oxygenation
Branch of the thoracic aorta; supplies blood to the
pericardium
Branch of the thoracic aorta; supplies blood to the
esophagus
Branch of the thoracic aorta; supplies blood to the
mediastinum
Also called somatic branches, a group of arterial
branches of the thoracic aorta; include those that
supply blood to the thoracic wall, vertebral column,
and the superior surface of the diaphragm
Branch of the thoracic aorta; supplies blood to the
muscles of the thoracic cavity and vertebral column
Branch of the thoracic aorta; supplies blood to the
superior surface of the diaphragm
Abdominal Aorta and Major Branches
After crossing through the diaphragm at the aortic hiatus, the thoracic aorta
is called the abdominal aorta (see [link]). This vessel remains to the left of
the vertebral column and is embedded in adipose tissue behind the
peritoneal cavity. It formally ends at approximately the level of vertebra L4,
where it bifurcates to form the common iliac arteries. Before this division,
the abdominal aorta gives rise to several important branches. A single
celiac trunk (artery) emerges and divides into the left gastric artery to
supply blood to the stomach and esophagus, the splenic artery to supply
blood to the spleen, and the common hepatic artery, which in turn gives
rise to the hepatic artery proper to supply blood to the liver, the right
gastric artery to supply blood to the stomach, the cystic artery to supply
blood to the gall bladder, and several branches, one to supply blood to the
duodenum and another to supply blood to the pancreas. Two additional
single vessels arise from the abdominal aorta. These are the superior and
inferior mesenteric arteries. The superior mesenteric artery arises
approximately 2.5 cm after the celiac trunk and branches into several major
vessels that supply blood to the small intestine (duodenum, jejunum, and
ileum), the pancreas, and a majority of the large intestine. The inferior
mesenteric artery supplies blood to the distal segment of the large
intestine, including the rectum. It arises approximately 5 cm superior to the
common iliac arteries.
In addition to these single branches, the abdominal aorta gives rise to
several significant paired arteries along the way. These include the inferior
phrenic arteries, the adrenal arteries, the renal arteries, the gonadal arteries,
and the lumbar arteries. Each inferior phrenic artery is a counterpart of a
superior phrenic artery and supplies blood to the inferior surface of the
diaphragm. The adrenal artery supplies blood to the adrenal (suprarenal)
glands and arises near the superior mesenteric artery. Each renal artery
branches approximately 2.5 cm inferior to the superior mesenteric arteries
and supplies a kidney. The right renal artery is longer than the left since the
aorta lies to the left of the vertebral column and the vessel must travel a
greater distance to reach its target. Renal arteries branch repeatedly to
supply blood to the kidneys. Each gonadal artery supplies blood to the
gonads, or reproductive organs, and is also described as either an ovarian
artery or a testicular artery (internal spermatic), depending upon the sex of
the individual. An ovarian artery supplies blood to an ovary, uterine
(Fallopian) tube, and the uterus, and is located within the suspensory
ligament of the uterus. It is considerably shorter than a testicular artery,
which ultimately travels outside the body cavity to the testes, forming one
component of the spermatic cord. The gonadal arteries arise inferior to the
renal arteries and are generally retroperitoneal. The ovarian artery continues
to the uterus where it forms an anastomosis with the uterine artery that
supplies blood to the uterus. Both the uterine arteries and vaginal arteries,
which distribute blood to the vagina, are branches of the internal iliac artery.
The four paired lumbar arteries are the counterparts of the intercostal
arteries and supply blood to the lumbar region, the abdominal wall, and the
spinal cord. In some instances, a fifth pair of lumbar arteries emerges from
the median sacral artery.
The aorta divides at approximately the level of vertebra L4 into a left and a
right common iliac artery but continues as a small vessel, the median
sacral artery, into the sacrum. The common iliac arteries provide blood to
the pelvic region and ultimately to the lower limbs. They split into external
and internal iliac arteries approximately at the level of the lumbar-sacral
articulation. Each internal iliac artery sends branches to the urinary
bladder, the walls of the pelvis, the external genitalia, and the medial
portion of the femoral region. In females, they also provide blood to the
uterus and vagina. The much larger external iliac artery supplies blood to
each of the lower limbs. [link] shows the distribution of the major branches
of the aorta into the thoracic and abdominal regions. [link] shows the
distribution of the major branches of the common iliac arteries. [link]
summarizes the major branches of the abdominal aorta.
Major Branches of the Aorta
UNPAIRED E
Thoracic
aorta
Air passages of
Bronchial respiratory tract, lung
arteries tissue
PAIRED
Vertebrae, spinal
Pericardial Pericardium Intercostal cord, back muscles,
arteries arteries body wall, skin
Vertebrae, spinal
Esophageal Esophagus I'elilelmdictiiem! Cord, back muscles,
arteries arteries body wall, skin,
diaphragm
Mediastinal Mediastinal structures
arteries
Abdominal
aorta
Stomach,
SiMe r-T ce adjacent ities lalg-tltemg) Diaphragm, inferior
artery portion of arteries portion of esophagus
esophagus
Spleen,
Splenic stomach, Adrenal Adrenal glands
artery pancreas arteries
Liver, stomach,
gallbladder, Renal Kidneys
duodenum, arteries
pancreas
Common
hepatic
artery
Pancreas, small
Superior intestine, appendix, Gonadal Testes or ovaries
Hitt Cdlerlaciall first two-thirds of arteries
large intestine
Vetebrae, spinal cord,
abdominal wall,
lumbar region
Inferior Last third of large
mesenteric artery Bintsun-)
Common iliac
arteries
Median
sacral
artery
The flow chart summarizes the distribution of the
major branches of the aorta into the thoracic and
abdominal regions.
Major Branches of the Iliac Arteries
Abdominal
aorta
Left common
iliac
(follows pattern
similar to right
common iliac)
Right common Pelvis and right lower
iliac limb
Pelvic muscles, skin,
Right lower (Uaifeln ata) tel aiiiceacciam viscera of pelvis,
iliac perineum, gluteal
region, medial thigh
Superior
gluteal Hip muscles, hip joint
Rectum, anus,
Internal perineal muscles,
pudendal external genitalia,
lateral rotators of hip
Ilium, hip and thigh
Obturator muscles, hip joint,
femoral head
Lateral Skin and muscles of
sacral sacrum
The flow chart summarizes the distribution of the
major branches of the common iliac arteries into the
pelvis and lower limbs. The left side follows a similar
pattern to the right.
Vessels of the Abdominal Aorta
Vessel Description
Vessels of the Abdominal Aorta
Vessel
Celiac
trunk
Left
gastric
artery
Splenic
artery
Common
hepatic
artery
Hepatic
artery
proper
Right
gastric
artery
Cystic
artery
Superior
mesenteric
artery
Description
Also called the celiac artery; a major branch of the
abdominal aorta; gives rise to the left gastric artery,
the splenic artery, and the common hepatic artery that
forms the hepatic artery to the liver, the right gastric
artery to the stomach, and the cystic artery to the gall
bladder
Branch of the celiac trunk; supplies blood to the
stomach
Branch of the celiac trunk; supplies blood to the
spleen
Branch of the celiac trunk that forms the hepatic
artery, the right gastric artery, and the cystic artery
Branch of the common hepatic artery; supplies
systemic blood to the liver
Branch of the common hepatic artery; supplies blood
to the stomach
Branch of the common hepatic artery; supplies blood
to the gall bladder
Branch of the abdominal aorta; supplies blood to the
small intestine (duodenum, jejunum, and ileum), the
pancreas, and a majority of the large intestine
Vessels of the Abdominal Aorta
Vessel
Inferior
mesenteric
artery
Inferior
phrenic
arteries
Adrenal
artery
Renal
artery
Gonadal
artery
Ovarian
artery
Testicular
artery
Lumbar
arteries
Common
iliac artery
Description
Branch of the abdominal aorta; supplies blood to the
distal segment of the large intestine and rectum
Branches of the abdominal aorta; supply blood to the
inferior surface of the diaphragm
Branch of the abdominal aorta; supplies blood to the
adrenal (suprarenal) glands
Branch of the abdominal aorta; supplies each kidney
Branch of the abdominal aorta; supplies blood to the
gonads or reproductive organs; also described as
Ovarian arteries or testicular arteries, depending upon
the sex of the individual
Branch of the abdominal aorta; supplies blood to
ovary, uterine (Fallopian) tube, and uterus
Branch of the abdominal aorta; ultimately travels
outside the body cavity to the testes and forms one
component of the spermatic cord
Branches of the abdominal aorta; supply blood to the
lumbar region, the abdominal wall, and spinal cord
Branch of the aorta that leads to the internal and
external iliac arteries
Vessels of the Abdominal Aorta
Vessel Description
Median
sacral Continuation of the aorta into the sacrum
artery
Branch from the common iliac arteries; supplies
blood to the urinary bladder, walls of the pelvis,
extemal genitalia, and the medial portion of the
femoral region; in females, also provides blood to the
uterus and vagina
Internal
iliac artery
Branch of the common iliac artery that leaves the
body cavity and becomes a femoral artery; supplies
blood to the lower limbs
External
iliac artery
Arteries Serving the Upper Limbs
As the subclavian artery exits the thorax into the axillary region, it is
renamed the axillary artery. Although it does branch and supply blood to
the region near the head of the humerus (via the humeral circumflex
arteries), the majority of the vessel continues into the upper arm, or
brachium, and becomes the brachial artery ({link]). The brachial artery
supplies blood to much of the brachial region and divides at the elbow into
several smaller branches, including the deep brachial arteries, which
provide blood to the posterior surface of the arm, and the ulnar collateral
arteries, which supply blood to the region of the elbow. As the brachial
artery approaches the coronoid fossa, it bifurcates into the radial and ulnar
arteries, which continue into the forearm, or antebrachium. The radial
artery and ulnar artery parallel their namesake bones, giving off smaller
branches until they reach the wrist, or carpal region. At this level, they fuse
to form the superficial and deep palmar arches that supply blood to the
hand, as well as the digital arteries that supply blood to the digits. [link]
shows the distribution of systemic arteries from the heart into the upper
limb. [link] summarizes the arteries serving the upper limbs.
Major Arteries Serving the Thorax and Upper Limb
Right subclavian
Axillary i ——
Y ” ea
Humeral circumflex —~L Ae
be
| i ——
Deep brachial > TF
Brachial ||
Ulnar collateral Tf] PS
ll WA
Radial ny ee
Anterior crural
interosseous
Ulnar
Deep palmar arch
Superficial palmar
[~ arch
| [; Digital
The arteries that supply blood
to the arms and hands are
extensions of the subclavian
arteries.
Major Arteries of the Upper Limb
Spinal cord, cervical F
vertebrae, fuses with left ipifelnis ight Het
vertebral to form basilar EWS sé]
artery in cranium
Left
vertebral
common common
carotid carotid
Muscles, tissues A
. 7 ween Right - 5 ; Left
skin of neck, thyroid = Right Brachiocephalic Left A
gland, shoulders, diesen ASS ETEY anal mUbclavian pial
upper back
Right Skin and muscles of
pectoral Right chest and abdomen, ae Ap Aortic eile al
and axilla JEYCIETS mammary gland, thoracic arch craareetete
muscles pericardium
Left
axillary
Structures
of the arm (isttliis Left
and elbow MverHie|| brachial
Thoracic
Left aorta
ventricle
Radial Ulnar
side of side of
forearm forearm
Digital Digital
arteries arteries
The flow chart summarizes the distribution of the major
arteries from the heart into the upper limb.
Arteries Serving the Upper Limbs
Description
Arteries Serving the Upper Limbs
Vessel
Axillary
artery
Brachial
artery
Radial
artery
Ulnar
artery
Palmar
arches
(superficial
and deep)
Description
Continuation of the subclavian artery as it penetrates
the body wall and enters the axillary region; supplies
blood to the region near the head of the humerus
(humeral circumflex arteries); the majority of the
vessel continues into the brachium and becomes the
brachial artery
Continuation of the axillary artery in the brachium;
supplies blood to much of the brachial region; gives
off several smaller branches that provide blood to the
posterior surface of the arm in the region of the
elbow; bifurcates into the radial and ulnar arteries at
the coronoid fossa
Formed at the bifurcation of the brachial artery;
parallels the radius; gives off smaller branches until
it reaches the carpal region where it fuses with the
ulnar artery to form the superficial and deep palmar
arches; supplies blood to the lower arm and carpal
region
Formed at the bifurcation of the brachial artery;
parallels the ulna; gives off smaller branches until it
reaches the carpal region where it fuses with the
radial artery to form the superficial and deep palmar
arches; supplies blood to the lower arm and carpal
region
Formed from anastomosis of the radial and ulnar
arteries; supply blood to the hand and digital arteries
Arteries Serving the Upper Limbs
Vessel Description
Digital Formed from the superficial and deep palmar arches;
arteries supply blood to the digits
Arteries Serving the Lower Limbs
The external iliac artery exits the body cavity and enters the femoral region
of the lower leg ({link]). As it passes through the body wall, it is renamed
the femoral artery. It gives off several smaller branches as well as the
lateral deep femoral artery that in turn gives rise to a lateral circumflex
artery. These arteries supply blood to the deep muscles of the thigh as well
as ventral and lateral regions of the integument. The femoral artery also
gives rise to the genicular artery, which provides blood to the region of the
knee. As the femoral artery passes posterior to the knee near the popliteal
fossa, it is called the popliteal artery. The popliteal artery branches into the
anterior and posterior tibial arteries.
The anterior tibial artery is located between the tibia and fibula, and
supplies blood to the muscles and integument of the anterior tibial region.
Upon reaching the tarsal region, it becomes the dorsalis pedis artery,
which branches repeatedly and provides blood to the tarsal and dorsal
regions of the foot. The posterior tibial artery provides blood to the
muscles and integument on the posterior surface of the tibial region. The
fibular or peroneal artery branches from the posterior tibial artery. It
bifurcates and becomes the medial plantar artery and lateral plantar
artery, providing blood to the plantar surfaces. There is an anastomosis
with the dorsalis pedis artery, and the medial and lateral plantar arteries
form two arches called the dorsal arch (also called the arcuate arch) and
the plantar arch, which provide blood to the remainder of the foot and
toes. [link] shows the distribution of the major systemic arteries in the lower
limb. [link] summarizes the major systemic arteries discussed in the text.
Major Arteries Serving the Lower Limb
External iliac ld = Common iliac
1
Internal iliac
Inguinal ligament Lateral sacral
Internal pudendal
Deep femoral Suiaise
Lateral femoral
circumflex
Femoral
Genicular
Popliteal
Dorsalis pedis
Dorsal arch
Anterior view
Right external
iliac
Deep femoral
Lateral femoral
circumflex
Femoral
Genicular
Popliteal
Peroneal
Anterior tibial
Posterior
tibial
Fibular
Lateral plantar
Medial plantar
Plantar arch
Posterior view
Major arteries serving the lower limb are shown in
anterior and posterior views.
Systemic Arteries of the Lower Limb
External iliac
Hip joint,
femoral head, Deep
Femoral Thigh
deep thigh femoral
muscles
Decending Knee joint,
genicular skin of leg
Adductor Median Lateral .
muscles, femoral femoral Quadriceps Popliteal Leg and
obturator circumflex circumflex muscles foot
muscles, hip joint
Anterior Posterior f
tibial tibial AlSOEL
Dorsalis pedis,
dorsal arch, Peroneal
plantar arch
Dorsal ‘
metatarsal, Distal foot,
dorsal digital [imsaed
The flow chart summarizes the distribution of the systemic
arteries from the external iliac artery into the lower limb.
Arteries Serving the Lower Limbs
Vessel Description
Arteries Serving the Lower Limbs
Vessel
Femoral
artery
Deep
femoral
artery
Lateral
circumflex
artery
Genicular
artery
Popliteal
artery
Anterior
tibial
artery
Dorsalis
pedis
artery
Posterior
tibial
artery
Description
Continuation of the external iliac artery after it passes
through the body cavity; divides into several smaller
branches, the lateral deep femoral artery, and the
genicular artery; becomes the popliteal artery as it
passes posterior to the knee
Branch of the femoral artery; gives rise to the lateral
circumflex arteries
Branch of the deep femoral artery; supplies blood to
the deep muscles of the thigh and the ventral and
lateral regions of the integument
Branch of the femoral artery; supplies blood to the
region of the knee
Continuation of the femoral artery posterior to the
knee; branches into the anterior and posterior tibial
arteries
Branches from the popliteal artery; supplies blood to
the anterior tibial region; becomes the dorsalis pedis
artery
Forms from the anterior tibial artery; branches
repeatedly to supply blood to the tarsal and dorsal
regions of the foot
Branches from the popliteal artery and gives rise to
the fibular or peroneal artery; supplies blood to the
posterior tibial region
Arteries Serving the Lower Limbs
Vessel Description
Medial Arises from the bifurcation of the posterior tibial
plantar arteries; supplies blood to the medial plantar surfaces
artery of the foot
Lateral Arises from the bifurcation of the posterior tibial
plantar arteries; supplies blood to the lateral plantar surfaces
artery of the foot
Dorsal or Formed from the anastomosis of the dorsalis pedis
arcuate artery and the medial and plantar arteries; branches
arch supply the distal portions of the foot and digits
Formed from the anastomosis of the dorsalis pedis
Plantar : ;
aah artery and the medial and plantar arteries; branches
supply the distal portions of the foot and digits
Overview of Systemic Veins
Systemic veins return blood to the right atrium. Since the blood has already
passed through the systemic capillaries, it will be relatively low in oxygen
concentration. In many cases, there will be veins draining organs and
regions of the body with the same name as the arteries that supplied these
regions and the two often parallel one another. This is often described as a
“complementary” pattern. However, there is a great deal more variability in
the venous circulation than normally occurs in the arteries. For the sake of
brevity and clarity, this text will discuss only the most commonly
encountered patterns. However, keep this variation in mind when you move
from the classroom to clinical practice.
In both the neck and limb regions, there are often both superficial and
deeper levels of veins. The deeper veins generally correspond to the
complementary arteries. The superficial veins do not normally have direct
arterial counterparts, but in addition to returning blood, they also make
contributions to the maintenance of body temperature. When the ambient
temperature is warm, more blood is diverted to the superficial veins where
heat can be more easily dissipated to the environment. In colder weather,
there is more constriction of the superficial veins and blood is diverted
deeper where the body can retain more of the heat.
The “Voyage of Discovery” analogy and stick drawings mentioned earlier
remain valid techniques for the study of systemic veins, but veins present a
more difficult challenge because there are numerous anastomoses and
multiple branches. It is like following a river with many tributaries and
channels, several of which interconnect. Tracing blood flow through
arteries follows the current in the direction of blood flow, so that we move
from the heart through the large arteries and into the smaller arteries to the
capillaries. From the capillaries, we move into the smallest veins and follow
the direction of blood flow into larger veins and back to the heart. [link]
outlines the path of the major systemic veins.
Note:
Visit this site for a brief online summary of the veins.
Major Systemic Veins of the Body
External jugular
Subclavian <=
fi ay >
Axillary ee }
Cephalic ——————_f/_// |
Brachial T \ / J
Basilic I) ST
Hepatic if | A
Median cubital
Radial
Median antebrachial
Ulnar
Palmar venous
arches
Digital
vA y
AK
Popliteal
Small saphenous ——y ||)
Fibular
Plantar venous arch
Dorsal venous arch
Internal jugular
Brachiocephalic
Superior vena cava
Intercostal
Inferior vena cava
Saas . imvy Renal
, aa. Gonadal
Tt ..— Lumbar
Right and left
common iliac
External iliac
Internal iliac
Posterior tibial
Anterior tibial
The major systemic veins of the body are shown
here in an anterior view.
The right atrium receives all of the systemic venous return. Most of the
blood flows into either the superior vena cava or inferior vena cava. If you
draw an imaginary line at the level of the diaphragm, systemic venous
circulation from above that line will generally flow into the superior vena
cava; this includes blood from the head, neck, chest, shoulders, and upper
limbs. The exception to this is that most venous blood flow from the
coronary veins flows directly into the coronary sinus and from there directly
into the right atrium. Beneath the diaphragm, systemic venous flow enters
the inferior vena cava, that is, blood from the abdominal and pelvic regions
and the lower limbs.
The Superior Vena Cava
The superior vena cava drains most of the body superior to the diaphragm
({link]). On both the left and right sides, the subclavian vein forms when
the axillary vein passes through the body wall from the axillary region. It
fuses with the external and internal jugular veins from the head and neck to
form the brachiocephalic vein. Each vertebral vein also flows into the
brachiocephalic vein close to this fusion. These veins arise from the base of
the brain and the cervical region of the spinal cord, and flow largely through
the intervertebral foramina in the cervical vertebrae. They are the
counterparts of the vertebral arteries. Each internal thoracic vein, also
known as an internal mammary vein, drains the anterior surface of the chest
wall and flows into the brachiocephalic vein.
The remainder of the blood supply from the thorax drains into the azygos
vein. Each intercostal vein drains muscles of the thoracic wall, each
esophageal vein delivers blood from the inferior portions of the esophagus,
each bronchial vein drains the systemic circulation from the lungs, and
several smaller veins drain the mediastinal region. Bronchial veins carry
approximately 13 percent of the blood that flows into the bronchial arteries;
the remainder intermingles with the pulmonary circulation and returns to
the heart via the pulmonary veins. These veins flow into the azygos vein,
and with the smaller hemiazygos vein (hemi- = “half”) on the left of the
vertebral column, drain blood from the thoracic region. The hemiazygos
vein does not drain directly into the superior vena cava but enters the
brachiocephalic vein via the superior intercostal vein.
The azygos vein passes through the diaphragm from the thoracic cavity on
the right side of the vertebral column and begins in the lumbar region of the
thoracic cavity. It flows into the superior vena cava at approximately the
level of T2, making a significant contribution to the flow of blood. It
combines with the two large left and right brachiocephalic veins to form the
superior vena Cava.
[link] summarizes the veins of the thoracic region that flow into the
superior vena Cava.
Veins of the Thoracic and Abdominal Regions
Vertebral
Internal jugular
Superior .
vena cava External jugular
Subclavian
iad Brachiocephalic
Axillary
Esophageal Bsonaie
Internal
werdes Hemiazygos
Azygos
Intercostal
Hepatic
Inferior vena cava
Renal Phrenic
Gonadal
Adrenal
Lumbar
Common iliac
Internal iliac
External iliac
Veins of the thoracic and abdominal regions drain
blood from the area above the diaphragm, returning it
to the right atrium via the superior vena cava.
Veins of the Thoracic Region
Vessel
Superior vena
cava
Subclavian vein
Brachiocephalic
veins
Vertebral vein
Description
Large systemic vein; drains blood from most
areas superior to the diaphragm; empties into the
right atrium
Located deep in the thoracic cavity; formed by
the axillary vein as it enters the thoracic cavity
from the axillary region; drains the axillary and
smaller local veins near the scapular region and
leads to the brachiocephalic vein
Pair of veins that form from a fusion of the
external and internal jugular veins and the
subclavian vein; subclavian, external and
internal jugulars, vertebral, and internal thoracic
veins flow into it; drain the upper thoracic
region and lead to the superior vena cava
Arises from the base of the brain and the
cervical region of the spinal cord; passes
through the intervertebral foramina in the
cervical vertebrae; drains smaller veins from the
cranium, spinal cord, and vertebrae, and leads to
the brachiocephalic vein; counterpart of the
vertebral artery
Veins of the Thoracic Region
Vessel Description
Also called internal mammary veins; drain the
anterior surface of the chest wall and lead to the
brachiocephalic vein
Internal
thoracic veins
Drains the muscles of the thoracic wall and
Intercostal vein
leads to the azygos vein
Esophageal Drains the inferior portions of the esophagus
vein and leads to the azygos vein
Drains the systemic circulation from the lungs
Bronchial vein .
and leads to the azygos vein
Originates in the lumbar region and passes
through the diaphragm into the thoracic cavity
on the right side of the vertebral column; drains
AZzygos vein blood from the intercostal veins, esophageal
veins, bronchial veins, and other veins draining
the mediastinal region, and leads to the superior
vena cava
Smaller vein complementary to the azygos vein;
drains the esophageal veins from the esophagus
and the left intercostal veins, and leads to the
brachiocephalic vein via the superior intercostal
vein
Hemiazygos
vein
Veins of the Head and Neck
Blood from the brain and the superficial facial vein flow into each internal
jugular vein ({link]). Blood from the more superficial portions of the head,
scalp, and cranial regions, including the temporal vein and maxillary vein,
flow into each external jugular vein. Although the external and internal
jugular veins are separate vessels, there are anastomoses between them
close to the thoracic region. Blood from the external jugular vein empties
into the subclavian vein. [link] summarizes the major veins of the head and
neck.
Major Veins of the Head and Neck
Vessel Description
Parallel to the common carotid artery, which is more or
Internal less its counterpart, and passes through the jugular
jugular foramen and canal; primarily drains blood from the
vein brain, receives the superficial facial vein, and empties
into the subclavian vein
Temporal Drains blood from the temporal region and flows into
vein the external jugular vein
Maxillary Drains blood from the maxillary region and flows into
vein the external jugular vein
External Drains blood from the more superficial portions of the
jugular head, scalp, and cranial regions, and leads to the
vein subclavian vein
Venous Drainage of the Brain
Circulation to the brain is both critical and complex (see [link]). Many
smaller veins of the brain stem and the superficial veins of the cerebrum
lead to larger vessels referred to as intracranial sinuses. These include the
superior and inferior sagittal sinuses, straight sinus, cavernous sinuses, left
and right sinuses, the petrosal sinuses, and the occipital sinuses. Ultimately,
sinuses will lead back to either the inferior jugular vein or vertebral vein.
Most of the veins on the superior surface of the cerebrum flow into the
largest of the sinuses, the superior sagittal sinus. It is located midsagittally
between the meningeal and periosteal layers of the dura mater within the
falx cerebri and, at first glance in images or models, can be mistaken for the
subarachnoid space. Most reabsorption of cerebrospinal fluid occurs via the
chorionic villi (arachnoid granulations) into the superior sagittal sinus.
Blood from most of the smaller vessels originating from the inferior
cerebral veins flows into the great cerebral vein and into the straight
sinus. Other cerebral veins and those from the eye socket flow into the
cavernous sinus, which flows into the petrosal sinus and then into the
internal jugular vein. The occipital sinus, sagittal sinus, and straight sinuses
all flow into the left and right transverse sinuses near the lambdoid suture.
The transverse sinuses in turn flow into the sigmoid sinuses that pass
through the jugular foramen and into the internal jugular vein. The internal
jugular vein flows parallel to the common carotid artery and is more or less
its counterpart. It empties into the brachiocephalic vein. The veins draining
the cervical vertebrae and the posterior surface of the skull, including some
blood from the occipital sinus, flow into the vertebral veins. These parallel
the vertebral arteries and travel through the transverse foramina of the
cervical vertebrae. The vertebral veins also flow into the brachiocephalic
veins. [link] summarizes the major veins of the brain.
Veins of the Head and Neck
Superior sagittal sinus
Inferior sagittal sinus
Straight sinus
Occipital
Temporal
Cavernous sinus
Right transverse sinus
Maxillary
Occipital sinus
Facial
Sigmoid sinus
Petrosal sinus
External jugular
Internal jugular
Vertebral
Right subclavian
Axillary
Superior vena cava
This left lateral view shows the veins of the head
and neck, including the intercranial sinuses.
Major Veins of the Brain
Vessel Description
Major Veins of the Brain
Vessel
Superior
Sagittal
sinus
Great
cerebral
vein
Straight
sinus
Cavernous
sinus
Petrosal
sinus
Occipital
sinus
Transverse
sinuses
Description
Enlarged vein located midsagittally between the
meningeal and periosteal layers of the dura mater
within the falx cerebri; receives most of the blood
drained from the superior surface of the cerebrum and
leads to the inferior jugular vein and the vertebral
vein
Receives most of the smaller vessels from the inferior
cerebral veins and leads to the straight sinus
Enlarged vein that drains blood from the brain;
receives most of the blood from the great cerebral
vein and leads to the left or right transverse sinus
Enlarged vein that receives blood from most of the
other cerebral veins and the eye socket, and leads to
the petrosal sinus
Enlarged vein that receives blood from the cavernous
sinus and leads into the internal jugular veins
Enlarged vein that drains the occipital region near the
falx cerebelli and leads to the left and right transverse
sinuses, and also the vertebral veins
Pair of enlarged veins near the lambdoid suture that
drains the occipital, sagittal, and straight sinuses, and
leads to the sigmoid sinuses
Major Veins of the Brain
Vessel Description
Enlarged vein that receives blood from the transverse
sinuses and leads through the jugular foramen to the
internal jugular vein
Sigmoid
sinuses
Veins Draining the Upper Limbs
The digital veins in the fingers come together in the hand to form the
palmar venous arches ((link]). From here, the veins come together to form
the radial vein, the ulnar vein, and the median antebrachial vein. The radial
vein and the ulnar vein parallel the bones of the forearm and join together
at the antebrachium to form the brachial vein, a deep vein that flows into
the axillary vein in the brachium.
The median antebrachial vein parallels the ulnar vein, is more medial in
location, and joins the basilic vein in the forearm. As the basilic vein
reaches the antecubital region, it gives off a branch called the median
cubital vein that crosses at an angle to join the cephalic vein. The median
cubital vein is the most common site for drawing venous blood in humans.
The basilic vein continues through the arm medially and superficially to the
axillary vein.
The cephalic vein begins in the antebrachium and drains blood from the
superficial surface of the arm into the axillary vein. It is extremely
superficial and easily seen along the surface of the biceps brachii muscle in
individuals with good muscle tone and in those without excessive
subcutaneous adipose tissue in the arms.
The subscapular vein drains blood from the subscapular region and joins
the cephalic vein to form the axillary vein. As it passes through the body
wall and enters the thorax, the axillary vein becomes the subclavian vein.
Many of the larger veins of the thoracic and abdominal region and upper
limb are further represented in the flow chart in [link]. [link] summarizes
the veins of the upper limbs.
Veins of the Upper Limb
Subclavian
Axillary
Cephalic
Subscapular
Brachial
Basilic
Median cubital
Cephalic
Radial
Median
antebrachial
Why)
“i
ik
Cae
LAA!
Al
} ay Palmar venous arches
YW
pr)
j Deep veins Uy rd / 7
[i Superficial veins Digital
This anterior view shows the veins that
drain the upper limb.
KEY
Veins Flowing into the Superior Vena Cava
Collects blood
from cranium, |§gile]n3
spinal cord, vertebral
vertebrae
Collects blood ; A
Right Right Collects blood
atl ie bath external cule from cranium,
se Se jugular jugular face, neck
Right Right Left Left
subclavian brachiocephalic brachiocephalic subclavian
F Collects ;
Right upper Right Left
limb RUE internal internal Lee roe
axillary brachial
(see left limb) Ia peaalens thoracic thoracic
thoracic wall
Collects Collects
wacti Suporte blood from (ULES FS) blood from
Mediaeunal Le arm medial ESS (sdiElla) arm lateral
vena cava
surface surface
Right Median cubital,
atrium median
antebrachial
Left
radial
Hemiazygos
Esophageal
Left
intercostal |
Digital
The flow chart summarizes the distribution of the veins
flowing into the superior vena cava.
Veins of the Upper Limbs
Veiselof the UppBekdrmbdon
Vessel
Digital
veins
Palmar
venous
arches
Radial vein
Ulnar vein
Brachial
vein
Median
antebrachial
vein
Basilic vein
Description
Drain the digits and lead to the palmar arches of the
hand and dorsal venous arch of the foot
Drain the hand and digits, and lead to the radial
vein, ulnar veins, and the median antebrachial vein
Vein that parallels the radius and radial artery; arises
from the palmar venous arches and leads to the
brachial vein
Vein that parallels the ulna and ulnar artery; arises
from the palmar venous arches and leads to the
brachial vein
Deeper vein of the arm that forms from the radial
and ulnar veins in the lower arm; leads to the
axillary vein
Vein that parallels the ulnar vein but is more medial
in location; intertwines with the palmar venous
arches; leads to the basilic vein
Superficial vein of the arm that arises from the
median antebrachial vein, intersects with the median
cubital vein, parallels the ulnar vein, and continues
into the upper arm; along with the brachial vein, it
leads to the axillary vein
Veins of the Upper Limbs
Vessel Description
Superficial vessel located in the antecubital region
Median that links the cephalic vein to the basilic vein in the
cubital vein form of a v; a frequent site from which to draw
blood
Cephalic Superficial vessel in the upper arm; leads to the
vein axillary vein
Subscapular Drains blood from the subscapular region and leads
vein to the axillary vein
Axillary The major vein in the axillary region; drains the
vein upper limb and becomes the subclavian vein
The Inferior Vena Cava
Other than the small amount of blood drained by the azygos and
hemiazygos veins, most of the blood inferior to the diaphragm drains into
the inferior vena cava before it is returned to the heart (see [link]). Lying
just beneath the parietal peritoneum in the abdominal cavity, the inferior
vena Cava parallels the abdominal aorta, where it can receive blood from
abdominal veins. The lumbar portions of the abdominal wall and spinal
cord are drained by a series of lumbar veins, usually four on each side. The
ascending lumbar veins drain into either the azygos vein on the right or the
hemiazygos vein on the left, and return to the superior vena cava. The
remaining lumbar veins drain directly into the inferior vena cava.
Blood supply from the kidneys flows into each renal vein, normally the
largest veins entering the inferior vena cava. A number of other, smaller
veins empty into the left renal vein. Each adrenal vein drains the adrenal or
suprarenal glands located immediately superior to the kidneys. The right
adrenal vein enters the inferior vena cava directly, whereas the left adrenal
vein enters the left renal vein.
From the male reproductive organs, each testicular vein flows from the
scrotum, forming a portion of the spermatic cord. Each ovarian vein drains
an ovary in females. Each of these veins is generically called a gonadal
vein. The right gonadal vein empties directly into the inferior vena cava,
and the left gonadal vein empties into the left renal vein.
Each side of the diaphragm drains into a phrenic vein; the right phrenic
vein empties directly into the inferior vena cava, whereas the left phrenic
vein empties into the left renal vein. Blood supply from the liver drains into
each hepatic vein and directly into the inferior vena cava. Since the inferior
vena cava lies primarily to the right of the vertebral column and aorta, the
left renal vein is longer, as are the left phrenic, adrenal, and gonadal veins.
The longer length of the left renal vein makes the left kidney the primary
target of surgeons removing this organ for donation. [link] provides a flow
chart of the veins flowing into the inferior vena cava. [link] summarizes the
major veins of the abdominal region.
Venous Flow into Inferior Vena Cava
Inferior
vena cava
Hepatic
veins
Phrenic
wena Diaphragm
Gonads (testes or Gonadal
ovaries) veins
Adrenal
F Adrenal glands
veins
Spinal cord and body
wall
Kidneys
Right common Left common
iliac iliac
Pelvic muscles, skin,
viscera of pelvis,
perineum, gluteal region
Left internal Left external Rater
iliac iliac limb
Right lower figile aon Cine! Right internal
limb iliac iliac
Lateral
sacral
veins
Superior Internal
gluteal pudendal
veins veins
Obturator
veins
The flow chart summarizes veins that deliver blood to the
inferior vena cava.
Major Veins of the Abdominal Region
Vessel
Inferior
vena
cava
Lumbar
veins
Renal
vein
Adrenal
vein
Testicular
vein
Ovarian
vein
Gonadal
vein
Description
Large systemic vein that drains blood from areas
largely inferior to the diaphragm; empties into the right
atrium
Series of veins that drain the lumbar portion of the
abdominal wall and spinal cord; the ascending lumbar
veins drain into the azygos vein on the right or the
hemiazygos vein on the left; the remaining lumbar
veins drain directly into the inferior vena cava
Largest vein entering the inferior vena cava; drains the
kidneys and flows into the inferior vena cava
Drains the adrenal or suprarenal; the right adrenal vein
enters the inferior vena cava directly and the left
adrenal vein enters the left renal vein
Drains the testes and forms part of the spermatic cord;
the right testicular vein empties directly into the
inferior vena cava and the left testicular vein empties
into the left renal vein
Drains the ovary; the right ovarian vein empties
directly into the inferior vena cava and the left ovarian
vein empties into the left renal vein
Generic term for a vein draining a reproductive organ;
may be either an ovarian vein or a testicular vein,
depending on the sex of the individual
Major Veins of the Abdominal Region
Vessel Description
Drains the diaphragm; the right phrenic vein flows into
Phrenic ener ane
oh the inferior vena cava and the left phrenic vein empties
into the left renal vein
Hepatic Drains systemic blood from the liver and flows into
vein the inferior vena cava
Veins Draining the Lower Limbs
The superior surface of the foot drains into the digital veins, and the inferior
surface drains into the plantar veins, which flow into a complex series of
anastomoses in the feet and ankles, including the dorsal venous arch and
the plantar venous arch ((link]). From the dorsal venous arch, blood
supply drains into the anterior and posterior tibial veins. The anterior tibial
vein drains the area near the tibialis anterior muscle and combines with the
posterior tibial vein and the fibular vein to form the popliteal vein. The
posterior tibial vein drains the posterior surface of the tibia and joins the
popliteal vein. The fibular vein drains the muscles and integument in
proximity to the fibula and also joins the popliteal vein. The small
saphenous vein located on the lateral surface of the leg drains blood from
the superficial regions of the lower leg and foot, and flows into to the
popliteal vein. As the popliteal vein passes behind the knee in the popliteal
region, it becomes the femoral vein. It is palpable in patients without
excessive adipose tissue.
Close to the body wall, the great saphenous vein, the deep femoral vein, and
the femoral circumflex vein drain into the femoral vein. The great
saphenous vein is a prominent surface vessel located on the medial surface
of the leg and thigh that collects blood from the superficial portions of these
areas. The deep femoral vein, as the name suggests, drains blood from the
deeper portions of the thigh. The femoral circumflex vein forms a loop
around the femur just inferior to the trochanters and drains blood from the
areas in proximity to the head and neck of the femur.
As the femoral vein penetrates the body wall from the femoral portion of
the upper limb, it becomes the external iliac vein, a large vein that drains
blood from the leg to the common iliac vein. The pelvic organs and
integument drain into the internal iliac vein, which forms from several
smaller veins in the region, including the umbilical veins that run on either
side of the bladder. The external and internal iliac veins combine near the
inferior portion of the sacroiliac joint to form the common iliac vein. In
addition to blood supply from the external and internal iliac veins, the
middle sacral vein drains the sacral region into the common iliac vein.
Similar to the common iliac arteries, the common iliac veins come together
at the level of L5 to form the inferior vena cava.
[link] is a flow chart of veins flowing into the lower limb. [link]
summarizes the major veins of the lower limbs.
Major Veins Serving the Lower Limbs
Common iliac
External iliac
| Internal iliac
External iliac = 42 Gluteal Internal
; Lateral sacral pudendal Gluteal
Internal pudendal Obturator
Obturator Femoral
Femoral
Deep femoral Deep femoral
Femoral Femoral
circumflex circumflex
Femoral
Femoral
Great saphenous Great
saphenous
Popliteal
Popliteal
Small saphenous Small
Anterior tibial eppnenGys
Posterior tibial Anterior tibial
Posterior
Fibular tibial
Fibular
Lateral plantar
Medial plantar
Plantar venous
Dorsal venous arch
arch
Digital
Anterior view Posterior view
Anterior and posterior views show the major veins that
drain the lower limb into the inferior vena cava.
Major Veins of the Lower Limb
External iliac
Collects blood from
Collects blood Deep Eemoral Great the superficial
from the thigh femoral EY-)9)il-JaleltESa) veins of the lower
limb
Collects blood
from superficial Small
veins of the leg saphenous
and foot
Popliteal
Anterior Posterior
tibial tibial AES
Lateral and medial plantar,
dorsal arch, plantar arch
Metatarsal, Distal foot,
digital toes
The flow chart summarizes venous flow from the
lower limb.
Veins of the Lower Limbs
Vessel
Plantar
veins
Dorsal
venous
arch
Plantar
venous
arch
Anterior
tibial vein
Posterior
tibial vein
Fibular
vein
Small
saphenous
vein
Popliteal
vein
Description
Drain the foot and flow into the plantar venous arch
Drains blood from digital veins and vessels on the
superior surface of the foot
Formed from the plantar veins; flows into the anterior
and posterior tibial veins through anastomoses
Formed from the dorsal venous arch; drains the area
near the tibialis anterior muscle and flows into the
popliteal vein
Formed from the dorsal venous arch; drains the area
near the posterior surface of the tibia and flows into
the popliteal vein
Drains the muscles and integument near the fibula
and flows into the popliteal vein
Located on the lateral surface of the leg; drains blood
from the superficial regions of the lower leg and foot,
and flows into the popliteal vein
Drains the region behind the knee and forms from the
fusion of the fibular, anterior, and posterior tibial
veins; flows into the femoral vein
Veins of the Lower Limbs
Vessel
Great
saphenous
vein
Deep
femoral
vein
Femoral
circumflex
vein
Femoral
vein
External
iliac vein
Internal
iliac vein
Middle
sacral vein
Description
Prominent surface vessel located on the medial
surface of the leg and thigh; drains the superficial
portions of these areas and flows into the femoral
vein
Drains blood from the deeper portions of the thigh
and flows into the femoral vein
Forms a loop around the femur just inferior to the
trochanters; drains blood from the areas around the
head and neck of the femur; flows into the femoral
vein
Drains the upper leg; receives blood from the great
saphenous vein, the deep femoral vein, and the
femoral circumflex vein; becomes the external iliac
vein when it crosses the body wall
Formed when the femoral vein passes into the body
cavity; drains the legs and flows into the common
iliac vein
Drains the pelvic organs and integument; formed
from several smaller veins in the region; flows into
the common iliac vein
Drains the sacral region and flows into the left
common iliac vein
Veins of the Lower Limbs
Vessel Description
Flows into the inferior vena cava at the level of L5;
the left common iliac vein drains the sacral region;
formed from the union of the external and internal
iliac veins near the inferior portion of the sacroiliac
joint
Common
iliac vein
Hepatic Portal System
The liver is a complex biochemical processing plant. It packages nutrients
absorbed by the digestive system; produces plasma proteins, clotting
factors, and bile; and disposes of worn-out cell components and waste
products. Instead of entering the circulation directly, absorbed nutrients and
certain wastes (for example, materials produced by the spleen) travel to the
liver for processing. They do so via the hepatic portal system ((Link)).
Portal systems begin and end in capillaries. In this case, the initial
capillaries from the stomach, small intestine, large intestine, and spleen lead
to the hepatic portal vein and end in specialized capillaries within the liver,
the hepatic sinusoids. You saw the only other portal system with the
hypothalamic-hypophyseal portal vessel in the endocrine chapter.
The hepatic portal system consists of the hepatic portal vein and the veins
that drain into it. The hepatic portal vein itself is relatively short, beginning
at the level of L2 with the confluence of the superior mesenteric and splenic
veins. It also receives branches from the inferior mesenteric vein, plus the
splenic veins and all their tributaries. The superior mesenteric vein receives
blood from the small intestine, two-thirds of the large intestine, and the
stomach. The inferior mesenteric vein drains the distal third of the large
intestine, including the descending colon, the sigmoid colon, and the
rectum. The splenic vein is formed from branches from the spleen,
pancreas, and portions of the stomach, and the inferior mesenteric vein.
After its formation, the hepatic portal vein also receives branches from the
gastric veins of the stomach and cystic veins from the gall bladder. The
hepatic portal vein delivers materials from these digestive and circulatory
organs directly to the liver for processing.
Because of the hepatic portal system, the liver receives its blood supply
from two different sources: from normal systemic circulation via the hepatic
artery and from the hepatic portal vein. The liver processes the blood from
the portal system to remove certain wastes and excess nutrients, which are
stored for later use. This processed blood, as well as the systemic blood that
came from the hepatic artery, exits the liver via the right, left, and middle
hepatic veins, and flows into the inferior vena cava. Overall systemic blood
composition remains relatively stable, since the liver is able to metabolize
the absorbed digestive components.
Hepatic Portal System
Hepatic portal
Cystic ey
Gall bladder
Superior mesenteric
Gastro-omental ()
Right gastric
Splenic
Gastroepiploic
. Pancreatic
Pancreaticoduodenal
Middle colic
Right colic cis ~~ Inferior mesenteric
i X Left colic
Ileocolic J > A
Intestinal ( on s
xeeyl
The liver receives blood from the normal systemic
circulation via the hepatic artery. It also receives and
processes blood from other organs, delivered via the
veins of the hepatic portal system. All blood exits the
liver via the hepatic vein, which delivers the blood to
the inferior vena cava. (Different colors are used to
help distinguish among the different vessels in the
system.)
Sigmoid
Superior rectal
Chapter Review
The right ventricle pumps oxygen-depleted blood into the pulmonary trunk
and right and left pulmonary arteries, which carry it to the right and left
lungs for gas exchange. Oxygen-rich blood is transported by pulmonary
veins to the left atrium. The left ventricle pumps this blood into the aorta.
The main regions of the aorta are the ascending aorta, aortic arch, and
descending aorta, which is further divided into the thoracic and abdominal
aorta. The coronary arteries branch from the ascending aorta. After
oxygenating tissues in the capillaries, systemic blood is returned to the right
atrium from the venous system via the superior vena cava, which drains
most of the veins superior to the diaphragm, the inferior vena cava, which
drains most of the veins inferior to the diaphragm, and the coronary veins
via the coronary sinus. The hepatic portal system carries blood to the liver
for processing before it enters circulation. Review the figures provided in
this section for circulation of blood through the blood vessels.
Review Questions
Exercise:
Problem:The coronary arteries branch off of the
a. aortic valve
b. ascending aorta
c. aortic arch
d. thoracic aorta
Solution:
B
Exercise:
Problem: Which of the following statements is true?
a. The left and right common carotid arteries both branch off of the
brachiocephalic trunk.
b. The brachial artery is the distal branch of the axillary artery.
c. The radial and ulnar arteries join to form the palmar arch.
d. All of the above are true.
Solution:
C
Exercise:
Problem:
Arteries serving the stomach, pancreas, and liver all branch from the
a. Superior mesenteric artery
b. inferior mesenteric artery
c. celiac trunk
d. splenic artery
Solution:
C
Exercise:
Problem:The right and left brachiocephalic veins
a. drain blood from the right and left internal jugular veins
b. drain blood from the right and left subclavian veins
c. drain into the superior vena cava
d. all of the above are true
Solution:
D
Exercise:
Problem:
The hepatic portal system delivers blood from the digestive organs to
the
a. liver
b. hypothalamus
c. spleen
d. left atrium
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Identify the ventricle of the heart that pumps oxygen-depleted blood
and the arteries of the body that carry oxygen-depleted blood.
Solution:
The right ventricle of the heart pumps oxygen-depleted blood to the
pulmonary arteries.
Exercise:
Problem:What organs do the gonadal veins drain?
Solution:
The gonadal veins drain the testes in males and the ovaries in females.
Exercise:
Problem:
What arteries play the leading roles in supplying blood to the brain?
Solution:
The internal carotid arteries and the vertebral arteries provide most of
the brain’s blood supply.
Glossary
abdominal aorta
portion of the aorta inferior to the aortic hiatus and superior to the
common iliac arteries
adrenal artery
branch of the abdominal aorta; supplies blood to the adrenal
(suprarenal) glands
adrenal vein
drains the adrenal or suprarenal glands that are immediately superior to
the kidneys; the right adrenal vein enters the inferior vena cava directly
and the left adrenal vein enters the left renal vein
anterior cerebral artery
arises from the internal carotid artery; supplies the frontal lobe of the
cerebrum
anterior communicating artery
anastomosis of the right and left internal carotid arteries; supplies
blood to the brain
anterior tibial artery
branches from the popliteal artery; supplies blood to the anterior tibial
region; becomes the dorsalis pedis artery
anterior tibial vein
forms from the dorsal venous arch; drains the area near the tibialis
anterior muscle and leads to the popliteal vein
aorta
largest artery in the body, originating from the left ventricle and
descending to the abdominal region where it bifurcates into the
common iliac arteries at the level of the fourth lumbar vertebra;
arteries originating from the aorta distribute blood to virtually all
tissues of the body
aortic arch
arc that connects the ascending aorta to the descending aorta; ends at
the intervertebral disk between the fourth and fifth thoracic vertebrae
aortic hiatus
opening in the diaphragm that allows passage of the thoracic aorta into
the abdominal region where it becomes the abdominal aorta
arterial circle
(also, circle of Willis) anastomosis located at the base of the brain that
ensures continual blood supply; formed from branches of the internal
carotid and vertebral arteries; supplies blood to the brain
ascending aorta
initial portion of the aorta, rising from the left ventricle for a distance
of approximately 5 cm
axillary artery
continuation of the subclavian artery as it penetrates the body wall and
enters the axillary region; supplies blood to the region near the head of
the humerus (humeral circumflex arteries); the majority of the vessel
continues into the brachium and becomes the brachial artery
axillary vein
major vein in the axillary region; drains the upper limb and becomes
the subclavian vein
azygos vein
originates in the lumbar region and passes through the diaphragm into
the thoracic cavity on the right side of the vertebral column; drains
blood from the intercostal veins, esophageal veins, bronchial veins,
and other veins draining the mediastinal region; leads to the superior
vena cava
basilar artery
formed from the fusion of the two vertebral arteries; sends branches to
the cerebellum, brain stem, and the posterior cerebral arteries; the main
blood supply to the brain stem
basilic vein
superficial vein of the arm that arises from the palmar venous arches,
intersects with the median cubital vein, parallels the ulnar vein, and
continues into the upper arm; along with the brachial vein, it leads to
the axillary vein
brachial artery
continuation of the axillary artery in the brachium; supplies blood to
much of the brachial region; gives off several smaller branches that
provide blood to the posterior surface of the arm in the region of the
elbow; bifurcates into the radial and ulnar arteries at the coronoid fossa
brachial vein
deeper vein of the arm that forms from the radial and ulnar veins in the
lower arm; leads to the axillary vein
brachiocephalic artery
single vessel located on the right side of the body; the first vessel
branching from the aortic arch; gives rise to the right subclavian artery
and the right common carotid artery; supplies blood to the head, neck,
upper limb, and wall of the thoracic region
brachiocephalic vein
one of a pair of veins that form from a fusion of the external and
internal jugular veins and the subclavian vein; subclavian, external and
internal jugulars, vertebral, and internal thoracic veins lead to it; drains
the upper thoracic region and flows into the superior vena cava
bronchial artery
systemic branch from the aorta that provides oxygenated blood to the
lungs in addition to the pulmonary circuit
bronchial vein
drains the systemic circulation from the lungs and leads to the azygos
vein
cavernous sinus
enlarged vein that receives blood from most of the other cerebral veins
and the eye socket, and leads to the petrosal sinus
celiac trunk
(also, celiac artery) major branch of the abdominal aorta; gives rise to
the left gastric artery, the splenic artery, and the common hepatic artery
that forms the hepatic artery to the liver, the right gastric artery to the
stomach, and the cystic artery to the gall bladder
cephalic vein
superficial vessel in the upper arm; leads to the axillary vein
cerebrovascular accident (CVA)
blockage of blood flow to the brain; also called a stroke
circle of Willis
(also, arterial circle) anastomosis located at the base of the brain that
ensures continual blood supply; formed from branches of the internal
carotid and vertebral arteries; supplies blood to the brain
common carotid artery
right common carotid artery arises from the brachiocephalic artery, and
the left common carotid arises from the aortic arch; gives rise to the
external and internal carotid arteries; supplies the respective sides of
the head and neck
common hepatic artery
branch of the celiac trunk that forms the hepatic artery, the right gastric
artery, and the cystic artery
common iliac artery
branch of the aorta that leads to the internal and external iliac arteries
common iliac vein
one of a pair of veins that flows into the inferior vena cava at the level
of L5; the left common iliac vein drains the sacral region; divides into
external and internal iliac veins near the inferior portion of the
sacroiliac joint
cystic artery
branch of the common hepatic artery; supplies blood to the gall
bladder
deep femoral artery
branch of the femoral artery; gives rise to the lateral circumflex
arteries
deep femoral vein
drains blood from the deeper portions of the thigh and leads to the
femoral vein
descending aorta
portion of the aorta that continues downward past the end of the aortic
arch; subdivided into the thoracic aorta and the abdominal aorta
digital arteries
formed from the superficial and deep palmar arches; supply blood to
the digits
digital veins
drain the digits and feed into the palmar arches of the hand and dorsal
venous arch of the foot
dorsal arch
(also, arcuate arch) formed from the anastomosis of the dorsalis pedis
artery and medial and plantar arteries; branches supply the distal
portions of the foot and digits
dorsal venous arch
drains blood from digital veins and vessels on the superior surface of
the foot
dorsalis pedis artery
forms from the anterior tibial artery; branches repeatedly to supply
blood to the tarsal and dorsal regions of the foot
esophageal artery
branch of the thoracic aorta; supplies blood to the esophagus
esophageal vein
drains the inferior portions of the esophagus and leads to the azygos
vein
external carotid artery
arises from the common carotid artery; supplies blood to numerous
structures within the face, lower jaw, neck, esophagus, and larynx
external iliac artery
branch of the common iliac artery that leaves the body cavity and
becomes a femoral artery; supplies blood to the lower limbs
external iliac vein
formed when the femoral vein passes into the body cavity; drains the
legs and leads to the common iliac vein
external jugular vein
one of a pair of major veins located in the superficial neck region that
drains blood from the more superficial portions of the head, scalp, and
cranial regions, and leads to the subclavian vein
femoral artery
continuation of the external iliac artery after it passes through the body
cavity; divides into several smaller branches, the lateral deep femoral
artery, and the genicular artery; becomes the popliteal artery as it
passes posterior to the knee
femoral circumflex vein
forms a loop around the femur just inferior to the trochanters; drains
blood from the areas around the head and neck of the femur; leads to
the femoral vein
femoral vein
drains the upper leg; receives blood from the great saphenous vein, the
deep femoral vein, and the femoral circumflex vein; becomes the
external iliac vein when it crosses the body wall
fibular vein
drains the muscles and integument near the fibula and leads to the
popliteal vein
genicular artery
branch of the femoral artery; supplies blood to the region of the knee
gonadal artery
branch of the abdominal aorta; supplies blood to the gonads or
reproductive organs; also described as ovarian arteries or testicular
arteries, depending upon the sex of the individual
gonadal vein
generic term for a vein draining a reproductive organ; may be either an
ovarian vein or a testicular vein, depending on the sex of the individual
great cerebral vein
receives most of the smaller vessels from the inferior cerebral veins
and leads to the straight sinus
great saphenous vein
prominent surface vessel located on the medial surface of the leg and
thigh; drains the superficial portions of these areas and leads to the
femoral vein
hemiazygos vein
smaller vein complementary to the azygos vein; drains the esophageal
veins from the esophagus and the left intercostal veins, and leads to the
brachiocephalic vein via the superior intercostal vein
hepatic artery proper
branch of the common hepatic artery; supplies systemic blood to the
liver
hepatic portal system
specialized circulatory pathway that carries blood from digestive
organs to the liver for processing before being sent to the systemic
circulation
hepatic vein
drains systemic blood from the liver and flows into the inferior vena
cava
inferior mesenteric artery
branch of the abdominal aorta; supplies blood to the distal segment of
the large intestine and rectum
inferior phrenic artery
branch of the abdominal aorta; supplies blood to the inferior surface of
the diaphragm
inferior vena cava
large systemic vein that drains blood from areas largely inferior to the
diaphragm; empties into the right atrium
intercostal artery
branch of the thoracic aorta; supplies blood to the muscles of the
thoracic cavity and vertebral column
intercostal vein
drains the muscles of the thoracic wall and leads to the azygos vein
internal carotid artery
arises from the common carotid artery and begins with the carotid
sinus; goes through the carotid canal of the temporal bone to the base
of the brain; combines with branches of the vertebral artery forming
the arterial circle; supplies blood to the brain
internal iliac artery
branch from the common iliac arteries; supplies blood to the urinary
bladder, walls of the pelvis, external genitalia, and the medial portion
of the femoral region; in females, also provide blood to the uterus and
vagina
internal iliac vein
drains the pelvic organs and integument; formed from several smaller
veins in the region; leads to the common iliac vein
internal jugular vein
one of a pair of major veins located in the neck region that passes
through the jugular foramen and canal, flows parallel to the common
carotid artery that is more or less its counterpart; primarily drains
blood from the brain, receives the superficial facial vein, and empties
into the subclavian vein
internal thoracic artery
(also, mammary artery) arises from the subclavian artery; supplies
blood to the thymus, pericardium of the heart, and the anterior chest
wall
internal thoracic vein
(also, internal mammary vein) drains the anterior surface of the chest
wall and leads to the brachiocephalic vein
lateral circumflex artery
branch of the deep femoral artery; supplies blood to the deep muscles
of the thigh and the ventral and lateral regions of the integument
lateral plantar artery
arises from the bifurcation of the posterior tibial arteries; supplies
blood to the lateral plantar surfaces of the foot
left gastric artery
branch of the celiac trunk; supplies blood to the stomach
lumbar arteries
branches of the abdominal aorta; supply blood to the lumbar region,
the abdominal wall, and spinal cord
lumbar veins
drain the lumbar portion of the abdominal wall and spinal cord; the
superior lumbar veins drain into the azygos vein on the right or the
hemiazygos vein on the left; blood from these vessels is returned to the
superior vena cava rather than the inferior vena cava
maxillary vein
drains blood from the maxillary region and leads to the external
jugular vein
medial plantar artery
arises from the bifurcation of the posterior tibial arteries; supplies
blood to the medial plantar surfaces of the foot
median antebrachial vein
vein that parallels the ulnar vein but is more medial in location;
intertwines with the palmar venous arches
median cubital vein
superficial vessel located in the antecubital region that links the
cephalic vein to the basilic vein in the form of a v; a frequent site for a
blood draw
median sacral artery
continuation of the aorta into the sacrum
mediastinal artery
branch of the thoracic aorta; supplies blood to the mediastinum
middle cerebral artery
another branch of the internal carotid artery; supplies blood to the
temporal and parietal lobes of the cerebrum
middle sacral vein
drains the sacral region and leads to the left common iliac vein
occipital sinus
enlarged vein that drains the occipital region near the falx cerebelli and
flows into the left and right transverse sinuses, and also into the
vertebral veins
ophthalmic artery
branch of the internal carotid artery; supplies blood to the eyes
ovarian artery
branch of the abdominal aorta; supplies blood to the ovary, uterine
(Fallopian) tube, and uterus
ovarian vein
drains the ovary; the right ovarian vein leads to the inferior vena cava
and the left ovarian vein leads to the left renal vein
palmar arches
superficial and deep arches formed from anastomoses of the radial and
ulnar arteries; supply blood to the hand and digital arteries
palmar venous arches
drain the hand and digits, and feed into the radial and ulnar veins
parietal branches
(also, somatic branches) group of arterial branches of the thoracic
aorta; includes those that supply blood to the thoracic cavity, vertebral
column, and the superior surface of the diaphragm
pericardial artery
branch of the thoracic aorta; supplies blood to the pericardium
petrosal sinus
enlarged vein that receives blood from the cavernous sinus and flows
into the internal jugular vein
phrenic vein
drains the diaphragm; the right phrenic vein flows into the inferior
vena cava and the left phrenic vein leads to the left renal vein
plantar arch
formed from the anastomosis of the dorsalis pedis artery and medial
and plantar arteries; branches supply the distal portions of the foot and
digits
plantar veins
drain the foot and lead to the plantar venous arch
plantar venous arch
formed from the plantar veins; leads to the anterior and posterior tibial
veins through anastomoses
popliteal artery
continuation of the femoral artery posterior to the knee; branches into
the anterior and posterior tibial arteries
popliteal vein
continuation of the femoral vein behind the knee; drains the region
behind the knee and forms from the fusion of the fibular and anterior
and posterior tibial veins
posterior cerebral artery
branch of the basilar artery that forms a portion of the posterior
segment of the arterial circle; supplies blood to the posterior portion of
the cerebrum and brain stem
posterior communicating artery
branch of the posterior cerebral artery that forms part of the posterior
portion of the arterial circle; supplies blood to the brain
posterior tibial artery
branch from the popliteal artery that gives rise to the fibular or
peroneal artery; supplies blood to the posterior tibial region
posterior tibial vein
forms from the dorsal venous arch; drains the area near the posterior
surface of the tibia and leads to the popliteal vein
pulmonary artery
one of two branches, left and right, that divides off from the pulmonary
trunk and leads to smaller arterioles and eventually to the pulmonary
capillaries
pulmonary circuit
system of blood vessels that provide gas exchange via a network of
arteries, veins, and capillaries that run from the heart, through the
body, and back to the lungs
pulmonary trunk
single large vessel exiting the right ventricle that divides to form the
right and left pulmonary arteries
pulmonary veins
two sets of paired vessels, one pair on each side, that are formed from
the small venules leading away from the pulmonary capillaries that
flow into the left atrium
radial artery
formed at the bifurcation of the brachial artery; parallels the radius;
gives off smaller branches until it reaches the carpal region where it
fuses with the ulnar artery to form the superficial and deep palmar
arches; supplies blood to the lower arm and carpal region
radial vein
parallels the radius and radial artery; arises from the palmar venous
arches and leads to the brachial vein
renal artery
branch of the abdominal aorta; supplies each kidney
renal vein
largest vein entering the inferior vena cava; drains the kidneys and
leads to the inferior vena cava
right gastric artery
branch of the common hepatic artery; supplies blood to the stomach
sigmoid sinuses
enlarged veins that receive blood from the transverse sinuses; flow
through the jugular foramen and into the internal jugular vein
small saphenous vein
located on the lateral surface of the leg; drains blood from the
superficial regions of the lower leg and foot, and leads to the popliteal
vein
splenic artery
branch of the celiac trunk; supplies blood to the spleen
straight sinus
enlarged vein that drains blood from the brain; receives most of the
blood from the great cerebral vein and flows into the left or right
transverse sinus
subclavian artery
right subclavian arises from the brachiocephalic artery, whereas the left
subclavian artery arises from the aortic arch; gives rise to the internal
thoracic, vertebral, and thyrocervical arteries; supplies blood to the
arms, chest, shoulders, back, and central nervous system
subclavian vein
located deep in the thoracic cavity; becomes the axillary vein as it
enters the axillary region; drains the axillary and smaller local veins
near the scapular region; leads to the brachiocephalic vein
subscapular vein
drains blood from the subscapular region and leads to the axillary vein
superior mesenteric artery
branch of the abdominal aorta; supplies blood to the small intestine
(duodenum, jejunum, and ileum), the pancreas, and a majority of the
large intestine
superior phrenic artery
branch of the thoracic aorta; supplies blood to the superior surface of
the diaphragm
superior sagittal sinus
enlarged vein located midsagittally between the meningeal and
periosteal layers of the dura mater within the falx cerebri; receives
most of the blood drained from the superior surface of the cerebrum
and leads to the inferior jugular vein and the vertebral vein
superior vena cava
large systemic vein; drains blood from most areas superior to the
diaphragm; empties into the right atrium
temporal vein
drains blood from the temporal region and leads to the external jugular
vein
testicular artery
branch of the abdominal aorta; will ultimately travel outside the body
cavity to the testes and form one component of the spermatic cord
testicular vein
drains the testes and forms part of the spermatic cord; the right
testicular vein empties directly into the inferior vena cava and the left
testicular vein empties into the left renal vein
thoracic aorta
portion of the descending aorta superior to the aortic hiatus
thyrocervical artery
arises from the subclavian artery; supplies blood to the thyroid, the
cervical region, the upper back, and shoulder
transient ischemic attack (TIA)
temporary loss of neurological function caused by a brief interruption
in blood flow; also known as a mini-stroke
transverse sinuses
pair of enlarged veins near the lambdoid suture that drain the occipital,
Sagittal, and straight sinuses, and leads to the sigmoid sinuses
trunk
large vessel that gives rise to smaller vessels
ulnar artery
formed at the bifurcation of the brachial artery; parallels the ulna;
gives off smaller branches until it reaches the carpal region where it
fuses with the radial artery to form the superficial and deep palmar
arches; supplies blood to the lower arm and carpal region
ulnar vein
parallels the ulna and ulnar artery; arises from the palmar venous
arches and leads to the brachial vein
vertebral artery
arises from the subclavian artery and passes through the vertebral
foramen through the foramen magnum to the brain; joins with the
internal carotid artery to form the arterial circle; supplies blood to the
brain and spinal cord
vertebral vein
arises from the base of the brain and the cervical region of the spinal
cord; passes through the intervertebral foramina in the cervical
vertebrae; drains smaller veins from the cranium, spinal cord, and
vertebrae, and leads to the brachiocephalic vein; counterpart of the
vertebral artery
visceral branches
branches of the descending aorta that supply blood to the viscera
An Overview of Blood
By the end of this section, you will be able to:
e Identify the primary functions of blood in transportation, defense, and
maintenance of homeostasis
e Name the fluid component of blood and the three major types of
formed elements, and identify their relative proportions in a blood
sample
e Discuss the unique physical characteristics of blood
e Identify the composition of blood plasma, including its most important
solutes and plasma proteins
Recall that blood is a connective tissue. Like all connective tissues, it is
made up of cellular elements and an extracellular matrix. The cellular
elements—referred to as the formed elements—include red blood cells
(RBCs), white blood cells (WBCs), and cell fragments called platelets.
The extracellular matrix, called plasma, makes blood unique among
connective tissues because it is fluid. This fluid, which is mostly water,
perpetually suspends the formed elements and enables them to circulate
throughout the body within the cardiovascular system.
Functions of Blood
The primary function of blood is to deliver oxygen and nutrients to and
remove wastes from body cells, but that is only the beginning of the story.
The specific functions of blood also include defense, distribution of heat,
and maintenance of homeostasis.
Transportation
Nutrients from the foods you eat are absorbed in the digestive tract. Most of
these travel in the bloodstream directly to the liver, where they are
processed and released back into the bloodstream for delivery to body cells.
Oxygen from the air you breathe diffuses into the blood, which moves from
the lungs to the heart, which then pumps it out to the rest of the body.
Moreover, endocrine glands scattered throughout the body release their
products, called hormones, into the bloodstream, which carries them to
distant target cells. Blood also picks up cellular wastes and byproducts, and
transports them to various organs for removal. For instance, blood moves
carbon dioxide to the lungs for exhalation from the body, and various waste
products are transported to the kidneys and liver for excretion from the
body in the form of urine or bile.
Defense
Many types of WBCs protect the body from external threats, such as
disease-causing bacteria that have entered the bloodstream in a wound.
Other WBCs seek out and destroy internal threats, such as cells with
mutated DNA that could multiply to become cancerous, or body cells
infected with viruses.
When damage to the vessels results in bleeding, blood platelets and certain
proteins dissolved in the plasma, the fluid portion of the blood, interact to
block the ruptured areas of the blood vessels involved. This protects the
body from further blood loss.
Maintenance of Homeostasis
Recall that body temperature is regulated via a classic negative-feedback
loop. If you were exercising on a warm day, your rising core body
temperature would trigger several homeostatic mechanisms, including
increased transport of blood from your core to your body periphery, which
is typically cooler. As blood passes through the vessels of the skin, heat
would be dissipated to the environment, and the blood returning to your
body core would be cooler. In contrast, on a cold day, blood is diverted
away from the skin to maintain a warmer body core. In extreme cases, this
may result in frostbite.
Blood also helps to maintain the chemical balance of the body. Proteins and
other compounds in blood act as buffers, which thereby help to regulate the
pH of body tissues. Blood also helps to regulate the water content of body
cells.
Composition of Blood
You have probably had blood drawn from a superficial vein in your arm,
which was then sent to a lab for analysis. Some of the most common blood
tests—for instance, those measuring lipid or glucose levels in plasma—
determine which substances are present within blood and in what quantities.
Other blood tests check for the composition of the blood itself, including
the quantities and types of formed elements.
One such test, called a hematocrit, measures the percentage of RBCs,
clinically known as erythrocytes, in a blood sample. It is performed by
spinning the blood sample in a specialized centrifuge, a process that causes
the heavier elements suspended within the blood sample to separate from
the lightweight, liquid plasma ({link]). Because the heaviest elements in
blood are the erythrocytes, these settle at the very bottom of the hematocrit
tube. Located above the erythrocytes is a pale, thin layer composed of the
remaining formed elements of blood. These are the WBCGs, clinically
known as leukocytes, and the platelets, cell fragments also called
thrombocytes. This layer is referred to as the buffy coat because of its
color; it normally constitutes less than 1 percent of a blood sample. Above
the buffy coat is the blood plasma, normally a pale, straw-colored fluid,
which constitutes the remainder of the sample.
The volume of erythrocytes after centrifugation is also commonly referred
to as packed cell volume (PCV). In normal blood, about 45 percent of a
sample is erythrocytes. The hematocrit of any one sample can vary
significantly, however, about 36—50 percent, according to gender and other
factors. Normal hematocrit values for females range from 37 to 47, with a
mean value of 41; for males, hematocrit ranges from 42 to 52, with a mean
of 47. The percentage of other formed elements, the WBCs and platelets, is
extremely small so it is not normally considered with the hematocrit. So the
mean plasma percentage is the percent of blood that is not erythrocytes: for
females, it is approximately 59 (or 100 minus 41), and for males, it is
approximately 53 (or 100 minus 47).
Composition of Blood
Plasma:
- Water, proteins,
nutrients, hormones,
etc.
Buffy coat:
- White blood cells,
platelets
Hematocrit:
- Red blood cells
Normal Blood: Anemia: Polycythemia:
Q. 37%-47% hematocrit Depressed Elevated
O" 42%-52% hematocrit hematocrit % hematocrit %
The cellular elements of blood include a
vast number of erythrocytes and
comparatively fewer leukocytes and
platelets. Plasma is the fluid in which the
formed elements are suspended. A sample
of blood spun in a centrifuge reveals that
plasma is the lightest component. It floats
at the top of the tube separated from the
heaviest elements, the erythrocytes, by a
buffy coat of leukocytes and platelets.
Hematocrit is the percentage of the total
sample that is comprised of erythrocytes.
Depressed and elevated hematocrit levels
are shown for comparison.
Characteristics of Blood
When you think about blood, the first characteristic that probably comes to
mind is its color. Blood that has just taken up oxygen in the lungs is bright
red, and blood that has released oxygen in the tissues is a more dusky red.
This is because hemoglobin is a pigment that changes color, depending
upon the degree of oxygen saturation.
Blood is viscous and somewhat sticky to the touch. It has a viscosity
approximately five times greater than water. Viscosity is a measure of a
fluid’s thickness or resistance to flow, and is influenced by the presence of
the plasma proteins and formed elements within the blood. The viscosity of
blood has a dramatic impact on blood pressure and flow. Consider the
difference in flow between water and honey. The more viscous honey
would demonstrate a greater resistance to flow than the less viscous water.
The same principle applies to blood.
The normal temperature of blood is slightly higher than normal body
temperature—about 38 °C (or 100.4 °F), compared to 37 °C (or 98.6 °F) for
an internal body temperature reading, although daily variations of 0.5 °C
are normal. Although the surface of blood vessels is relatively smooth, as
blood flows through them, it experiences some friction and resistance,
especially as vessels age and lose their elasticity, thereby producing heat.
This accounts for its slightly higher temperature.
The pH of blood averages about 7.4; however, it can range from 7.35 to
7.45 in a healthy person. Blood is therefore somewhat more basic (alkaline)
on a chemical scale than pure water, which has a pH of 7.0. Blood contains
numerous buffers that actually help to regulate pH.
Blood constitutes approximately 8 percent of adult body weight. Adult
males typically average about 5 to 6 liters of blood. Females average 4—5
liters.
Blood Plasma
Like other fluids in the body, plasma is composed primarily of water: In
fact, it is about 92 percent water. Dissolved or suspended within this water
is a mixture of substances, most of which are proteins. There are literally
hundreds of substances dissolved or suspended in the plasma, although
many of them are found only in very small quantities.
Note:
Ase
een en
ara
inten
Visit this site for a list of normal levels established for many of the
substances found in a sample of blood. Serum, one of the specimen types
included, refers to a sample of plasma after clotting factors have been
removed. What types of measurements are given for levels of glucose in
the blood?
Plasma Proteins
About 7 percent of the volume of plasma—nearly all that is not water—is
made of proteins. These include several plasma proteins (proteins that are
unique to the plasma), plus a much smaller number of regulatory proteins,
including enzymes and some hormones. The major components of plasma
are summarized in [link].
The three major groups of plasma proteins are as follows:
e Albumin is the most abundant of the plasma proteins. Manufactured
by the liver, albumin molecules serve as binding proteins—transport
vehicles for fatty acids and steroid hormones. Recall that lipids are
hydrophobic; however, their binding to albumin enables their transport
in the watery plasma. Albumin is also the most significant contributor
to the osmotic pressure of blood; that is, its presence holds water inside
the blood vessels and draws water from the tissues, across blood vessel
walls, and into the bloodstream. This in turn helps to maintain both
blood volume and blood pressure. Albumin normally accounts for
approximately 54 percent of the total plasma protein content, in
clinical levels of 3.5-5.0 g/dL blood.
e The second most common plasma proteins are the globulins. A
heterogeneous group, there are three main subgroups known as alpha,
beta, and gamma globulins. The alpha and beta globulins transport
iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells;
like albumin, they also contribute to osmotic pressure. The gamma
globulins are proteins involved in immunity and are better known as an
antibodies or immunoglobulins. Although other plasma proteins are
produced by the liver, immunoglobulins are produced by specialized
leukocytes known as plasma cells. (Seek additional content for more
information about immunoglobulins.) Globulins make up
approximately 38 percent of the total plasma protein volume, in
clinical levels of 1.0—1.5 g/dL blood.
e The least abundant plasma protein is fibrinogen. Like albumin and the
alpha and beta globulins, fibrinogen is produced by the liver. It is
essential for blood clotting, a process described later in this chapter.
Fibrinogen accounts for about 7 percent of the total plasma protein
volume, in clinical levels of 0.2—-0.45 g/dL blood.
Other Plasma Solutes
In addition to proteins, plasma contains a wide variety of other substances.
These include various electrolytes, such as sodium, potassium, and calcium
ions; dissolved gases, such as oxygen, carbon dioxide, and nitrogen; various
organic nutrients, such as vitamins, lipids, glucose, and amino acids; and
metabolic wastes. All of these nonprotein solutes combined contribute
approximately 1 percent to the total volume of plasma.
Major Blood Components
Component Subcomponent Type and %
and % and % of (where Site of production Major function(s)
of blood component appropriate)
Absorbed by intestinal
tract or produced by
metabolism
Water
92 percent
Transport
medium
Maintain osmotic
Albumin concentration,
54-60 percent transport lipid
molecules
Transport,
maintain osmotic
concentration
Alpha globulins—
liver
Plasma proteins
Plasma 7 percent Globulins Beta globulins—
46-63 35-38 percent liver
percent
Transport,
maintain osmotic
concentration
Gamma globulins
(immunoglobulins)
—plasma cells
Immune
responses
Fibrinogen Blood clotting in
4-7 percent hemostasis
Regulatory proteins Hormones : Regulate various
and enzymes Various sources body functions
Absorbed by intestinal
Other solutes Nutrients, gases, tract, exchanged in Numerous
1 percent and wastes respiratory system, and varied
or produced by cells
Transport gases,
Erythrocytes primarily oxygen
99 percent Erytwocyive Red bone marrow and some carbon
dioxide
Granular
leukocytes:
neutrophils Red bone marrow
eosinophils
basophils
Nonspecific
Formed immunity
elements Leukocytes
37-54 <1 percent
percent Platelets Lymphocytes: Lymphocytes:
<1 percent Agranular bone marrow and specific
leukocytes: lymphatic tissue immunity
lymphocytes
monocytes Monocytes: Monocytes:
red bone marrow nonspecific immunity
Platelets Megakaryocytes: :
Note:
Career Connection
Phlebotomy and Medical Lab Technology
Phlebotomists are professionals trained to draw blood (phleb- = “a blood
vessel”; -tomy = “to cut”). When more than a few drops of blood are
required, phlebotomists perform a venipuncture, typically of a surface vein
in the arm. They perform a capillary stick on a finger, an earlobe, or the
heel of an infant when only a small quantity of blood is required. An
arterial stick is collected from an artery and used to analyze blood gases.
After collection, the blood may be analyzed by medical laboratories or
perhaps used for transfusions, donations, or research. While many allied
health professionals practice phlebotomy, the American Society of
Phlebotomy Technicians issues certificates to individuals passing a
national examination, and some large labs and hospitals hire individuals
expressly for their skill in phlebotomy.
Medical or clinical laboratories employ a variety of individuals in technical
positions:
e Medical technologists (MT), also known as clinical laboratory
technologists (CLT), typically hold a bachelor’s degree and
certification from an accredited training program. They perform a
wide variety of tests on various body fluids, including blood. The
information they provide is essential to the primary care providers in
determining a diagnosis and in monitoring the course of a disease and
response to treatment.
¢ Medical laboratory technicians (MLT) typically have an associate’s
degree but may perform duties similar to those of an MT.
e Medical laboratory assistants (MLA) spend the majority of their time
processing samples and carrying out routine assignments within the
lab. Clinical training is required, but a degree may not be essential to
obtaining a position.
Chapter Review
Blood is a fluid connective tissue critical to the transportation of nutrients,
gases, and wastes throughout the body; to defend the body against infection
and other threats; and to the homeostatic regulation of pH, temperature, and
other internal conditions. Blood is composed of formed elements—
erythrocytes, leukocytes, and cell fragments called platelets—and a fluid
extracellular matrix called plasma. More than 90 percent of plasma is water.
The remainder is mostly plasma proteins—mainly albumin, globulins, and
fibrinogen—and other dissolved solutes such as glucose, lipids, electrolytes,
and dissolved gases. Because of the formed elements and the plasma
proteins and other solutes, blood is sticky and more viscous than water. It is
also slightly alkaline, and its temperature is slightly higher than normal
body temperature.
Interactive Link Questions
Exercise:
Problem:
Visit this site for a list of normal levels established for many of the
substances found in a sample of blood. Serum, one of the specimen
types included, refers to a sample of plasma after clotting factors have
been removed. What types of measurements are given for levels of
glucose in the blood?
Solution:
There are values given for percent saturation, tension, and blood gas,
and there are listings for different types of hemoglobin.
Review Questions
Exercise:
Problem: Which of the following statements about blood is true?
a. Blood is about 92 percent water.
b. Blood is slightly more acidic than water.
c. Blood is slightly more viscous than water.
d. Blood is slightly more salty than seawater.
Solution:
C
Exercise:
Problem: Which of the following statements about albumin is true?
a. It draws water out of the blood vessels and into the body’s tissues.
b. It is the most abundant plasma protein.
c. It is produced by specialized leukocytes called plasma cells.
d. All of the above are true.
Solution:
B
Exercise:
Problem:
Which of the following plasma proteins is not produced by the liver?
a. fibrinogen
b. alpha globulin
c. beta globulin
d. immunoglobulin
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
A patient’s hematocrit is 42 percent. Approximately what percentage
of the patient’s blood is plasma?
Solution:
The patient’s blood is approximately 58 percent plasma (since the
buffy coat is less than 1 percent).
Exercise:
Problem:
Why would it be incorrect to refer to the formed elements as cells?
Solution:
The formed elements include erythrocytes and leukocytes, which are
cells (although mature erythrocytes do not have a nucleus); however,
the formed elements also include platelets, which are not true cells but
cell fragments.
Exercise:
Problem:
True or false: The buffy coat is the portion of a blood sample that is
made up of its proteins.
Solution:
False. The buffy coat is the portion of blood that is made up of its
leukocytes and platelets.
Glossary
albumin
most abundant plasma protein, accounting for most of the osmotic
pressure of plasma
antibodies
(also, immunoglobulins or gamma globulins) antigen-specific proteins
produced by specialized B lymphocytes that protect the body by
binding to foreign objects such as bacteria and viruses
blood
liquid connective tissue composed of formed elements—erythrocytes,
leukocytes, and platelets—and a fluid extracellular matrix called
plasma; component of the cardiovascular system
buffy coat
thin, pale layer of leukocytes and platelets that separates the
erythrocytes from the plasma in a sample of centrifuged blood
fibrinogen
plasma protein produced in the liver and involved in blood clotting
formed elements
cellular components of blood; that is, erythrocytes, leukocytes, and
platelets
globulins
heterogeneous group of plasma proteins that includes transport
proteins, clotting factors, immune proteins, and others
hematocrit
(also, packed cell volume) volume percentage of erythrocytes in a
sample of centrifuged blood
immunoglobulins
(also, antibodies or gamma globulins) antigen-specific proteins
produced by specialized B lymphocytes that protect the body by
binding to foreign objects such as bacteria and viruses
packed cell volume (PCV)
(also, hematocrit) volume percentage of erythrocytes present in a
sample of centrifuged blood
plasma
in blood, the liquid extracellular matrix composed mostly of water that
circulates the formed elements and dissolved materials throughout the
cardiovascular system
platelets
(also, thrombocytes) one of the formed elements of blood that consists
of cell fragments broken off from megakaryocytes
red blood cells (RBCs)
(also, erythrocytes) one of the formed elements of blood that transports
oxygen
white blood cells (WBCs)
(also, leukocytes) one of the formed elements of blood that provides
defense against disease agents and foreign materials
Production of the Formed Elements
By the end of this section, you will be able to:
e Trace the generation of the formed elements of blood from bone
marrow stem cells
e Discuss the role of hemopoietic growth factors in promoting the
production of the formed elements
The lifespan of the formed elements is very brief. Although one type of
leukocyte called memory cells can survive for years, most erythrocytes,
leukocytes, and platelets normally live only a few hours to a few weeks.
Thus, the body must form new blood cells and platelets quickly and
continuously. When you donate a unit of blood during a blood drive
(approximately 475 mL, or about 1 pint), your body typically replaces the
donated plasma within 24 hours, but it takes about 4 to 6 weeks to replace
the blood cells. This restricts the frequency with which donors can
contribute their blood. The process by which this replacement occurs is
called hemopoiesis, or hematopoiesis (from the Greek root haima- =
“blood”; -poiesis = “production”).
Sites of Hemopoiesis
Prior to birth, hemopoiesis occurs in a number of tissues, beginning with
the yolk sac of the developing embryo, and continuing in the fetal liver,
spleen, lymphatic tissue, and eventually the red bone marrow. Following
birth, most hemopoiesis occurs in the red marrow, a connective tissue
within the spaces of spongy (cancellous) bone tissue. In children,
hemopoiesis can occur in the medullary cavity of long bones; in adults, the
process is largely restricted to the cranial and pelvic bones, the vertebrae,
the sternum, and the proximal epiphyses of the femur and humerus.
Throughout adulthood, the liver and spleen maintain their ability to
generate the formed elements. This process is referred to as extramedullary
hemopoiesis (meaning hemopoiesis outside the medullary cavity of adult
bones). When a disease such as bone cancer destroys the bone marrow,
causing hemopoiesis to fail, extramedullary hemopoiesis may be initiated.
Differentiation of Formed Elements from Stem Cells
All formed elements arise from stem cells of the red bone marrow. Recall
that stem cells undergo mitosis plus cytokinesis (cellular division) to give
rise to new daughter cells: One of these remains a stem cell and the other
differentiates into one of any number of diverse cell types. Stem cells may
be viewed as occupying a hierarchal system, with some loss of the ability to
diversify at each step. The totipotent stem cell is the zygote, or fertilized
egg. The totipotent (toti- = “all”) stem cell gives rise to all cells of the
human body. The next level is the pluripotent stem cell, which gives rise
to multiple types of cells of the body and some of the supporting fetal
membranes. Beneath this level, the mesenchymal cell is a stem cell that
develops only into types of connective tissue, including fibrous connective
tissue, bone, cartilage, and blood, but not epithelium, muscle, and nervous
tissue. One step lower on the hierarchy of stem cells is the hemopoietic
stem cell, or hemocytoblast. All of the formed elements of blood originate
from this specific type of cell.
Hemopoiesis begins when the hemopoietic stem cell is exposed to
appropriate chemical stimuli collectively called hemopoietic growth
factors, which prompt it to divide and differentiate. One daughter cell
remains a hemopoietic stem cell, allowing hemopoiesis to continue. The
other daughter cell becomes either of two types of more specialized stem
cells ([link]):
¢ Lymphoid stem cells give rise to a class of leukocytes known as
lymphocytes, which include the various T cells, B cells, and natural
killer (NK) cells, all of which function in immunity. However,
hemopoiesis of lymphocytes progresses somewhat differently from the
process for the other formed elements. In brief, lymphoid stem cells
quickly migrate from the bone marrow to lymphatic tissues, including
the lymph nodes, spleen, and thymus, where their production and
differentiation continues. B cells are so named since they mature in the
bone marrow, while T cells mature in the thymus.
¢ Myeloid stem cells give rise to all the other formed elements,
including the erythrocytes; megakaryocytes that produce platelets; and
a myeloblast lineage that gives rise to monocytes and three forms of
granular leukocytes: neutrophils, eosinophils, and basophils.
Hematopoietic System of Bone Marrow
@
Multipotent hematopoietic
stem cell (hemocytoblast)
After division, some cells
remain stem cells.
@ i Ng The remaining cell goes down one of two paths
depending on the chemical signals received.
OF
Myeloid stem cell Lymphoid stem cell
rs rs
o ~) @ @ ‘
isn a a a a
Megakaryocyte Erythrocyte Basophil Neutrophil Eosinophil Monocyte Q.
T lymphocyte —_B lymphocyte
Megakaryoblast Proerythroblast Myeloblast Monoblast Lymphoblast
Reticulocyte = ak
rad | &
i_- Natural killer cell Small lymphocyte
~ (large granular
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Platelets
Hemopoiesis is the proliferation and differentiation of the formed
elements of blood.
Lymphoid and myeloid stem cells do not immediately divide and
differentiate into mature formed elements. As you can see in [link], there
are several intermediate stages of precursor cells (literally, forerunner cells),
many of which can be recognized by their names, which have the suffix -
blast. For instance, megakaryoblasts are the precursors of megakaryocytes,
and proerythroblasts become reticulocytes, which eject their nucleus and
most other organelles before maturing into erythrocytes.
Hemopoietic Growth Factors
Development from stem cells to precursor cells to mature cells is again
initiated by hemopoietic growth factors. These include the following:
e Erythropoietin (EPO) is a glycoprotein hormone secreted by the
interstitial fibroblast cells of the kidneys in response to low oxygen
levels. It prompts the production of erythrocytes. Some athletes use
synthetic EPO as a performance-enhancing drug (called blood doping)
to increase RBC counts and subsequently increase oxygen delivery to
tissues throughout the body. EPO is a banned substance in most
organized sports, but it is also used medically in the treatment of
certain anemia, specifically those triggered by certain types of cancer,
and other disorders in which increased erythrocyte counts and oxygen
levels are desirable.
e¢ Thrombopoietin, another glycoprotein hormone, is produced by the
liver and kidneys. It triggers the development of megakaryocytes into
platelets.
¢ Cytokines are glycoproteins secreted by a wide variety of cells,
including red bone marrow, leukocytes, macrophages, fibroblasts, and
endothelial cells. They act locally as autocrine or paracrine factors,
stimulating the proliferation of progenitor cells and helping to
stimulate both nonspecific and specific resistance to disease. There are
two major subtypes of cytokines known as colony-stimulating factors
and interleukins.
o Colony-stimulating factors (CSFs) are glycoproteins that act
locally, as autocrine or paracrine factors. Some trigger the
differentiation of myeloblasts into granular leukocytes, namely,
neutrophils, eosinophils, and basophils. These are referred to as
granulocyte CSFs. A different CSF induces the production of
monocytes, called monocyte CSFs. Both granulocytes and
monocytes are stimulated by GM-CSF; granulocytes, monocytes,
platelets, and erythrocytes are stimulated by multi-CSF. Synthetic
forms of these hormones are often administered to patients with
various forms of cancer who are receiving chemotherapy to
revive their WBC counts.
o Interleukins are another class of cytokine signaling molecules
important in hemopoiesis. They were initially thought to be
secreted uniquely by leukocytes and to communicate only with
other leukocytes, and were named accordingly, but are now
known to be produced by a variety of cells including bone
marrow and endothelium. Researchers now suspect that
interleukins may play other roles in body functioning, including
differentiation and maturation of cells, producing immunity and
inflammation. To date, more than a dozen interleukins have been
identified, with others likely to follow. They are generally
numbered IL-1, IL-2, IL-3, etc.
Note:
Everyday Connection
Blood Doping
In its original intent, the term blood doping was used to describe the
practice of injecting by transfusion supplemental RBCs into an individual,
typically to enhance performance in a sport. Additional RBCs would
deliver more oxygen to the tissues, providing extra aerobic capacity,
clinically referred to as VO» max. The source of the cells was either from
the recipient (autologous) or from a donor with compatible blood
(homologous). This practice was aided by the well-developed techniques
of harvesting, concentrating, and freezing of the RBCs that could be later
thawed and injected, yet still retain their functionality. These practices are
considered illegal in virtually all sports and run the risk of infection,
significantly increasing the viscosity of the blood and the potential for
transmission of blood-bome pathogens if the blood was collected from
another individual.
With the development of synthetic EPO in the 1980s, it became possible to
provide additional RBCs by artificially stimulating RBC production in the
bone marrow. Originally developed to treat patients suffering from anemia,
renal failure, or cancer treatment, large quantities of EPO can be generated
by recombinant DNA technology. Synthetic EPO is injected under the skin
and can increase hematocrit for many weeks. It may also induce
polycythemia and raise hematocrit to 70 or greater. This increased
viscosity raises the resistance of the blood and forces the heart to pump
more powerfully; in extreme cases, it has resulted in death. Other drugs
such as cobalt II chloride have been shown to increase natural EPO gene
expression. Blood doping has become problematic in many sports,
especially cycling. Lance Armstrong, winner of seven Tour de France and
many other cycling titles, was stripped of his victories and admitted to
blood doping in 2013.
Note:
we
—
meee OPENStAX COLLEGE
Watch this video to see doctors discuss the dangers of blood doping in
sports. What are the some potential side effects of blood doping?
Bone Marrow Sampling and Transplants
Sometimes, a healthcare provider will order a bone marrow biopsy, a
diagnostic test of a sample of red bone marrow, or a bone marrow
transplant, a treatment in which a donor’s healthy bone marrow—and its
stem cells—replaces the faulty bone marrow of a patient. These tests and
procedures are often used to assist in the diagnosis and treatment of various
severe forms of anemia, such as thalassemia major and sickle cell anemia,
as well as some types of cancer, specifically leukemia.
In the past, when a bone marrow sample or transplant was necessary, the
procedure would have required inserting a large-bore needle into the region
near the iliac crest of the pelvic bones (os coxae). This location was
preferred, since its location close to the body surface makes it more
accessible, and it is relatively isolated from most vital organs.
Unfortunately, the procedure is quite painful.
Now, direct sampling of bone marrow can often be avoided. In many cases,
stem cells can be isolated in just a few hours from a sample of a patient’s
blood. The isolated stem cells are then grown in culture using the
appropriate hemopoietic growth factors, and analyzed or sometimes frozen
for later use.
For an individual requiring a transplant, a matching donor is essential to
prevent the immune system from destroying the donor cells—a
phenomenon known as tissue rejection. To treat patients with bone marrow
transplants, it is first necessary to destroy the patient’s own diseased
marrow through radiation and/or chemotherapy. Donor bone marrow stem
cells are then intravenously infused. From the bloodstream, they establish
themselves in the recipient’s bone marrow.
Chapter Review
Through the process of hemopoiesis, the formed elements of blood are
continually produced, replacing the relatively short-lived erythrocytes,
leukocytes, and platelets. Hemopoiesis begins in the red bone marrow, with
hemopoietic stem cells that differentiate into myeloid and lymphoid
lineages. Myeloid stem cells give rise to most of the formed elements.
Lymphoid stem cells give rise only to the various lymphocytes designated
as B and T cells, and NK cells. Hemopoietic growth factors, including
erythropoietin, thrombopoietin, colony-stimulating factors, and interleukins,
promote the proliferation and differentiation of formed elements.
Interactive Link Questions
Exercise:
Problem:
Watch this video to see doctors discuss the dangers of blood doping in
sports. What are the some potential side effects of blood doping?
Solution:
Side effects can include heart disease, stroke, pulmonary embolism,
and virus transmission.
Review Questions
Exercise:
Problem:
Which of the formed elements arise from myeloid stem cells?
a. B cells
b. natural killer cells
c. platelets
d. all of the above
Solution:
C
Exercise:
Problem:
Which of the following statements about erythropoietin is true?
a. It facilitates the proliferation and differentiation of the erythrocyte
lineage.
b. It is a hormone produced by the thyroid gland.
c. It is a hemopoietic growth factor that prompts lymphoid stem
cells to leave the bone marrow.
d. Both a and b are true.
Solution:
A
Exercise:
Problem:
Interleukins are associated primarily with which of the following?
a. production of various lymphocytes
b. immune responses
c. inflammation
d. all of the above
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Myelofibrosis is a disorder in which inflammation and scar tissue
formation in the bone marrow impair hemopoiesis. One sign is an
enlarged spleen. Why?
Solution:
When disease impairs the ability of the bone marrow to participate in
hemopoiesis, extramedullary hemopoiesis begins in the patient’s liver
and spleen. This causes the spleen to enlarge.
Exercise:
Problem:
Would you expect a patient with a form of cancer called acute
myelogenous leukemia to experience impaired production of
erythrocytes, or impaired production of lymphocytes? Explain your
choice.
Solution:
The adjective myelogenous suggests a condition originating from
(generated by) myeloid cells. Acute myelogenous leukemia impairs the
production of erythrocytes and other mature formed elements of the
myeloid stem cell lineage. Lymphocytes arise from the lymphoid stem
cell line.
Glossary
bone marrow biopsy
diagnostic test of a sample of red bone marrow
bone marrow transplant
treatment in which a donor’s healthy bone marrow with its stem cells
replaces diseased or damaged bone marrow of a patient
colony-stimulating factors (CSFs)
glycoproteins that trigger the proliferation and differentiation of
myeloblasts into granular leukocytes (basophils, neutrophils, and
eosinophils)
cytokines
class of proteins that act as autocrine or paracrine signaling molecules;
in the cardiovascular system, they stimulate the proliferation of
progenitor cells and help to stimulate both nonspecific and specific
resistance to disease
erythropoietin (EPO)
glycoprotein that triggers the bone marrow to produce RBCs; secreted
by the kidney in response to low oxygen levels
hemocytoblast
hemopoietic stem cell that gives rise to the formed elements of blood
hemopoiesis
production of the formed elements of blood
hemopoietic growth factors
chemical signals including erythropoietin, thrombopoietin, colony-
stimulating factors, and interleukins that regulate the differentiation
and proliferation of particular blood progenitor cells
hemopoietic stem cell
type of pluripotent stem cell that gives rise to the formed elements of
blood (hemocytoblast)
interleukins
signaling molecules that may function in hemopoiesis, inflammation,
and specific immune responses
lymphoid stem cells
type of hemopoietic stem cells that gives rise to lymphocytes,
including various T cells, B cells, and NK cells, all of which function
in immunity
myeloid stem cells
type of hemopoietic stem cell that gives rise to some formed elements,
including erythrocytes, megakaryocytes that produce platelets, and a
myeloblast lineage that gives rise to monocytes and three forms of
granular leukocytes (neutrophils, eosinophils, and basophils)
pluripotent stem cell
stem cell that derives from totipotent stem cells and is capable of
differentiating into many, but not all, cell types
totipotent stem cell
embryonic stem cell that is capable of differentiating into any and all
cells of the body; enabling the full development of an organism
thrombopoietin
hormone secreted by the liver and kidneys that prompts the
development of megakaryocytes into thrombocytes (platelets)
Erythrocytes
By the end of this section, you will be able to:
e Describe the anatomy of erythrocytes
e Discuss the various steps in the lifecycle of an erythrocyte
e Explain the composition and function of hemoglobin
The erythrocyte, commonly known as a red blood cell (or RBC), is by far
the most common formed element: A single drop of blood contains millions
of erythrocytes and just thousands of leukocytes. Specifically, males have
about 5.4 million erythrocytes per microliter (uL) of blood, and females
have approximately 4.8 million per pL. In fact, erythrocytes are estimated
to make up about 25 percent of the total cells in the body. As you can
imagine, they are quite small cells, with a mean diameter of only about 7-8
micrometers (um) ({link]). The primary functions of erythrocytes are to
pick up inhaled oxygen from the lungs and transport it to the body’s tissues,
and to pick up some (about 24 percent) carbon dioxide waste at the tissues
and transport it to the lungs for exhalation. Erythrocytes remain within the
vascular network. Although leukocytes typically leave the blood vessels to
perform their defensive functions, movement of erythrocytes from the
blood vessels is abnormal.
Summary of Formed Elements in Blood
Formed
element
Numbers Comments
present per
microliter (uL)
and mean (range)
Major
subtypes
Appearance ina
standard
blood smear
Summary of
functions
Erythrocytes
(red blood
cells) .
Flattened biconcave
disk; no nucleus;
pale red color
5.2 million
(4.4-6.0 million)
Transport oxygen and
some carbon dioxide
between tissues and
lungs
Leukocytes
(white blood
cells)
7000
(5000—10,000)
Obvious dark-staining
nucleus
All function in body
defenses
Exit capillaries and
move into tissues;
lifespan of usually
a few hours or days
4360
(1800-9950)
Granulocytes
including
neutrophils,
eosinophils, and
basophils
Abundant granules in
cytoplasm; nucleus
normally lobed
Nonspecific (innate)
resistance to disease
Classified according
to membrane-bound
granules in cytoplasm
Neutrophils
4150
(1800-7300)
Nuclear lobes
increase with age;
pale lilac granules
Phagocytic;
particularly effective
against bacteria.
Release cytotoxic
chemicals from
Most common
leukocyte;
lifespan of minutes
to days
granules
Eosinophils 165 Nucleus generally Phagocytic cells; Lifespan of
(0-700) two-lobed; bright particularly effective minutes to days
La red-orange granules with antigen- antibody
2640
(1700-4950)
2185
(1500-4000)
455
(200-950)
Nucleus generally
two-lobed but difficult
to see due to
presence of heavy,
dense, dark purple
granules
Lack abundant
granules in cytoplasm;
have a simple-
shaped nucleus that
may be indented
Spherical cells with
a single often large
nucleus occupying
much of the cell’s
volume; stains purple;
seen in large
(natural killer cells) and
small (B and T cells)
variants
Largest leukocyte
with an indented or
horseshoe-shaped
nucleus
complexes. Release
antihistamines.
Increase in allergies
and parasitic infections
Promotes
inflammation
Body defenses
Primarily specific
(adaptive) immunity:
T cells directly attack
other cells (cellular
immunity); B cells
release antibodies
(humoral immunity);
natural killer cells are
similar to T cells but
nonspecific
Very effective
phagocytic cells
engulfing pathogens
or worn out cells; also
Least common
leukocyte; lifespan
unknown
Group consists of
two major cell types
from different
lineages
Initial cells originate
in bone marrow, but
secondary production
occurs in lymphatic
tissue; several distinct
subtypes; memory cells
form after exposure to
a pathogen and rapidly
increase responses to
subsequent exposure;
lifespan of many years
Produced in red bone
marrow; referred to as
macrophages after
leaving circulation
serve as antigen-
presenting cells (APCs)
for other components
of the immune system
350,000 Cellular fragments
(150,000—500,000) | surrounded by a
plasma membrane
and containing
granules; purple stain
Hemostasis plus Formed from
release growth factors }|megakaryocytes
for repair and healing |that remain in the red
of tissue bone marrow and shed
platelets into circulation
Platelets
Shape and Structure of Erythrocytes
As an erythrocyte matures in the red bone marrow, it extrudes its nucleus
and most of its other organelles. During the first day or two that it is in the
circulation, an immature erythrocyte, known as a reticulocyte, will still
typically contain remnants of organelles. Reticulocytes should comprise
approximately 1—2 percent of the erythrocyte count and provide a rough
estimate of the rate of RBC production, with abnormally low or high rates
indicating deviations in the production of these cells. These remnants,
primarily of networks (reticulum) of ribosomes, are quickly shed, however,
and mature, circulating erythrocytes have few internal cellular structural
components. Lacking mitochondria, for example, they rely on anaerobic
respiration. This means that they do not utilize any of the oxygen they are
transporting, so they can deliver it all to the tissues. They also lack
endoplasmic reticula and do not synthesize proteins. Erythrocytes do,
however, contain some structural proteins that help the blood cells maintain
their unique structure and enable them to change their shape to squeeze
through capillaries. This includes the protein spectrin, a cytoskeletal protein
element.
Erythrocytes are biconcave disks; that is, they are plump at their periphery
and very thin in the center ({link]). Since they lack most organelles, there is
more interior space for the presence of the hemoglobin molecules that, as
you will see shortly, transport gases. The biconcave shape also provides a
greater surface area across which gas exchange can occur, relative to its
volume; a sphere of a similar diameter would have a lower surface area-to-
volume ratio. In the capillaries, the oxygen carried by the erythrocytes can
diffuse into the plasma and then through the capillary walls to reach the
cells, whereas some of the carbon dioxide produced by the cells as a waste
product diffuses into the capillaries to be picked up by the erythrocytes.
Capillary beds are extremely narrow, slowing the passage of the
erythrocytes and providing an extended opportunity for gas exchange to
occur. However, the space within capillaries can be so minute that, despite
their own small size, erythrocytes may have to fold in on themselves if they
are to make their way through. Fortunately, their structural proteins like
spectrin are flexible, allowing them to bend over themselves to a surprising
degree, then spring back again when they enter a wider vessel. In wider
vessels, erythrocytes may stack up much like a roll of coins, forming a
rouleaux, from the French word for “roll.”
Shape of Red Blood Cells
Erythrocytes are biconcave
discs with very shallow centers.
This shape optimizes the ratio
of surface area to volume,
facilitating gas exchange. It also
enables them to fold up as they
move through narrow blood
vessels.
Hemoglobin
Hemoglobin is a large molecule made up of proteins and iron. It consists of
four folded chains of a protein called globin, designated alpha 1 and 2, and
beta 1 and 2 ({link]a). Each of these globin molecules is bound to a red
pigment molecule called heme, which contains an ion of iron (Fe**)
([link]b).
Hemoglobin
B chain 1 B chain 2
a chain 1
(a) A molecule of hemoglobin contains four globin
proteins, each of which is bound to one molecule of the
iron-containing pigment heme. (b) A single erythrocyte
can contain 300 million hemoglobin molecules, and thus
more than 1 billion oxygen molecules.
Each iron ion in the heme can bind to one oxygen molecule; therefore, each
hemoglobin molecule can transport four oxygen molecules. An individual
erythrocyte may contain about 300 million hemoglobin molecules, and
therefore can bind to and transport up to 1.2 billion oxygen molecules (see
[link]b).
In the lungs, hemoglobin picks up oxygen, which binds to the iron ions,
forming oxyhemoglobin. The bright red, oxygenated hemoglobin travels to
the body tissues, where it releases some of the oxygen molecules, becoming
darker red deoxyhemoglobin, sometimes referred to as reduced
hemoglobin. Oxygen release depends on the need for oxygen in the
surrounding tissues, so hemoglobin rarely if ever leaves all of its oxygen
behind. In the capillaries, carbon dioxide enters the bloodstream. About 76
percent dissolves in the plasma, some of it remaining as dissolved CO>, and
the remainder forming bicarbonate ion. About 23—24 percent of it binds to
the amino acids in hemoglobin, forming a molecule known as
carbaminohemoglobin. From the capillaries, the hemoglobin carries
carbon dioxide back to the lungs, where it releases it for exchange of
oxygen.
Changes in the levels of RBCs can have significant effects on the body’s
ability to effectively deliver oxygen to the tissues. Ineffective hematopoiesis
results in insufficient numbers of RBCs and results in one of several forms
of anemia. An overproduction of RBCs produces a condition called
polycythemia. The primary drawback with polycythemia is not a failure to
directly deliver enough oxygen to the tissues, but rather the increased
viscosity of the blood, which makes it more difficult for the heart to
circulate the blood.
In patients with insufficient hemoglobin, the tissues may not receive
sufficient oxygen, resulting in another form of anemia. In determining
oxygenation of tissues, the value of greatest interest in healthcare is the
percent saturation; that is, the percentage of hemoglobin sites occupied by
oxygen in a patient’s blood. Clinically this value is commonly referred to
simply as “percent sat.”
Percent saturation is normally monitored using a device known as a pulse
oximeter, which is applied to a thin part of the body, typically the tip of the
patient’s finger. The device works by sending two different wavelengths of
light (one red, the other infrared) through the finger and measuring the light
with a photodetector as it exits. Hemoglobin absorbs light differentially
depending upon its saturation with oxygen. The machine calibrates the
amount of light received by the photodetector against the amount absorbed
by the partially oxygenated hemoglobin and presents the data as percent
saturation. Normal pulse oximeter readings range from 95-100 percent.
Lower percentages reflect hypoxemia, or low blood oxygen. The term
hypoxia is more generic and simply refers to low oxygen levels. Oxygen
levels are also directly monitored from free oxygen in the plasma typically
following an arterial stick. When this method is applied, the amount of
oxygen present is expressed in terms of partial pressure of oxygen or simply
pO, and is typically recorded in units of millimeters of mercury, mm Hg.
The kidneys filter about 180 liters (~380 pints) of blood in an average adult
each day, or about 20 percent of the total resting volume, and thus serve as
ideal sites for receptors that determine oxygen saturation. In response to
hypoxemia, less oxygen will exit the vessels supplying the kidney, resulting
in hypoxia (low oxygen concentration) in the tissue fluid of the kidney
where oxygen concentration is actually monitored. Interstitial fibroblasts
within the kidney secrete EPO, thereby increasing erythrocyte production
and restoring oxygen levels. In a classic negative-feedback loop, as oxygen
saturation rises, EPO secretion falls, and vice versa, thereby maintaining
homeostasis. Populations dwelling at high elevations, with inherently lower
levels of oxygen in the atmosphere, naturally maintain a hematocrit higher
than people living at sea level. Consequently, people traveling to high
elevations may experience symptoms of hypoxemia, such as fatigue,
headache, and shortness of breath, for a few days after their arrival. In
response to the hypoxemia, the kidneys secrete EPO to step up the
production of erythrocytes until homeostasis is achieved once again. To
avoid the symptoms of hypoxemia, or altitude sickness, mountain climbers
typically rest for several days to a week or more at a series of camps
situated at increasing elevations to allow EPO levels and, consequently,
erythrocyte counts to rise. When climbing the tallest peaks, such as Mt.
Everest and K2 in the Himalayas, many mountain climbers rely upon
bottled oxygen as they near the summit.
Lifecycle of Erythrocytes
Production of erythrocytes in the marrow occurs at the staggering rate of
more than 2 million cells per second. For this production to occur, a number
of raw materials must be present in adequate amounts. These include the
same nutrients that are essential to the production and maintenance of any
cell, such as glucose, lipids, and amino acids. However, erythrocyte
production also requires several trace elements:
e Iron. We have said that each heme group in a hemoglobin molecule
contains an ion of the trace mineral iron. On average, less than 20
percent of the iron we consume is absorbed. Heme iron, from animal
foods such as meat, poultry, and fish, is absorbed more efficiently than
non-heme iron from plant foods. Upon absorption, iron becomes part
of the body’s total iron pool. The bone marrow, liver, and spleen can
store iron in the protein compounds ferritin and hemosiderin.
Ferroportin transports the iron across the intestinal cell plasma
membranes and from its storage sites into tissue fluid where it enters
the blood. When EPO stimulates the production of erythrocytes, iron is
released from storage, bound to transferrin, and carried to the red
marrow where it attaches to erythrocyte precursors.
e Copper. A trace mineral, copper is a component of two plasma
proteins, hephaestin and ceruloplasmin. Without these, hemoglobin
could not be adequately produced. Located in intestinal villi,
hephaestin enables iron to be absorbed by intestinal cells.
Ceruloplasmin transports copper. Both enable the oxidation of iron
from Fe** to Fe**, a form in which it can be bound to its transport
protein, transferrin, for transport to body cells. In a state of copper
deficiency, the transport of iron for heme synthesis decreases, and iron
can accumulate in tissues, where it can eventually lead to organ
damage.
e Zinc. The trace mineral zinc functions as a co-enzyme that facilitates
the synthesis of the heme portion of hemoglobin.
e B vitamins. The B vitamins folate and vitamin B,5 function as co-
enzymes that facilitate DNA synthesis. Thus, both are critical for the
synthesis of new cells, including erythrocytes.
Erythrocytes live up to 120 days in the circulation, after which the worn-out
cells are removed by a type of myeloid phagocytic cell called a
macrophage, located primarily within the bone marrow, liver, and spleen.
The components of the degraded erythrocytes’ hemoglobin are further
processed as follows:
¢ Globin, the protein portion of hemoglobin, is broken down into amino
acids, which can be sent back to the bone marrow to be used in the
production of new erythrocytes. Hemoglobin that is not phagocytized
is broken down in the circulation, releasing alpha and beta chains that
are removed from circulation by the kidneys.
e The iron contained in the heme portion of hemoglobin may be stored
in the liver or spleen, primarily in the form of ferritin or hemosiderin,
or carried through the bloodstream by transferrin to the red bone
marrow for recycling into new erythrocytes.
e The non-iron portion of heme is degraded into the waste product
biliverdin, a green pigment, and then into another waste product,
bilirubin, a yellow pigment. Bilirubin binds to albumin and travels in
the blood to the liver, which uses it in the manufacture of bile, a
compound released into the intestines to help emulsify dietary fats. In
the large intestine, bacteria breaks the bilirubin apart from the bile and
converts it to urobilinogen and then into stercobilin. It is then
eliminated from the body in the feces. Broad-spectrum antibiotics
typically eliminate these bacteria as well and may alter the color of
feces. The kidneys also remove any circulating bilirubin and other
related metabolic byproducts such as urobilins and secrete them into
the urine.
The breakdown pigments formed from the destruction of hemoglobin can
be seen in a variety of situations. At the site of an injury, biliverdin from
damaged RBCs produces some of the dramatic colors associated with
bruising. With a failing liver, bilirubin cannot be removed effectively from
circulation and causes the body to assume a yellowish tinge associated with
jaundice. Stercobilins within the feces produce the typical brown color
associated with this waste. And the yellow of urine is associated with the
urobilins.
The erythrocyte lifecycle is summarized in [link].
Erythrocyte Lifecycle
Hemopoiesis of erythrocytes begins
©) Unused heme groups can be recycled and used in in the hemopoietic bone marrow.
hemopoiesis, or can be converted into bilirubin
and used to make bile in the liver. Iron ions
can also be transferred to the protein ferritin
for storage in the liver.
Locations of hemopoietic
bone marrow
@ Stem cell
@ Erythroblast
Bilirubin Ferritin
Iron ions
Biliverdin bound
to transferrin
©) The heme portion
is broken down into
biliverdin for
transport in the
blood. The iron ions
bind to the blood
protein transferrin
for transport.
Globin
Heme oR ihS amino acids
groups and cell @) Reticulocytes are released into the
bloodstream, where they mature into
erythrocytes, which circulate for an
average of 120 days.
\/ components
Hemoglobin
protein
structure
@) Old and damaged
erythrocytes are
phagocytized by
is broken macrophages in
down into the bone marrow,
amino acids liver, and spleen.
Lysosome
@ The globin (protein) portion of hemoglobin
is metabolized into amino acids, which are
reused for protein synthesis.
Erythrocytes are produced in the bone marrow and sent into the
circulation. At the end of their lifecycle, they are destroyed by
macrophages, and their components are recycled.
Disorders of Erythrocytes
The size, shape, and number of erythrocytes, and the number of hemoglobin
molecules can have a major impact on a person’s health. When the number
of RBCs or hemoglobin is deficient, the general condition is called anemia.
There are more than 400 types of anemia and more than 3.5 million
Americans suffer from this condition. Anemia can be broken down into
three major groups: those caused by blood loss, those caused by faulty or
decreased RBC production, and those caused by excessive destruction of
RBCs. Clinicians often use two groupings in diagnosis: The kinetic
approach focuses on evaluating the production, destruction, and removal of
RBCs, whereas the morphological approach examines the RBCs
themselves, paying particular emphasis to their size. A common test is the
mean corpuscle volume (MCV), which measures size. Normal-sized cells
are referred to as normocytic, smaller-than-normal cells are referred to as
microcytic, and larger-than-normal cells are referred to as macrocytic.
Reticulocyte counts are also important and may reveal inadequate
production of RBCs. The effects of the various anemias are widespread,
because reduced numbers of RBCs or hemoglobin will result in lower levels
of oxygen being delivered to body tissues. Since oxygen is required for
tissue functioning, anemia produces fatigue, lethargy, and an increased risk
for infection. An oxygen deficit in the brain impairs the ability to think
clearly, and may prompt headaches and irritability. Lack of oxygen leaves
the patient short of breath, even as the heart and lungs work harder in
response to the deficit.
Blood loss anemias are fairly straightforward. In addition to bleeding from
wounds or other lesions, these forms of anemia may be due to ulcers,
hemorrhoids, inflammation of the stomach (gastritis), and some cancers of
the gastrointestinal tract. The excessive use of aspirin or other nonsteroidal
anti-inflammatory drugs such as ibuprofen can trigger ulceration and
gastritis. Excessive menstruation and loss of blood during childbirth are
also potential causes.
Anemias caused by faulty or decreased RBC production include sickle cell
anemia, iron deficiency anemia, vitamin deficiency anemia, and diseases of
the bone marrow and stem cells.
e A characteristic change in the shape of erythrocytes is seen in sickle
cell disease (also referred to as sickle cell anemia). A genetic disorder,
it is caused by production of an abnormal type of hemoglobin, called
hemoglobin S, which delivers less oxygen to tissues and causes
erythrocytes to assume a sickle (or crescent) shape, especially at low
oxygen concentrations ({link]). These abnormally shaped cells can
then become lodged in narrow capillaries because they are unable to
fold in on themselves to squeeze through, blocking blood flow to
tissues and causing a variety of serious problems from painful joints to
delayed growth and even blindness and cerebrovascular accidents
(strokes). Sickle cell anemia is a genetic condition particularly found
in individuals of African descent.
Sickle Cells
Sickle cell anemia is caused by
a mutation in one of the
hemoglobin genes. Erythrocytes
produce an abnormal type of
hemoglobin, which causes the
cell to take on a sickle or
crescent shape. (credit: Janice
Haney Carr)
e Iron deficiency anemia is the most common type and results when the
amount of available iron is insufficient to allow production of
sufficient heme. This condition can occur in individuals with a
deficiency of iron in the diet and is especially common in teens and
children as well as in vegans and vegetarians. Additionally, iron
deficiency anemia may be caused by either an inability to absorb and
transport iron or slow, chronic bleeding.
¢ Vitamin-deficient anemias generally involve insufficient vitamin B12
and folate.
o Megaloblastic anemia involves a deficiency of vitamin B12
and/or folate, and often involves diets deficient in these essential
nutrients. Lack of meat or a viable alternate source, and
overcooking or eating insufficient amounts of vegetables may
lead to a lack of folate.
o Pernicious anemia is caused by poor absorption of vitamin B12
and is often seen in patients with Crohn’s disease (a severe
intestinal disorder often treated by surgery), surgical removal of
the intestines or stomach (common in some weight loss
surgeries), intestinal parasites, and AIDS.
o Pregnancies, some medications, excessive alcohol consumption,
and some diseases such as celiac disease are also associated with
vitamin deficiencies. It is essential to provide sufficient folic acid
during the early stages of pregnancy to reduce the risk of
neurological defects, including spina bifida, a failure of the neural
tube to close.
e Assorted disease processes can also interfere with the production and
formation of RBCs and hemoglobin. If myeloid stem cells are
defective or replaced by cancer cells, there will be insufficient
quantities of RBCs produced.
o Aplastic anemia is the condition in which there are deficient
numbers of RBC stem cells. Aplastic anemia is often inherited, or
it may be triggered by radiation, medication, chemotherapy, or
infection.
o Thalassemia is an inherited condition typically occurring in
individuals from the Middle East, the Mediterranean, African, and
Southeast Asia, in which maturation of the RBCs does not
proceed normally. The most severe form is called Cooley’s
anemia.
o Lead exposure from industrial sources or even dust from paint
chips of iron-containing paints or pottery that has not been
properly glazed may also lead to destruction of the red marrow.
e Various disease processes also can lead to anemias. These include
chronic kidney diseases often associated with a decreased production
of EPO, hypothyroidism, some forms of cancer, lupus, and rheumatoid
arthritis.
In contrast to anemia, an elevated RBC count is called polycythemia and is
detected in a patient’s elevated hematocrit. It can occur transiently in a
person who is dehydrated; when water intake is inadequate or water losses
are excessive, the plasma volume falls. As a result, the hematocrit rises. For
reasons mentioned earlier, a mild form of polycythemia is chronic but
normal in people living at high altitudes. Some elite athletes train at high
elevations specifically to induce this phenomenon. Finally, a type of bone
marrow disease called polycythemia vera (from the Greek vera = “true”)
causes an excessive production of immature erythrocytes. Polycythemia
vera can dangerously elevate the viscosity of blood, raising blood pressure
and making it more difficult for the heart to pump blood throughout the
body. It is a relatively rare disease that occurs more often in men than
women, and is more likely to be present in elderly patients those over 60
years of age.
Chapter Review
The most abundant formed elements in blood, erythrocytes are red,
biconcave disks packed with an oxygen-carrying compound called
hemoglobin. The hemoglobin molecule contains four globin proteins bound
to a pigment molecule called heme, which contains an ion of iron. In the
bloodstream, iron picks up oxygen in the lungs and drops it off in the
tissues; the amino acids in hemoglobin then transport carbon dioxide from
the tissues back to the lungs. Erythrocytes live only 120 days on average,
and thus must be continually replaced. Worn-out erythrocytes are
phagocytized by macrophages and their hemoglobin is broken down. The
breakdown products are recycled or removed as wastes: Globin is broken
down into amino acids for synthesis of new proteins; iron is stored in the
liver or spleen or used by the bone marrow for production of new
erythrocytes; and the remnants of heme are converted into bilirubin, or
other waste products that are taken up by the liver and excreted in the bile
or removed by the kidneys. Anemia is a deficiency of RBCs or hemoglobin,
whereas polycythemia is an excess of RBCs.
Review Questions
Exercise:
Problem:
Which of the following statements about mature, circulating
erythrocytes is true?
a. They have no nucleus.
b. They are packed with mitochondria.
c. They survive for an average of 4 days.
d. All of the above
Solution:
A
Exercise:
Problem:A molecule of hemoglobin
a. is shaped like a biconcave disk packed almost entirely with iron
b. contains four glycoprotein units studded with oxygen
c. consists of four globin proteins, each bound to a molecule of
heme
d. can carry up to 120 molecules of oxygen
Solution:
C
Exercise:
Problem:
The production of healthy erythrocytes depends upon the availability
of
a. Copper
b. zinc
c. vitamin By
d. copper, zinc, and vitamin By
Solution:
D
Exercise:
Problem:
Aging and damaged erythrocytes are removed from the circulation by
a. myeoblasts
b. monocytes
c. macrophages
d. mast cells
Solution:
C
Exercise:
Problem:
A patient has been suffering for 2 months with a chronic, watery
diarrhea. A blood test is likely to reveal
a. a hematocrit below 30 percent
b. hypoxemia
c. anemia
d. polycythemia
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
A young woman has been experiencing unusually heavy menstrual
bleeding for several years. She follows a strict vegan diet (no animal
foods). She is at risk for what disorder, and why?
Solution:
She is at risk for anemia, because her unusually heavy menstrual
bleeding results in excessive loss of erythrocytes each month. At the
same time, her vegan diet means that she does not have dietary sources
of heme iron. The non-heme iron she consumes in plant foods is not as
well absorbed as heme iron.
Exercise:
Problem:
A patient has thalassemia, a genetic disorder characterized by
abnormal synthesis of globin proteins and excessive destruction of
erythrocytes. This patient is jaundiced and is found to have an
excessive level of bilirubin in his blood. Explain the connection.
Solution:
Bilirubin is a breakdown product of the non-iron component of heme,
which is cleaved from globin when erythrocytes are degraded.
Excessive erythrocyte destruction would deposit excessive bilirubin in
the blood. Bilirubin is a yellowish pigment, and high blood levels can
manifest as yellowed skin.
Glossary
anemia
deficiency of red blood cells or hemoglobin
bilirubin
yellowish bile pigment produced when iron is removed from heme and
is further broken down into waste products
biliverdin
green bile pigment produced when the non-iron portion of heme is
degraded into a waste product; converted to bilirubin in the liver
carbaminohemoglobin
compound of carbon dioxide and hemoglobin, and one of the ways in
which carbon dioxide is carried in the blood
deoxyhemoglobin
molecule of hemoglobin without an oxygen molecule bound to it
erythrocyte
(also, red blood cell) mature myeloid blood cell that is composed
mostly of hemoglobin and functions primarily in the transportation of
oxygen and carbon dioxide
ferritin
protein-containing storage form of iron found in the bone marrow,
liver, and spleen
globin
heme-containing globular protein that is a constituent of hemoglobin
heme
red, iron-containing pigment to which oxygen binds in hemoglobin
hemoglobin
oxygen-carrying compound in erythrocytes
hemosiderin
protein-containing storage form of iron found in the bone marrow,
liver, and spleen
hypoxemia
below-normal level of oxygen saturation of blood (typically <95
percent)
macrophage
phagocytic cell of the myeloid lineage; a matured monocyte
oxyhemoglobin
molecule of hemoglobin to which oxygen is bound
polycythemia
elevated level of hemoglobin, whether adaptive or pathological
reticulocyte
immature erythrocyte that may still contain fragments of organelles
sickle cell disease
(also, sickle cell anemia) inherited blood disorder in which
hemoglobin molecules are malformed, leading to the breakdown of
RBCs that take on a characteristic sickle shape
thalassemia
inherited blood disorder in which maturation of RBCs does not
proceed normally, leading to abnormal formation of hemoglobin and
the destruction of RBCs
transferrin
plasma protein that binds reversibly to iron and distributes it
throughout the body
Leukocytes and Platelets
By the end of this section, you will be able to:
e Describe the general characteristics of leukocytes
¢ Classify leukocytes according to their lineage, their main structural
features, and their primary functions
e Discuss the most common malignancies involving leukocytes
e Identify the lineage, basic structure, and function of platelets
The leukocyte, commonly known as a white blood cell (or WBC), is a
major component of the body’s defenses against disease. Leukocytes
protect the body against invading microorganisms and body cells with
mutated DNA, and they clean up debris. Platelets are essential for the repair
of blood vessels when damage to them has occurred; they also provide
growth factors for healing and repair. See [link] for a summary of
leukocytes and platelets.
Characteristics of Leukocytes
Although leukocytes and erythrocytes both originate from hematopoietic
stem cells in the bone marrow, they are very different from each other in
many significant ways. For instance, leukocytes are far less numerous than
erythrocytes: Typically there are only 5000 to 10,000 per pL. They are also
larger than erythrocytes and are the only formed elements that are complete
cells, possessing a nucleus and organelles. And although there is just one
type of erythrocyte, there are many types of leukocytes. Most of these types
have a much shorter lifespan than that of erythrocytes, some as short as a
few hours or even a few minutes in the case of acute infection.
One of the most distinctive characteristics of leukocytes is their movement.
Whereas erythrocytes spend their days circulating within the blood vessels,
leukocytes routinely leave the bloodstream to perform their defensive
functions in the body’s tissues. For leukocytes, the vascular network is
simply a highway they travel and soon exit to reach their true destination.
When they arrive, they are often given distinct names, such as macrophage
or microglia, depending on their function. As shown in [link], they leave the
capillaries—the smallest blood vessels—or other small vessels through a
process known as emigration (from the Latin for “removal”) or diapedesis
(dia- = “through”; -pedan = “to leap”) in which they squeeze through
adjacent cells in a blood vessel wall.
Once they have exited the capillaries, some leukocytes will take up fixed
positions in lymphatic tissue, bone marrow, the spleen, the thymus, or other
organs. Others will move about through the tissue spaces very much like
amoebas, continuously extending their plasma membranes, sometimes
wandering freely, and sometimes moving toward the direction in which they
are drawn by chemical signals. This attracting of leukocytes occurs because
of positive chemotaxis (literally “movement in response to chemicals”), a
phenomenon in which injured or infected cells and nearby leukocytes emit
the equivalent of a chemical “911” call, attracting more leukocytes to the
site. In clinical medicine, the differential counts of the types and
percentages of leukocytes present are often key indicators in making a
diagnosis and selecting a treatment.
Emigration
GQ) Leukocytes in the blood
respond to chemical Eosinophil
attractants released by
pathogens and
Injured/infected cells
secrete chemical signals
chemical signals from into the blood.
nearby injured cells.
Monocyte
Neutrophil : Pathogens
Leukocytes emigrate
Q@) The leukocytes squeeze to site of injury and
between the cells of infection.
the capillary wall as they
follow the chemical
signals to where they
are most concentrated
(positive chemotaxis).
Eosinophil releases
cytotoxic chemicals
from granules into
tissue.
@) Within the damaged tissue,
monocytes differentiate into
macrophages that phagocytize the
pathogens. The eosinophils and
neutrophils release chemicals that
break apart pathogens. They are
also capable of phagocytosis.
Macrophage engulfs
pathogen.
Leukocytes exit the blood vessel and then move through the
connective tissue of the dermis toward the site of a wound. Some
leukocytes, such as the eosinophil and neutrophil, are characterized
as granular leukocytes. They release chemicals from their granules
that destroy pathogens; they are also capable of phagocytosis. The
monocyte, an agranular leukocyte, differentiates into a macrophage
that then phagocytizes the pathogens.
Classification of Leukocytes
When scientists first began to observe stained blood slides, it quickly
became evident that leukocytes could be divided into two groups, according
to whether their cytoplasm contained highly visible granules:
¢ Granular leukocytes contain abundant granules within the cytoplasm.
They include neutrophils, eosinophils, and basophils (you can view
their lineage from myeloid stem cells in [link]).
e While granules are not totally lacking in agranular leukocytes, they
are far fewer and less obvious. Agranular leukocytes include
monocytes, which mature into macrophages that are phagocytic, and
lymphocytes, which arise from the lymphoid stem cell line.
Granular Leukocytes
We will consider the granular leukocytes in order from most common to
least common. All of these are produced in the red bone marrow and have a
short lifespan of hours to days. They typically have a lobed nucleus and are
classified according to which type of stain best highlights their granules
({link]).
Granular Leukocytes
és
Neutrophil Eosinophil Basophil
A neutrophil has small granules that stain
light lilac and a nucleus with two to five
lobes. An eosinophil’s granules are
slightly larger and stain reddish-orange,
and its nucleus has two to three lobes. A
basophil has large granules that stain dark
blue to purple and a two-lobed nucleus.
The most common of all the leukocytes, neutrophils will normally
comprise 50—70 percent of total leukocyte count. They are 10-12 pm in
diameter, significantly larger than erythrocytes. They are called neutrophils
because their granules show up most clearly with stains that are chemically
neutral (neither acidic nor basic). The granules are numerous but quite fine
and normally appear light lilac. The nucleus has a distinct lobed appearance
and may have two to five lobes, the number increasing with the age of the
cell. Older neutrophils have increasing numbers of lobes and are often
referred to as polymorphonuclear (a nucleus with many forms), or simply
“polys.” Younger and immature neutrophils begin to develop lobes and are
known as “bands.”
Neutrophils are rapid responders to the site of infection and are efficient
phagocytes with a preference for bacteria. Their granules include lysozyme,
an enzyme capable of lysing, or breaking down, bacterial cell walls;
oxidants such as hydrogen peroxide; and defensins, proteins that bind to
and puncture bacterial and fungal plasma membranes, so that the cell
contents leak out. Abnormally high counts of neutrophils indicate infection
and/or inflammation, particularly triggered by bacteria, but are also found
in burn patients and others experiencing unusual stress. A burn injury
increases the proliferation of neutrophils in order to fight off infection that
can result from the destruction of the barrier of the skin. Low counts may be
caused by drug toxicity and other disorders, and may increase an
individual’s susceptibility to infection.
Eosinophils typically represent 2—4 percent of total leukocyte count. They
are also 10—12 pm in diameter. The granules of eosinophils stain best with
an acidic stain known as eosin. The nucleus of the eosinophil will typically
have two to three lobes and, if stained properly, the granules will have a
distinct red to orange color.
The granules of eosinophils include antihistamine molecules, which
counteract the activities of histamines, inflammatory chemicals produced by
basophils and mast cells. Some eosinophil granules contain molecules toxic
to parasitic worms, which can enter the body through the integument, or
when an individual consumes raw or undercooked fish or meat. Eosinophils
are also capable of phagocytosis and are particularly effective when
antibodies bind to the target and form an antigen-antibody complex. High
counts of eosinophils are typical of patients experiencing allergies, parasitic
wor! infestations, and some autoimmune diseases. Low counts may be due
to drug toxicity and stress.
Basophils are the least common leukocytes, typically comprising less than
one percent of the total leukocyte count. They are slightly smaller than
neutrophils and eosinophils at 8-10 yum in diameter. The granules of
basophils stain best with basic (alkaline) stains. Basophils contain large
granules that pick up a dark blue stain and are so common they may make it
difficult to see the two-lobed nucleus.
In general, basophils intensify the inflammatory response. They share this
trait with mast cells. In the past, mast cells were considered to be basophils
that left the circulation. However, this appears not to be the case, as the two
cell types develop from different lineages.
The granules of basophils release histamines, which contribute to
inflammation, and heparin, which opposes blood clotting. High counts of
basophils are associated with allergies, parasitic infections, and
hypothyroidism. Low counts are associated with pregnancy, stress, and
hyperthyroidism.
Agranular Leukocytes
Agranular leukocytes contain smaller, less-visible granules in their
cytoplasm than do granular leukocytes. The nucleus is simple in shape,
sometimes with an indentation but without distinct lobes. There are two
major types of agranulocytes: lymphocytes and monocytes (see [link]).
Lymphocytes are the only formed element of blood that arises from
lymphoid stem cells. Although they form initially in the bone marrow,
much of their subsequent development and reproduction occurs in the
lymphatic tissues. Lymphocytes are the second most common type of
leukocyte, accounting for about 20—30 percent of all leukocytes, and are
essential for the immune response. The size range of lymphocytes is quite
extensive, with some authorities recognizing two size classes and others
three. Typically, the large cells are 10-14 ym and have a smaller nucleus-to-
cytoplasm ratio and more granules. The smaller cells are typically 6-9 pm
with a larger volume of nucleus to cytoplasm, creating a “halo” effect. A
few cells may fall outside these ranges, at 14-17 pm. This finding has led to
the three size range classification.
The three major groups of lymphocytes include natural killer cells, B cells,
and T cells. Natural killer (NK) cells are capable of recognizing cells that
do not express “self” proteins on their plasma membrane or that contain
foreign or abnormal markers. These “nonself” cells include cancer cells,
cells infected with a virus, and other cells with atypical surface proteins.
Thus, they provide generalized, nonspecific immunity. The larger
lymphocytes are typically NK cells.
B cells and T cells, also called B lymphocytes and T lymphocytes, play
prominent roles in defending the body against specific pathogens (disease-
causing microorganisms) and are involved in specific immunity. One form
of B cells (plasma cells) produces the antibodies or immunoglobulins that
bind to specific foreign or abnormal components of plasma membranes.
This is also referred to as humoral (body fluid) immunity. T cells provide
cellular-level immunity by physically attacking foreign or diseased cells. A
memory cell is a variety of both B and T cells that forms after exposure to
a pathogen and mounts rapid responses upon subsequent exposures. Unlike
other leukocytes, memory cells live for many years. B cells undergo a
maturation process in the bone marrow, whereas T cells undergo maturation
in the thymus. This site of the maturation process gives rise to the name B
and T cells. The functions of lymphocytes are complex and will be covered
in detail in the chapter covering the lymphatic system and immunity.
Smaller lymphocytes are either B or T cells, although they cannot be
differentiated in a normal blood smear.
Abnormally high lymphocyte counts are characteristic of viral infections as
well as some types of cancer. Abnormally low lymphocyte counts are
characteristic of prolonged (chronic) illness or immunosuppression,
including that caused by HIV infection and drug therapies that often involve
steroids.
Monocytes originate from myeloid stem cells. They normally represent 2—8
percent of the total leukocyte count. They are typically easily recognized by
their large size of 12—20 ym and indented or horseshoe-shaped nuclei.
Macrophages are monocytes that have left the circulation and phagocytize
debris, foreign pathogens, worn-out erythrocytes, and many other dead,
worn out, or damaged cells. Macrophages also release antimicrobial
defensins and chemotactic chemicals that attract other leukocytes to the site
of an infection. Some macrophages occupy fixed locations, whereas others
wander through the tissue fluid.
Abnormally high counts of monocytes are associated with viral or fungal
infections, tuberculosis, and some forms of leukemia and other chronic
diseases. Abnormally low counts are typically caused by suppression of the
bone marrow.
Lifecycle of Leukocytes
Most leukocytes have a relatively short lifespan, typically measured in
hours or days. Production of all leukocytes begins in the bone marrow
under the influence of CSFs and interleukins. Secondary production and
maturation of lymphocytes occurs in specific regions of lymphatic tissue
known as germinal centers. Lymphocytes are fully capable of mitosis and
may produce clones of cells with identical properties. This capacity enables
an individual to maintain immunity throughout life to many threats that
have been encountered in the past.
Disorders of Leukocytes
Leukopenia is a condition in which too few leukocytes are produced. If this
condition is pronounced, the individual may be unable to ward off disease.
Excessive leukocyte proliferation is known as leukocytosis. Although
leukocyte counts are high, the cells themselves are often nonfunctional,
leaving the individual at increased risk for disease.
Leukemia is a cancer involving an abundance of leukocytes. It may involve
only one specific type of leukocyte from either the myeloid line (myelocytic
leukemia) or the lymphoid line (lymphocytic leukemia). In chronic
leukemia, mature leukocytes accumulate and fail to die. In acute leukemia,
there is an overproduction of young, immature leukocytes. In both
conditions the cells do not function properly.
Lymphoma is a form of cancer in which masses of malignant T and/or B
lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues.
As in leukemia, the malignant leukocytes do not function properly, and the
patient is vulnerable to infection. Some forms of lymphoma tend to
progress slowly and respond well to treatment. Others tend to progress
quickly and require aggressive treatment, without which they are rapidly
fatal.
Platelets
You may occasionally see platelets referred to as thrombocytes, but
because this name suggests they are a type of cell, it is not accurate. A
platelet is not a cell but rather a fragment of the cytoplasm of a cell called a
megakaryocyte that is surrounded by a plasma membrane. Megakaryocytes
are descended from myeloid stem cells (see [link]) and are large, typically
50-100 pm in diameter, and contain an enlarged, lobed nucleus. As noted
earlier, thrombopoietin, a glycoprotein secreted by the kidneys and liver,
stimulates the proliferation of megakaryoblasts, which mature into
megakaryocytes. These remain within bone marrow tissue ([link]) and
ultimately form platelet-precursor extensions that extend through the walls
of bone matrow capillaries to release into the circulation thousands of
cytoplasmic fragments, each enclosed by a bit of plasma membrane. These
enclosed fragments are platelets. Each megakarocyte releases 2000—3000
platelets during its lifespan. Following platelet release, megakaryocyte
remnants, which are little more than a cell nucleus, are consumed by
macrophages.
Platelets are relatively small, 2-4 ym in diameter, but numerous, with
typically 150,000—160,000 per uL of blood. After entering the circulation,
approximately one-third migrate to the spleen for storage for later release in
response to any rupture in a blood vessel. They then become activated to
perform their primary function, which is to limit blood loss. Platelets
remain only about 10 days, then are phagocytized by macrophages.
Platelets are critical to hemostasis, the stoppage of blood flow following
damage to a vessel. They also secrete a variety of growth factors essential
for growth and repair of tissue, particularly connective tissue. Infusions of
concentrated platelets are now being used in some therapies to stimulate
healing.
Disorders of Platelets
Thrombocytosis is a condition in which there are too many platelets. This
may trigger formation of unwanted blood clots (thrombosis), a potentially
fatal disorder. If there is an insufficient number of platelets, called
thrombocytopenia, blood may not clot properly, and excessive bleeding
may result.
Platelets
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Leukocytes
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Basophil Eosinophil Neutrophil Monocyte Lymphocyte
(Micrographs provided by the Regents of
University of Michigan Medical School ©
2012)
View University of Michigan Webscopes at
http://virtualslides.med.umich.edu/Histology/Cardiovascular%20System/0
81-2 HISTO 40X.svs/view.apml?
cwidth=860&cheight=733&chost=virtualslides.med.umich.edu&dlistview=
1&title=&csis=1 and explore the blood slides in greater detail. The
Webscope feature allows you to move the slides as you would with a
mechanical stage. You can increase and decrease the magnification. There
is a chance to review each of the leukocytes individually after you have
attempted to identify them from the first two blood smears. In addition,
there are a few multiple choice questions.
Are you able to recognize and identify the various formed elements? You
will need to do this is a systematic manner, scanning along the image. The
standard method is to use a grid, but this is not possible with this resource.
Try constructing a simple table with each leukocyte type and then making a
mark for each cell type you identify. Attempt to classify at least 50 and
perhaps as many as 100 different cells. Based on the percentage of cells
that you count, do the numbers represent a normal blood smear or does
something appear to be abnormal?
Chapter Review
Leukocytes function in body defenses. They squeeze out of the walls of
blood vessels through emigration or diapedesis, then may move through
tissue fluid or become attached to various organs where they fight against
pathogenic organisms, diseased cells, or other threats to health. Granular
leukocytes, which include neutrophils, eosinophils, and basophils, originate
with myeloid stem cells, as do the agranular monocytes. The other
agranular leukocytes, NK cells, B cells, and T cells, arise from the
lymphoid stem cell line. The most abundant leukocytes are the neutrophils,
which are first responders to infections, especially with bacteria. About 20—
30 percent of all leukocytes are lymphocytes, which are critical to the
body’s defense against specific threats. Leukemia and lymphoma are
malignancies involving leukocytes. Platelets are fragments of cells known
as megakaryocytes that dwell within the bone marrow. While many
platelets are stored in the spleen, others enter the circulation and are
essential for hemostasis; they also produce several growth factors important
for repair and healing.
Interactive Link Questions
Exercise:
Problem:
[link] Are you able to recognize and identify the various formed
elements? You will need to do this is a systematic manner, scanning
along the image. The standard method is to use a grid, but this is not
possible with this resource. Try constructing a simple table with each
leukocyte type and then making a mark for each cell type you identify.
Attempt to classify at least 50 and perhaps as many as 100 different
cells. Based on the percentage of cells that you count, do the numbers
represent a normal blood smear or does something appear to be
abnormal?
Solution:
[link] This should appear to be a normal blood smear.
Review Questions
Exercise:
Problem:
The process by which leukocytes squeeze through adjacent cells in a
blood vessel wall is called
a. leukocytosis
b. positive chemotaxis
c. emigration
d. cytoplasmic extending
Solution:
CG
Exercise:
Problem: Which of the following describes a neutrophil?
a. abundant, agranular, especially effective against cancer cells
b. abundant, granular, especially effective against bacteria
c. rare, agranular, releases antimicrobial defensins
d. rare, granular, contains multiple granules packed with histamine
Solution:
B
Exercise:
Problem:T and B lymphocytes
a. are polymorphonuclear
b. are involved with specific immune function
c. proliferate excessively in leukopenia
d. are most active against parasitic worms
Solution:
B
Exercise:
Problem:
A patient has been experiencing severe, persistent allergy symptoms
that are reduced when she takes an antihistamine. Before the treatment,
this patient was likely to have had increased activity of which
leukocyte?
a. basophils
b. neutrophils
c. monocytes
d. natural killer cells
Solution:
A
Exercise:
Problem:Thrombocytes are more accurately called
a. clotting factors
b. megakaryoblasts
c. megakaryocytes
d. platelets
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
One of the more common adverse effects of cancer chemotherapy is
the destruction of leukocytes. Before his next scheduled chemotherapy
treatment, a patient undergoes a blood test called an absolute
neutrophil count (ANC), which reveals that his neutrophil count is
1900 cells per microliter. Would his healthcare team be likely to
proceed with his chemotherapy treatment? Why?
Solution:
A neutrophil count below 1800 cells per microliter is considered
abnormal. Thus, this patient’s ANC is at the low end of the normal
range and there would be no reason to delay chemotherapy. In clinical
practice, most patients are given chemotherapy if their ANC is above
1000.
Exercise:
Problem:
A patient was admitted to the burn unit the previous evening suffering
from a severe burn involving his left upper extremity and shoulder. A
blood test reveals that he is experiencing leukocytosis. Why is this an
expected finding?
Solution:
Any severe stress can increase the leukocyte count, resulting in
leukocytosis. A burn is especially likely to increase the proliferation of
leukocytes in order to ward off infection, a significant risk when the
barrier function of the skin is destroyed.
Glossary
agranular leukocytes
leukocytes with few granules in their cytoplasm; specifically,
monocytes, lymphocytes, and NK cells
B lymphocytes
(also, B cells) lymphocytes that defend the body against specific
pathogens and thereby provide specific immunity
basophils
granulocytes that stain with a basic (alkaline) stain and store histamine
and heparin
defensins
antimicrobial proteins released from neutrophils and macrophages that
create openings in the plasma membranes to kill cells
diapedesis
(also, emigration) process by which leukocytes squeeze through
adjacent cells in a blood vessel wall to enter tissues
emigration
(also, diapedesis) process by which leukocytes squeeze through
adjacent cells in a blood vessel wall to enter tissues
eosinophils
granulocytes that stain with eosin; they release antihistamines and are
especially active against parasitic worms
granular leukocytes
leukocytes with abundant granules in their cytoplasm; specifically,
neutrophils, eosinophils, and basophils
leukemia
cancer involving leukocytes
leukocyte
(also, white blood cell) colorless, nucleated blood cell, the chief
function of which is to protect the body from disease
leukocytosis
excessive leukocyte proliferation
leukopenia
below-normal production of leukocytes
lymphocytes
agranular leukocytes of the lymphoid stem cell line, many of which
function in specific immunity
lymphoma
form of cancer in which masses of malignant T and/or B lymphocytes
collect in lymph nodes, the spleen, the liver, and other tissues
lysozyme
digestive enzyme with bactericidal properties
megakaryocyte
bone marrow cell that produces platelets
memory cell
type of B or T lymphocyte that forms after exposure to a pathogen
monocytes
agranular leukocytes of the myeloid stem cell line that circulate in the
bloodstream; tissue monocytes are macrophages
natural killer (NK) cells
cytotoxic lymphocytes capable of recognizing cells that do not express
“self” proteins on their plasma membrane or that contain foreign or
abnormal markers; provide generalized, nonspecific immunity
neutrophils
granulocytes that stain with a neutral dye and are the most numerous
of the leukocytes; especially active against bacteria
polymorphonuclear
having a lobed nucleus, as seen in some leukocytes
positive chemotaxis
process in which a cell is attracted to move in the direction of chemical
stimuli
T lymphocytes
(also, T cells) lymphocytes that provide cellular-level immunity by
physically attacking foreign or diseased cells
thrombocytes
platelets, one of the formed elements of blood that consists of cell
fragments broken off from megakaryocytes
thrombocytopenia
condition in which there are too few platelets, resulting in abnormal
bleeding (hemophilia)
thrombocytosis
condition in which there are too many platelets, resulting in abnormal
clotting (thrombosis)
Basic Structure and Function of the Nervous System
By the end of this section, you will be able to:
e Identify the anatomical and functional divisions of the nervous system
¢ Relate the functional and structural differences between gray matter
and white matter structures of the nervous system to the structure of
neurons
e List the basic functions of the nervous system
The picture you have in your mind of the nervous system probably includes
the brain, the nervous tissue contained within the cranium, and the spinal
cord, the extension of nervous tissue within the vertebral column. That
suggests it is made of two organs—and you may not even think of the
spinal cord as an organ—but the nervous system is a very complex
structure. Within the brain, many different and separate regions are
responsible for many different and separate functions. It is as if the nervous
system is composed of many organs that all look similar and can only be
differentiated using tools such as the microscope or electrophysiology. In
comparison, it is easy to see that the stomach is different than the esophagus
or the liver, so you can imagine the digestive system as a collection of
specific organs.
The Central and Peripheral Nervous Systems
The nervous system can be divided into two major regions: the central and
peripheral nervous systems. The central nervous system (CNS) is the
brain and spinal cord, and the peripheral nervous system (PNS) is
everything else ([link]). The brain is contained within the cranial cavity of
the skull, and the spinal cord is contained within the vertebral cavity of the
vertebral column. It is a bit of an oversimplification to say that the CNS is
what is inside these two cavities and the peripheral nervous system is
outside of them, but that is one way to start to think about it. In actuality,
there are some elements of the peripheral nervous system that are within the
cranial or vertebral cavities. The peripheral nervous system is so named
because it is on the periphery—meaning beyond the brain and spinal cord.
Depending on different aspects of the nervous system, the dividing line
between central and peripheral is not necessarily universal.
Central and Peripheral Nervous System
Central Nervous System
Brain
Spinal cord
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The structures of the PNS are referred to
as ganglia and nerves, which can be seen
as distinct structures. The equivalent
structures in the CNS are not obvious
from this overall perspective and are best
examined in prepared tissue under the
microscope.
Nervous tissue, present in both the CNS and PNS, contains two basic types
of cells: neurons and glial cells. A glial cell is one of a variety of cells that
provide a framework of tissue that supports the neurons and their activities.
The neuron is the more functionally important of the two, in terms of the
communicative function of the nervous system. To describe the functional
divisions of the nervous system, it is important to understand the structure
of aneuron. Neurons are cells and therefore have a soma, or cell body, but
they also have extensions of the cell; each extension is generally referred to
as a process. There is one important process that every neuron has called an
axon, which is the fiber that connects a neuron with its target. Another type
of process that branches off from the soma is the dendrite. Dendrites are
responsible for receiving most of the input from other neurons. Looking at
nervous tissue, there are regions that predominantly contain cell bodies and
regions that are largely composed of just axons. These two regions within
nervous system structures are often referred to as gray matter (the regions
with many cell bodies and dendrites) or white matter (the regions with
many axons). [link] demonstrates the appearance of these regions in the
brain and spinal cord. The colors ascribed to these regions are what would
be seen in “fresh,” or unstained, nervous tissue. Gray matter is not
necessarily gray. It can be pinkish because of blood content, or even slightly
tan, depending on how long the tissue has been preserved. But white matter
is white because axons are insulated by a lipid-rich substance called
myelin. Lipids can appear as white (“fatty”) material, much like the fat on a
raw piece of chicken or beef. Actually, gray matter may have that color
ascribed to it because next to the white matter, it is just darker—hence,
gray.
The distinction between gray matter and white matter is most often applied
to central nervous tissue, which has large regions that can be seen with the
unaided eye. When looking at peripheral structures, often a microscope is
used and the tissue is stained with artificial colors. That is not to say that
central nervous tissue cannot be stained and viewed under a microscope,
but unstained tissue is most likely from the CNS—for example, a frontal
section of the brain or cross section of the spinal cord.
Gray Matter and White Matter
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A brain removed during an autopsy, with
a partial section removed, shows white
matter surrounded by gray matter. Gray
matter makes up the outer cortex of the
brain. (credit: modification of work by
“Suseno”/Wikimedia Commons)
Regardless of the appearance of stained or unstained tissue, the cell bodies
of neurons or axons can be located in discrete anatomical structures that
need to be named. Those names are specific to whether the structure is
central or peripheral. A localized collection of neuron cell bodies in the
CNS is referred to as a nucleus. In the PNS, a cluster of neuron cell bodies
is referred to as a ganglion. [link] indicates how the term nucleus has a few
different meanings within anatomy and physiology. It is the center of an
atom, where protons and neutrons are found; it is the center of a cell, where
the DNA is found; and it is a center of some function in the CNS. There is
also a potentially confusing use of the word ganglion (plural = ganglia) that
has a historical explanation. In the central nervous system, there is a group
of nuclei that are connected together and were once called the basal ganglia
before “ganglion” became accepted as a description for a peripheral
structure. Some sources refer to this group of nuclei as the “basal nuclei” to
avoid confusion.
What Is a Nucleus?
Nucleus
cee
Helium atom
(®) Proton
O Neutron
Electron cloud
(a) Nucleus of an atom (b) Nucleus of a cell (c) Nucleus in the brain
(a) The nucleus of an atom contains its
protons and neutrons. (b) The nucleus of a
cell is the organelle that contains DNA. (c)
A nucleus in the CNS is a localized center
of function with the cell bodies of several
neurons, shown here circled in red. (credit
c: “Was a bee”/Wikimedia Commons)
Terminology applied to bundles of axons also differs depending on location.
A bundle of axons, or fibers, found in the CNS is called a tract whereas the
same thing in the PNS would be called a nerve. There is an important point
to make about these terms, which is that they can both be used to refer to
the same bundle of axons. When those axons are in the PNS, the term is
nerve, but if they are CNS, the term is tract. The most obvious example of
this is the axons that project from the retina into the brain. Those axons are
called the optic nerve as they leave the eye, but when they are inside the
cranium, they are referred to as the optic tract. There is a specific place
where the name changes, which is the optic chiasm, but they are still the
same axons ({link]). A similar situation outside of science can be described
for some roads. Imagine a road called “Broad Street” in a town called
“Anyville.” The road leaves Anyville and goes to the next town over, called
“Hometown.” When the road crosses the line between the two towns and is
in Hometown, its name changes to “Main Street.” That is the idea behind
the naming of the retinal axons. In the PNS, they are called the optic nerve,
and in the CNS, they are the optic tract. [link] helps to clarify which of
these terms apply to the central or peripheral nervous systems.
Optic Nerve Versus Optic Tract
Left eye
Optic nerve
Optic chiasma
Optic tract
Thalamus
Midbrain
Occipital
lobes
This drawing of the connections of the
eye to the brain shows the optic nerve
extending from the eye to the chiasm,
where the structure continues as the optic
tract. The same axons extend from the eye
to the brain through these two bundles of
fibers, but the chiasm represents the
border between peripheral and central.
Note:
ree pri
wees OPenstax COLLEGE”
In 2003, the Nobel Prize in Physiology or Medicine was awarded to Paul
C. Lauterbur and Sir Peter Mansfield for discoveries related to magnetic
resonance imaging (MRI). This is a tool to see the structures of the body
(not just the nervous system) that depends on magnetic fields associated
with certain atomic nuclei. The utility of this technique in the nervous
system is that fat tissue and water appear as different shades between black
and white. Because white matter is fatty (from myelin) and gray matter is
not, they can be easily distinguished in MRI images. Visit the Nobel Prize
web site to play an interactive game that demonstrates the use of this
technology and compares it with other types of imaging technologies.
Also, the results from an MRI session are compared with images obtained
from X-ray or computed tomography. How do the imaging techniques
shown in this game indicate the separation of white and gray matter
compared with the freshly dissected tissue shown earlier?
Structures of the CNS and PNS
CNS PNS
Group of Neuron Cell Bodies (i.e., gray Mncleds Ganglion
matter)
Bundle of Axons (i.e., white matter) Tract Nerve
Functional Divisions of the Nervous System
The nervous system can also be divided on the basis of its functions, but
anatomical divisions and functional divisions are different. The CNS and
the PNS both contribute to the same functions, but those functions can be
attributed to different regions of the brain (such as the cerebral cortex or the
hypothalamus) or to different ganglia in the periphery. The problem with
trying to fit functional differences into anatomical divisions is that
sometimes the same structure can be part of several functions. For example,
the optic nerve carries signals from the retina that are either used for the
conscious perception of visual stimuli, which takes place in the cerebral
cortex, or for the reflexive responses of smooth muscle tissue that are
processed through the hypothalamus.
There are two ways to consider how the nervous system is divided
functionally. First, the basic functions of the nervous system are sensation,
integration, and response. Secondly, control of the body can be somatic or
autonomic—divisions that are largely defined by the structures that are
involved in the response. There is also a region of the peripheral nervous
system that is called the enteric nervous system that is responsible for a
specific set of the functions within the realm of autonomic control related to
gastrointestinal functions.
Basic Functions
The nervous system is involved in receiving information about the
environment around us (sensation) and generating responses to that
information (motor responses). The nervous system can be divided into
regions that are responsible for sensation (sensory functions) and for the
response (motor functions). But there is a third function that needs to be
included. Sensory input needs to be integrated with other sensations, as well
as with memories, emotional state, or learning (cognition). Some regions of
the nervous system are termed integration or association areas. The process
of integration combines sensory perceptions and higher cognitive functions
such as memories, learning, and emotion to produce a response.
Sensation. The first major function of the nervous system is sensation—
receiving information about the environment to gain input about what is
happening outside the body (or, sometimes, within the body). The sensory
functions of the nervous system register the presence of a change from
homeostasis or a particular event in the environment, known as a stimulus.
The senses we think of most are the “big five”: taste, smell, touch, sight,
and hearing. The stimuli for taste and smell are both chemical substances
(molecules, compounds, ions, etc.), touch is physical or mechanical stimuli
that interact with the skin, sight is light stimuli, and hearing is the
perception of sound, which is a physical stimulus similar to some aspects of
touch. There are actually more senses than just those, but that list represents
the major senses. Those five are all senses that receive stimuli from the
outside world, and of which there is conscious perception. Additional
sensory stimuli might be from the internal environment (inside the body),
such as the stretch of an organ wall or the concentration of certain ions in
the blood.
Response. The nervous system produces a response on the basis of the
stimuli perceived by sensory structures. An obvious response would be the
movement of muscles, such as withdrawing a hand from a hot stove, but
there are broader uses of the term. The nervous system can cause the
contraction of all three types of muscle tissue. For example, skeletal muscle
contracts to move the skeleton, cardiac muscle is influenced as heart rate
increases during exercise, and smooth muscle contracts as the digestive
system moves food along the digestive tract. Responses also include the
neural control of glands in the body as well, such as the production and
secretion of sweat by the eccrine and merocrine sweat glands found in the
skin to lower body temperature.
Responses can be divided into those that are voluntary or conscious
(contraction of skeletal muscle) and those that are involuntary (contraction
of smooth muscles, regulation of cardiac muscle, activation of glands).
Voluntary responses are governed by the somatic nervous system and
involuntary responses are governed by the autonomic nervous system,
which are discussed in the next section.
Integration. Stimuli that are received by sensory structures are
communicated to the nervous system where that information is processed.
This is called integration. Stimuli are compared with, or integrated with,
other stimuli, memories of previous stimuli, or the state of a person at a
particular time. This leads to the specific response that will be generated.
Seeing a baseball pitched to a batter will not automatically cause the batter
to swing. The trajectory of the ball and its speed will need to be considered.
Maybe the count is three balls and one strike, and the batter wants to let this
pitch go by in the hope of getting a walk to first base. Or maybe the batter’s
team is so far ahead, it would be fun to just swing away.
Controlling the Body
The nervous system can be divided into two parts mostly on the basis of a
functional difference in responses. The somatic nervous system (SNS) is
responsible for conscious perception and voluntary motor responses.
Voluntary motor response means the contraction of skeletal muscle, but
those contractions are not always voluntary in the sense that you have to
want to perform them. Some somatic motor responses are reflexes, and
often happen without a conscious decision to perform them. If your friend
jumps out from behind a corner and yells “Boo!” you will be startled and
you might scream or leap back. You didn’t decide to do that, and you may
not have wanted to give your friend a reason to laugh at your expense, but it
is areflex involving skeletal muscle contractions. Other motor responses
become automatic (in other words, unconscious) as a person learns motor
skills (referred to as “habit learning” or “procedural memory”).
The autonomic nervous system (ANS) is responsible for involuntary
control of the body, usually for the sake of homeostasis (regulation of the
internal environment). Sensory input for autonomic functions can be from
sensory structures tuned to external or internal environmental stimuli. The
motor output extends to smooth and cardiac muscle as well as glandular
tissue. The role of the autonomic system is to regulate the organ systems of
the body, which usually means to control homeostasis. Sweat glands, for
example, are controlled by the autonomic system. When you are hot,
sweating helps cool your body down. That is a homeostatic mechanism. But
when you are nervous, you might start sweating also. That is not
homeostatic, it is the physiological response to an emotional state.
There is another division of the nervous system that describes functional
responses. The enteric nervous system (ENS) is responsible for
controlling the smooth muscle and glandular tissue in your digestive
system. It is a large part of the PNS, and is not dependent on the CNS. It is
sometimes valid, however, to consider the enteric system to be a part of the
autonomic system because the neural structures that make up the enteric
system are a component of the autonomic output that regulates digestion.
There are some differences between the two, but for our purposes here there
will be a good bit of overlap. See [link] for examples of where these
divisions of the nervous system can be found.
Somatic, Autonomic, and Enteric Structures of the Nervous System
Brain (CNS)
Perception and processing of sensory
stimuli (somatic/autonomic)
Execution of voluntary motor
responses (somatic)
Regulation of homeostatic
mechanisms (autonomic)
Spinal cord (CNS)
Initiation of reflexes from ventral
horn (somatic) and lateral horn
(autonomic) gray matter
Pathways for sensory and motor
functions between periphery
and brain (somatic/autonomic)
Nerves (PNS)
Fibers of sensory and motor
neurons (somatic/autonomic)
Ganglia (PNS)
Reception of sensory stimuli by
dorsal root and cranial ganglia
(somatic/autonomic)
Relay of visceral motor responses
Digestive tract (ENS) by autonomic ganglia (autonomic)
The enteric nervous system |
(ENS), located in the digestive
tract, is responsible for autonomous
functions and can operate independently
of the brain and spinal cord.
Somatic structures include the spinal nerves, both motor and
sensory fibers, as well as the sensory ganglia (posterior root
ganglia and cranial nerve ganglia). Autonomic structures are found
in the nerves also, but include the sympathetic and parasympathetic
ganglia. The enteric nervous system includes the nervous tissue
within the organs of the digestive tract.
De era
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meee OPENStAX COLLEGE
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Visit this site to read about a woman that notices that her daughter is
having trouble walking up the stairs. This leads to the discovery of a
hereditary condition that affects the brain and spinal cord. The
electromyography and MRI tests indicated deficiencies in the spinal cord
and cerebellum, both of which are responsible for controlling coordinated
movements. To what functional division of the nervous system would these
structures belong?
Note:
Everyday Connection
How Much of Your Brain Do You Use?
Have you ever heard the claim that humans only use 10 percent of their
brains? Maybe you have seen an advertisement on a website saying that
there is a secret to unlocking the full potential of your mind—as if there
were 90 percent of your brain sitting idle, just waiting for you to use it. If
you see an ad like that, don’t click. It isn’t true.
An easy way to see how much of the brain a person uses is to take
measurements of brain activity while performing a task. An example of
this kind of measurement is functional magnetic resonance imaging
(fMRI), which generates a map of the most active areas and can be
generated and presented in three dimensions ([link]). This procedure is
different from the standard MRI technique because it is measuring changes
in the tissue in time with an experimental condition or event.
fMRI
This {MRI shows activation of the visual
cortex in response to visual stimuli.
(credit: “Superborsuk”/Wikimedia
Commons)
The underlying assumption is that active nervous tissue will have greater
blood flow. By having the subject perform a visual task, activity all over
the brain can be measured. Consider this possible experiment: the subject
is told to look at a screen with a black dot in the middle (a fixation point).
A photograph of a face is projected on the screen away from the center.
The subject has to look at the photograph and decipher what it is. The
subject has been instructed to push a button if the photograph is of
someone they recognize. The photograph might be of a celebrity, so the
subject would press the button, or it might be of a random person unknown
to the subject, so the subject would not press the button.
In this task, visual sensory areas would be active, integrating areas would
be active, motor areas responsible for moving the eyes would be active,
and motor areas for pressing the button with a finger would be active.
Those areas are distributed all around the brain and the [MRI images
would show activity in more than just 10 percent of the brain (some
evidence suggests that about 80 percent of the brain is using energy—
based on blood flow to the tissue—during well-defined tasks similar to the
one suggested above). This task does not even include all of the functions
the brain performs. There is no language response, the body is mostly lying
still in the MRI machine, and it does not consider the autonomic functions
that would be ongoing in the background.
Chapter Review
The nervous system can be separated into divisions on the basis of anatomy
and physiology. The anatomical divisions are the central and peripheral
nervous systems. The CNS is the brain and spinal cord. The PNS is
everything else. Functionally, the nervous system can be divided into those
regions that are responsible for sensation, those that are responsible for
integration, and those that are responsible for generating responses. All of
these functional areas are found in both the central and peripheral anatomy.
Considering the anatomical regions of the nervous system, there are specific
names for the structures within each division. A localized collection of
neuron cell bodies is referred to as a nucleus in the CNS and as a ganglion
in the PNS. A bundle of axons is referred to as a tract in the CNS and as a
nerve in the PNS. Whereas nuclei and ganglia are specifically in the central
or peripheral divisions, axons can cross the boundary between the two. A
single axon can be part of a nerve and a tract. The name for that specific
structure depends on its location.
Nervous tissue can also be described as gray matter and white matter on the
basis of its appearance in unstained tissue. These descriptions are more
often used in the CNS. Gray matter is where nuclei are found and white
matter is where tracts are found. In the PNS, ganglia are basically gray
matter and nerves are white matter.
The nervous system can also be divided on the basis of how it controls the
body. The somatic nervous system (SNS) is responsible for functions that
result in moving skeletal muscles. Any sensory or integrative functions that
result in the movement of skeletal muscle would be considered somatic.
The autonomic nervous system (ANS) is responsible for functions that
affect cardiac or smooth muscle tissue, or that cause glands to produce their
secretions. Autonomic functions are distributed between central and
peripheral regions of the nervous system. The sensations that lead to
autonomic functions can be the same sensations that are part of initiating
somatic responses. Somatic and autonomic integrative functions may
overlap as well.
A special division of the nervous system is the enteric nervous system,
which is responsible for controlling the digestive organs. Parts of the
autonomic nervous system overlap with the enteric nervous system. The
enteric nervous system is exclusively found in the periphery because it is
the nervous tissue in the organs of the digestive system.
Interactive Link Questions
Exercise:
Problem:
In 2003, the Nobel Prize in Physiology or Medicine was awarded to
Paul C. Lauterbur and Sir Peter Mansfield for discoveries related to
magnetic resonance imaging (MRI). This is a tool to see the structures
of the body (not just the nervous system) that depends on magnetic
fields associated with certain atomic nuclei. The utility of this
technique in the nervous system is that fat tissue and water appear as
different shades between black and white. Because white matter is
fatty (from myelin) and gray matter is not, they can be easily
distinguished in MRI images. Visit the Nobel Prize website to play an
interactive game that demonstrates the use of this technology and
compares it with other types of imaging technologies. Also, the results
from an MRI session are compared with images obtained from x-ray or
computed tomography. How do the imaging techniques shown in this
game indicate the separation of white and gray matter compared with
the freshly dissected tissue shown earlier?
Solution:
MRI uses the relative amount of water in tissue to distinguish different
areas, SO gray and white matter in the nervous system can be seen
clearly in these images.
Exercise:
Problem:
Visit this site to read about a woman that notices that her daughter is
having trouble walking up the stairs. This leads to the discovery of a
hereditary condition that affects the brain and spinal cord. The
electromyography and MRI tests indicated deficiencies in the spinal
cord and cerebellum, both of which are responsible for controlling
coordinated movements. To what functional division of the nervous
system would these structures belong?
Solution:
They are part of the somatic nervous system, which is responsible for
voluntary movements such as walking or climbing the stairs.
Review Questions
Exercise:
Problem:
Which of the following cavities contains a component of the central
nervous system?
a. abdominal
b. pelvic
c. cranial
d. thoracic
Solution:
C
Exercise:
Problem:
Which structure predominates in the white matter of the brain?
a. myelinated axons
b. neuronal cell bodies
c. ganglia of the parasympathetic nerves
d. bundles of dendrites from the enteric nervous system
Solution:
A
Exercise:
Problem:
Which part of a neuron transmits an electrical signal to a target cell?
a. dendrites
b. soma
c. cell body
d. axon
Solution:
D
Exercise:
Problem:
Which term describes a bundle of axons in the peripheral nervous
system?
a. nucleus
b. ganglion
c. tract
d. nerve
Solution:
D
Exercise:
Problem:
Which functional division of the nervous system would be responsible
for the physiological changes seen during exercise (e.g., increased
heart rate and sweating)?
a. Somatic
b. autonomic
c. enteric
d. central
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
What responses are generated by the nervous system when you run on
a treadmill? Include an example of each type of tissue that is under
nervous system control.
Solution:
Running on a treadmill involves contraction of the skeletal muscles in
the legs, increase in contraction of the cardiac muscle of the heart, and
the production and secretion of sweat in the skin to stay cool.
Exercise:
Problem:
When eating food, what anatomical and functional divisions of the
nervous system are involved in the perceptual experience?
Solution:
The sensation of taste associated with eating is sensed by nerves in the
periphery that are involved in sensory and somatic functions.
References
Kramer, PD. Listening to prozac. 1st ed. New York (NY): Penguin Books;
1993:
Glossary
autonomic nervous system (ANS)
functional division of the nervous system that is responsible for
homeostatic reflexes that coordinate control of cardiac and smooth
muscle, as well as glandular tissue
axon
single process of the neuron that carries an electrical signal (action
potential) away from the cell body toward a target cell
brain
the large organ of the central nervous system composed of white and
gray matter, contained within the cranium and continuous with the
spinal cord
central nervous system (CNS)
anatomical division of the nervous system located within the cranial
and vertebral cavities, namely the brain and spinal cord
dendrite
one of many branchlike processes that extends from the neuron cell
body and functions as a contact for incoming signals (synapses) from
other neurons or sensory cells
enteric nervous system (ENS)
neural tissue associated with the digestive system that is responsible
for nervous control through autonomic connections
ganglion
localized collection of neuron cell bodies in the peripheral nervous
system
glial cell
one of the various types of neural tissue cells responsible for
maintenance of the tissue, and largely responsible for supporting
neurons
gray matter
regions of the nervous system containing cell bodies of neurons with
few or no myelinated axons; actually may be more pink or tan in color,
but called gray in contrast to white matter
integration
nervous system function that combines sensory perceptions and higher
cognitive functions (memories, learning, emotion, etc.) to produce a
response
myelin
lipid-rich insulating substance surrounding the axons of many neurons,
allowing for faster transmission of electrical signals
nerve
cord-like bundle of axons located in the peripheral nervous system that
transmits sensory input and response output to and from the central
nervous system
neuron
neural tissue cell that is primarily responsible for generating and
propagating electrical signals into, within, and out of the nervous
system
nucleus
in the nervous system, a localized collection of neuron cell bodies that
are functionally related; a “center” of neural function
peripheral nervous system (PNS)
anatomical division of the nervous system that is largely outside the
cranial and vertebral cavities, namely all parts except the brain and
spinal cord
process
in cells, an extension of a cell body; in the case of neurons, this
includes the axon and dendrites
response
nervous system function that causes a target tissue (muscle or gland) to
produce an event as a consequence to stimuli
sensation
nervous system function that receives information from the
environment and translates it into the electrical signals of nervous
tissue
soma
in neurons, that portion of the cell that contains the nucleus; the cell
body, as opposed to the cell processes (axons and dendrites)
somatic nervous system (SNS)
functional division of the nervous system that is concerned with
conscious perception, voluntary movement, and skeletal muscle
reflexes
spinal cord
organ of the central nervous system found within the vertebral cavity
and connected with the periphery through spinal nerves; mediates
reflex behaviors
stimulus
an event in the external or internal environment that registers as
activity in a sensory neuron
tract
bundle of axons in the central nervous system having the same
function and point of origin
white matter
regions of the nervous system containing mostly myelinated axons,
making the tissue appear white because of the high lipid content of
myelin
Nervous Tissue
By the end of this section, you will be able to:
e Describe the basic structure of a neuron
e Identify the different types of neurons on the basis of polarity
e List the glial cells of the CNS and describe their function
List the glial cells of the PNS and describe their function
Nervous tissue is composed of two types of cells, neurons and glial cells.
Neurons are the primary type of cell that most anyone associates with the
nervous system. They are responsible for the computation and
communication that the nervous system provides. They are electrically
active and release chemical signals to target cells. Glial cells, or glia, are
known to play a supporting role for nervous tissue. Ongoing research
pursues an expanded role that glial cells might play in signaling, but
neurons are still considered the basis of this function. Neurons are
important, but without glial support they would not be able to perform their
function.
Neurons
Neurons are the cells considered to be the basis of nervous tissue. They are
responsible for the electrical signals that communicate information about
sensations, and that produce movements in response to those stimuli, along
with inducing thought processes within the brain. An important part of the
function of neurons is in their structure, or shape. The three-dimensional
shape of these cells makes the immense numbers of connections within the
nervous system possible.
Parts of a Neuron
As you leamed in the first section, the main part of a neuron is the cell
body, which is also known as the soma (soma = “body”). The cell body
contains the nucleus and most of the major organelles. But what makes
neurons special is that they have many extensions of their cell membranes,
which are generally referred to as processes. Neurons are usually described
as having one, and only one, axon—a fiber that emerges from the cell body
and projects to target cells. That single axon can branch repeatedly to
communicate with many target cells. It is the axon that propagates the nerve
impulse, which is communicated to one or more cells. The other processes
of the neuron are dendrites, which receive information from other neurons
at specialized areas of contact called synapses. The dendrites are usually
highly branched processes, providing locations for other neurons to
communicate with the cell body. Information flows through a neuron from
the dendrites, across the cell body, and down the axon. This gives the
neuron a polarity—meaning that information flows in this one direction.
[link] shows the relationship of these parts to one another.
Parts of a Neuron
Cell body (soma)
Axon Oligodendrocyte
Cell membrane
Dendrites
Node of Ranvier
Myelin sheath
Synapse
The major parts of the neuron are labeled
on a multipolar neuron from the CNS.
Where the axon emerges from the cell body, there is a special region
referred to as the axon hillock. This is a tapering of the cell body toward
the axon fiber. Within the axon hillock, the cytoplasm changes to a solution
of limited components called axoplasm. Because the axon hillock
represents the beginning of the axon, it is also referred to as the initial
segment.
Many axons are wrapped by an insulating substance called myelin, which is
actually made from glial cells. Myelin acts as insulation much like the
plastic or rubber that is used to insulate electrical wires. A key difference
between myelin and the insulation on a wire is that there are gaps in the
myelin covering of an axon. Each gap is called a node of Ranvier and is
important to the way that electrical signals travel down the axon. The length
of the axon between each gap, which is wrapped in myelin, is referred to as
an axon segment. At the end of the axon is the axon terminal, where there
are usually several branches extending toward the target cell, each of which
ends in an enlargement called a synaptic end bulb. These bulbs are what
make the connection with the target cell at the synapse.
Note:
ae COLLEGE
FRR eT
" Lal
[a] ate ce
Visit this site to learn about how nervous tissue is composed of neurons
and glial cells. Neurons are dynamic cells with the ability to make a vast
number of connections, to respond incredibly quickly to stimuli, and to
initiate movements on the basis of those stimuli. They are the focus of
intense research because failures in physiology can lead to devastating
illnesses. Why are neurons only found in animals? Based on what this
article says about neuron function, why wouldn't they be helpful for plants
or microorganisms?
Types of Neurons
There are many neurons in the nervous system—a number in the trillions.
And there are many different types of neurons. They can be classified by
many different criteria. The first way to classify them is by the number of
processes attached to the cell body. Using the standard model of neurons,
one of these processes is the axon, and the rest are dendrites. Because
information flows through the neuron from dendrites or cell bodies toward
the axon, these names are based on the neuron's polarity ([link]).
Neuron Classification by Shape
Bipolar neuron
Unipolar neuron
Multipolar neuron
Unipolar cells have one process that includes
both the axon and dendrite. Bipolar cells have
two processes, the axon and a dendrite.
Multipolar cells have more than two processes,
the axon and two or more dendrites.
Unipolar cells have only one process emerging from the cell. True unipolar
cells are only found in invertebrate animals, so the unipolar cells in humans
are more appropriately called “pseudo-unipolar” cells. Invertebrate unipolar
cells do not have dendrites. Human unipolar cells have an axon that
emerges from the cell body, but it splits so that the axon can extend along a
very long distance. At one end of the axon are dendrites, and at the other
end, the axon forms synaptic connections with a target. Unipolar cells are
exclusively sensory neurons and have two unique characteristics. First, their
dendrites are receiving sensory information, sometimes directly from the
stimulus itself. Secondly, the cell bodies of unipolar neurons are always
found in ganglia. Sensory reception is a peripheral function (those dendrites
are in the periphery, perhaps in the skin) so the cell body is in the periphery,
though closer to the CNS in a ganglion. The axon projects from the dendrite
endings, past the cell body in a ganglion, and into the central nervous
system.
Bipolar cells have two processes, which extend from each end of the cell
body, opposite to each other. One is the axon and one the dendrite. Bipolar
cells are not very common. They are found mainly in the olfactory
epithelium (where smell stimuli are sensed), and as part of the retina.
Multipolar neurons are all of the neurons that are not unipolar or bipolar.
They have one axon and two or more dendrites (usually many more). With
the exception of the unipolar sensory ganglion cells, and the two specific
bipolar cells mentioned above, all other neurons are multipolar. Some
cutting edge research suggests that certain neurons in the CNS do not
conform to the standard model of “one, and only one” axon. Some sources
describe a fourth type of neuron, called an anaxonic neuron. The name
suggests that it has no axon (an- = “without”), but this is not accurate.
Anaxonic neurons are very small, and if you look through a microscope at
the standard resolution used in histology (approximately 400X to 1000X
total magnification), you will not be able to distinguish any process
specifically as an axon or a dendrite. Any of those processes can function as
an axon depending on the conditions at any given time. Nevertheless, even
if they cannot be easily seen, and one specific process is definitively the
axon, these neurons have multiple processes and are therefore multipolar.
Neurons can also be classified on the basis of where they are found, who
found them, what they do, or even what chemicals they use to communicate
with each other. Some neurons referred to in this section on the nervous
system are named on the basis of those sorts of classifications ((link]). For
example, a multipolar neuron that has a very important role to play in a part
of the brain called the cerebellum is known as a Purkinje (commonly
pronounced per-KIN-gee) cell. It is named after the anatomist who
discovered it (Jan Evangilista Purkinje, 1787-1869).
Other Neuron Classifications
(a) Pyramidal cell of the (b) Purkinje cell of the (c) Olfactory cells in the olfactory
cerebral cortex cerebellar cortex epithelium and olfactory bulbs
Three examples of neurons that are classified on
the basis of other criteria. (a) The pyramidal cell is
a multipolar cell with a cell body that is shaped
something like a pyramid. (b) The Purkinje cell in
the cerebellum was named after the scientist who
originally described it. (c) Olfactory neurons are
named for the functional group with which they
belong.
Glial Cells
Glial cells, or neuroglia or simply glia, are the other type of cell found in
nervous tissue. They are considered to be supporting cells, and many
functions are directed at helping neurons complete their function for
communication. The name glia comes from the Greek word that means
“glue,” and was coined by the German pathologist Rudolph Virchow, who
wrote in 1856: “This connective substance, which is in the brain, the spinal
cord, and the special sense nerves, is a kind of glue (neuroglia) in which the
nervous elements are planted.” Today, research into nervous tissue has
shown that there are many deeper roles that these cells play. And research
may find much more about them in the future.
There are six types of glial cells. Four of them are found in the CNS and
two are found in the PNS. [link] outlines some common characteristics and
functions.
Glial Cell Types by Location and Basic Function
CNS glia PNS glia Basic function
Satellite
Astrocyte cell Support
Oligodendrocyte all Insulation, myelination
; Immune surveillance and
Microglia - ;
phagocytosis
Ependymal cell - Creating CSF
Glial Cells of the CNS
One cell providing support to neurons of the CNS is the astrocyte, so
named because it appears to be star-shaped under the microscope (astro- =
“star”). Astrocytes have many processes extending from their main cell
body (not axons or dendrites like neurons, just cell extensions). Those
processes extend to interact with neurons, blood vessels, or the connective
tissue covering the CNS that is called the pia mater ({link]). Generally, they
are supporting cells for the neurons in the central nervous system. Some
ways in which they support neurons in the central nervous system are by
maintaining the concentration of chemicals in the extracellular space,
removing excess signaling molecules, reacting to tissue damage, and
contributing to the blood-brain barrier (BBB). The blood-brain barrier is a
physiological barrier that keeps many substances that circulate in the rest of
the body from getting into the central nervous system, restricting what can
cross from circulating blood into the CNS. Nutrient molecules, such as
glucose or amino acids, can pass through the BBB, but other molecules
cannot. This actually causes problems with drug delivery to the CNS.
Pharmaceutical companies are challenged to design drugs that can cross the
BBB as well as have an effect on the nervous system.
Glial Cells of the CNS
Microglial cell if aw
Astrocyte
7} Ependymal cells al Oligodendrocytes
The CNS has astrocytes, oligodendrocytes,
microglia, and ependymal cells that support the
neurons of the CNS in several ways.
Like a few other parts of the body, the brain has a privileged blood supply.
Very little can pass through by diffusion. Most substances that cross the
wall of a blood vessel into the CNS must do so through an active transport
process. Because of this, only specific types of molecules can enter the
CNS. Glucose—the primary energy source—is allowed, as are amino acids.
Water and some other small particles, like gases and ions, can enter. But
most everything else cannot, including white blood cells, which are one of
the body’s main lines of defense. While this barrier protects the CNS from
exposure to toxic or pathogenic substances, it also keeps out the cells that
could protect the brain and spinal cord from disease and damage. The BBB
also makes it harder for pharmaceuticals to be developed that can affect the
nervous system. Aside from finding efficacious substances, the means of
delivery is also crucial.
Also found in CNS tissue is the oligodendrocyte, sometimes called just
“oligo,” which is the glial cell type that insulates axons in the CNS. The
name means “cell of a few branches” (oligo- = “few”; dendro- =
“branches”; -cyte = “cell”). There are a few processes that extend from the
cell body. Each one reaches out and surrounds an axon to insulate it in
myelin. One oligodendrocyte will provide the myelin for multiple axon
segments, either for the same axon or for separate axons. The function of
myelin will be discussed below.
Microglia are, as the name implies, smaller than most of the other glial
cells. Ongoing research into these cells, although not entirely conclusive,
suggests that they may originate as white blood cells, called macrophages,
that become part of the CNS during early development. While their origin is
not conclusively determined, their function is related to what macrophages
do in the rest of the body. When macrophages encounter diseased or
damaged cells in the rest of the body, they ingest and digest those cells or
the pathogens that cause disease. Microglia are the cells in the CNS that can
do this in normal, healthy tissue, and they are therefore also referred to as
CNS-resident macrophages.
The ependymal cell is a glial cell that filters blood to make cerebrospinal
fluid (CSF), the fluid that circulates through the CNS. Because of the
privileged blood supply inherent in the BBB, the extracellular space in
nervous tissue does not easily exchange components with the blood.
Ependymal cells line each ventricle, one of four central cavities that are
remnants of the hollow center of the neural tube formed during the
embryonic development of the brain. The choroid plexus is a specialized
structure in the ventricles where ependymal cells come in contact with
blood vessels and filter and absorb components of the blood to produce
cerebrospinal fluid. Because of this, ependymal cells can be considered a
component of the BBB, or a place where the BBB breaks down. These glial
cells appear similar to epithelial cells, making a single layer of cells with
little intracellular space and tight connections between adjacent cells. They
also have cilia on their apical surface to help move the CSF through the
ventricular space. The relationship of these glial cells to the structure of the
CNS is seen in [link].
Glial Cells of the PNS
One of the two types of glial cells found in the PNS is the satellite cell.
Satellite cells are found in sensory and autonomic ganglia, where they
surround the cell bodies of neurons. This accounts for the name, based on
their appearance under the microscope. They provide support, performing
similar functions in the periphery as astrocytes do in the CNS—except, of
course, for establishing the BBB.
The second type of glial cell is the Schwann cell, which insulate axons with
myelin in the periphery. Schwann cells are different than oligodendrocytes,
in that a Schwann cell wraps around a portion of only one axon segment
and no others. Oligodendrocytes have processes that reach out to multiple
axon segments, whereas the entire Schwann cell surrounds just one axon
segment. The nucleus and cytoplasm of the Schwann cell are on the edge of
the myelin sheath. The relationship of these two types of glial cells to
ganglia and nerves in the PNS is seen in [link].
Glial Cells of the PNS
Peripheral ganglionic
neuron cell body
(unipolar cell)
Satellite cells
Schwann cells
Axon
The PNS has satellite cells and Schwann cells.
Myelin
The insulation for axons in the nervous system is provided by glial cells,
oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the
manner in which either cell is associated with the axon segment, or
segments, that it insulates is different, the means of myelinating an axon
segment is mostly the same in the two situations. Myelin is a lipid-rich
sheath that surrounds the axon and by doing so creates a myelin sheath that
facilitates the transmission of electrical signals along the axon. The lipids
are essentially the phospholipids of the glial cell membrane. Myelin,
however, is more than just the membrane of the glial cell. It also includes
important proteins that are integral to that membrane. Some of the proteins
help to hold the layers of the glial cell membrane closely together.
The appearance of the myelin sheath can be thought of as similar to the
pastry wrapped around a hot dog for “pigs in a blanket” or a similar food.
The glial cell is wrapped around the axon several times with little to no
cytoplasm between the glial cell layers. For oligodendrocytes, the rest of the
cell is separate from the myelin sheath as a cell process extends back
toward the cell body. A few other processes provide the same insulation for
other axon segments in the area. For Schwann cells, the outermost layer of
the cell membrane contains cytoplasm and the nucleus of the cell as a bulge
on one side of the myelin sheath. During development, the glial cell is
loosely or incompletely wrapped around the axon ([link]a). The edges of
this loose enclosure extend toward each other, and one end tucks under the
other. The inner edge wraps around the axon, creating several layers, and
the other edge closes around the outside so that the axon is completely
enclosed.
cs
mess Openstax COLLEGE
View the University of Michigan WebScope to see an electron micrograph
of a cross-section of a myelinated nerve fiber. The axon contains
microtubules and neurofilaments that are bounded by a plasma membrane
known as the axolemma. Outside the plasma membrane of the axon is the
myelin sheath, which is composed of the tightly wrapped plasma
membrane of a Schwann cell. What aspects of the cells in this image react
with the stain to make them a deep, dark, black color, such as the multiple
layers that are the myelin sheath?
Myelin sheaths can extend for one or two millimeters, depending on the
diameter of the axon. Axon diameters can be as small as 1 to 20
micrometers. Because a micrometer is 1/1000 of a millimeter, this means
that the length of a myelin sheath can be 100-1000 times the diameter of
the axon. [link], [link], and [link] show the myelin sheath surrounding an
axon segment, but are not to scale. If the myelin sheath were drawn to scale,
the neuron would have to be immense—possibly covering an entire wall of
the room in which you are sitting.
The Process of Myelination
Nucleus Axon Node of Ranvier
WebScopes
Wl 15704 x 16044 size 720.85MB mag 20X
> |Gau FP SSH d Pe Oo» s
Myelin sheath
External lamina
Endonerium
(collagen)
(b)
Myelinating glia wrap several layers of cell membrane around
the cell membrane of an axon segment. A single Schwann cell
insulates a segment of a peripheral nerve, whereas in the CNS,
an oligodendrocyte may provide insulation for a few separate
axon segments. EM x 1,460,000. (Micrograph provided by the
Regents of University of Michigan Medical School © 2012)
Note:
Disorders of the...
Nervous Tissue
Several diseases can result from the demyelination of axons. The causes of
these diseases are not the same; some have genetic causes, some are caused
by pathogens, and others are the result of autoimmune disorders. Though
the causes are varied, the results are largely similar. The myelin insulation
of axons is compromised, making electrical signaling slower.
Multiple sclerosis (MS) is one such disease. It is an example of an
autoimmune disease. The antibodies produced by lymphocytes (a type of
white blood cell) mark myelin as something that should not be in the body.
This causes inflammation and the destruction of the myelin in the central
nervous system. As the insulation around the axons is destroyed by the
disease, scarring becomes obvious. This is where the name of the disease
comes from; sclerosis means hardening of tissue, which is what a scar is.
Multiple scars are found in the white matter of the brain and spinal cord.
The symptoms of MS include both somatic and autonomic deficits. Control
of the musculature is compromised, as is control of organs such as the
bladder.
Guillain-Barré (pronounced gee- YAN bah-RAY) syndrome is an example
of a demyelinating disease of the peripheral nervous system. It is also the
result of an autoimmune reaction, but the inflammation is in peripheral
nerves. Sensory symptoms or motor deficits are common, and autonomic
failures can lead to changes in the heart rhythm or a drop in blood pressure,
especially when standing, which causes dizziness.
Chapter Review
Nervous tissue contains two major cell types, neurons and glial cells.
Neurons are the cells responsible for communication through electrical
signals. Glial cells are supporting cells, maintaining the environment around
the neurons.
Neurons are polarized cells, based on the flow of electrical signals along
their membrane. Signals are received at the dendrites, are passed along the
cell body, and propagate along the axon towards the target, which may be
another neuron, muscle tissue, or a gland. Many axons are insulated by a
lipid-rich substance called myelin. Specific types of glial cells provide this
insulation.
Several types of glial cells are found in the nervous system, and they can be
categorized by the anatomical division in which they are found. In the CNS,
astrocytes, oligodendrocytes, microglia, and ependymal cells are found.
Astrocytes are important for maintaining the chemical environment around
the neuron and are crucial for regulating the blood-brain barrier.
Oligodendrocytes are the myelinating glia in the CNS. Microglia act as
phagocytes and play a role in immune surveillance. Ependymal cells are
responsible for filtering the blood to produce cerebrospinal fluid, which is a
circulatory fluid that performs some of the functions of blood in the brain
and spinal cord because of the BBB. In the PNS, satellite cells are
supporting cells for the neurons, and Schwann cells insulate peripheral
axons.
Interactive Link Questions
Exercise:
Problem:
Visit this site to learn about how nervous tissue is composed of
neurons and glial cells. The neurons are dynamic cells with the ability
to make a vast number of connections and to respond incredibly
quickly to stimuli and to initiate movements based on those stimuli.
They are the focus of intense research as failures in physiology can
lead to devastating illnesses. Why are neurons only found in animals?
Based on what this article says about neuron function, why wouldn’t
they be helpful for plants or microorganisms?
Solution:
Neurons enable thought, perception, and movement. Plants do not
move, so they do not need this type of tissue. Microorganisms are too
small to have a nervous system. Many are single-celled, and therefore
have organelles for perception and movement.
Exercise:
Problem:
View the University of Michigan Webscope to see an electron
micrograph of a cross-section of a myelinated nerve fiber. The axon
contains microtubules and neurofilaments, bounded by a plasma
membrane known as the axolemma. Outside the plasma membrane of
the axon is the myelin sheath, which is composed of the tightly
wrapped plasma membrane of a Schwann cell. What aspects of the
cells in this image react with the stain that makes them the deep, dark,
black color, such as the multiple layers that are the myelin sheath?
Solution:
Lipid membranes, such as the cell membrane and organelle
membranes.
Review Questions
Exercise:
Problem:
What type of glial cell provides myelin for the axons in a tract?
a. oligodendrocyte
b. astrocyte
c. Schwann cell
d. satellite cell
Solution:
A
Exercise:
Problem: Which part of a neuron contains the nucleus?
a. dendrite
b. soma
c. axon
d. synaptic end bulb
Solution:
B
Exercise:
Problem:
Which of the following substances is least able to cross the blood-brain
barrier?
a. water
b. sodium ions
c. glucose
d. white blood cells
Solution:
D
Exercise:
Problem:
What type of glial cell is the resident macrophage behind the blood-
brain barrier?
a. microglia
b. astrocyte
c. Schwann cell
d. satellite cell
Solution:
A
Exercise:
Problem:
What two types of macromolecules are the main components of
myelin?
a. carbohydrates and lipids
b. proteins and nucleic acids
c. lipids and proteins
d. carbohydrates and nucleic acids
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Multiple sclerosis is a demyelinating disease affecting the central
nervous system. What type of cell would be the most likely target of
this disease? Why?
Solution:
The disease would target oligodendrocytes. In the CNS,
oligodendrocytes provide the myelin for axons.
Exercise:
Problem:
Which type of neuron, based on its shape, is best suited for relaying
information directly from one neuron to another? Explain why.
Solution:
Bipolar cells, because they have one dendrite that receives input and
one axon that provides output, would be a direct relay between two
other cells.
Glossary
astrocyte
glial cell type of the CNS that provides support for neurons and
maintains the blood-brain barrier
axon hillock
tapering of the neuron cell body that gives rise to the axon
axon segment
single stretch of the axon insulated by myelin and bounded by nodes of
Ranvier at either end (except for the first, which is after the initial
segment, and the last, which is followed by the axon terminal)
axon terminal
end of the axon, where there are usually several branches extending
toward the target cell
axoplasm
cytoplasm of an axon, which is different in composition than the
cytoplasm of the neuronal cell body
bipolar
shape of a neuron with two processes extending from the neuron cell
body—the axon and one dendrite
blood-brain barrier (BBB)
physiological barrier between the circulatory system and the central
nervous system that establishes a privileged blood supply, restricting
the flow of substances into the CNS
cerebrospinal fluid (CSF)
circulatory medium within the CNS that is produced by ependymal
cells in the choroid plexus filtering the blood
choroid plexus
specialized structure containing ependymal cells that line blood
capillaries and filter blood to produce CSF in the four ventricles of the
brain
ependymal cell
glial cell type in the CNS responsible for producing cerebrospinal fluid
initial segment
first part of the axon as it emerges from the axon hillock, where the
electrical signals known as action potentials are generated
microglia
glial cell type in the CNS that serves as the resident component of the
immune system
multipolar
shape of a neuron that has multiple processes—the axon and two or
more dendrites
myelin sheath
lipid-rich layer of insulation that surrounds an axon, formed by
oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates
the transmission of electrical signals
node of Ranvier
gap between two myelinated regions of an axon, allowing for
strengthening of the electrical signal as it propagates down the axon
oligodendrocyte
glial cell type in the CNS that provides the myelin insulation for axons
in tracts
satellite cell
glial cell type in the PNS that provides support for neurons in the
ganglia
Schwann cell
glial cell type in the PNS that provides the myelin insulation for axons
in nerves
synapse
narrow junction across which a chemical signal passes from neuron to
the next, initiating a new electrical signal in the target cell
synaptic end bulb
swelling at the end of an axon where neurotransmitter molecules are
released onto a target cell across a synapse
unipolar
shape of a neuron which has only one process that includes both the
axon and dendrite
ventricle
central cavity within the brain where CSF is produced and circulates
Anatomy of the CNS
By the end of this section, you will be able to:
e Name the major regions of the adult brain
e Describe the connections between the cerebrum and brain stem
through the diencephalon, and from those regions into the spinal cord
e Recognize the complex connections within the subcortical structures
of the basal nuclei
e Explain the arrangement of gray and white matter in the spinal cord
The brain and the spinal cord are the central nervous system, and they
represent the main organs of the nervous system. The spinal cord is a single
structure, whereas the adult brain is described in terms of four major
regions: the cerebrum, the diencephalon, the brain stem, and the
cerebellum. A person’s conscious experiences are based on neural activity
in the brain. The regulation of homeostasis is governed by a specialized
region in the brain. The coordination of reflexes depends on the integration
of sensory and motor pathways in the spinal cord.
The Cerebrum
The iconic gray mantle of the human brain, which appears to make up most
of the mass of the brain, is the cerebrum ((link]). The wrinkled portion is
the cerebral cortex, and the rest of the structure is beneath that outer
covering. There is a large separation between the two sides of the cerebrum
called the longitudinal fissure. It separates the cerebrum into two distinct
halves, a right and left cerebral hemisphere. Deep within the cerebrum, the
white matter of the corpus callosum provides the major pathway for
communication between the two hemispheres of the cerebral cortex.
The Cerebrum
Cerebrum
Corpus callosum
Longitudinal
fissure
Right hemisphere
Cerebral cortex hemisphere
Lateral view Anterior view
The cerebrum is a large component of the CNS in humans, and the
most obvious aspect of it is the folded surface called the cerebral
cortex.
Many of the higher neurological functions, such as memory, emotion, and
consciousness, are the result of cerebral function. The complexity of the
cerebrum is different across vertebrate species. The cerebrum of the most
primitive vertebrates is not much more than the connection for the sense of
smell. In mammals, the cerebrum comprises the outer gray matter that is the
cortex (from the Latin word meaning “bark of a tree”) and several deep
nuclei that belong to three important functional groups. The basal nuclei
are responsible for cognitive processing, the most important function being
that associated with planning movements. The basal forebrain contains
nuclei that are important in learning and memory. The limbic cortex is the
region of the cerebral cortex that is part of the limbic system, a collection
of structures involved in emotion, memory, and behavior.
Cerebral Cortex
The cerebrum is covered by a continuous layer of gray matter that wraps
around either side of the forebrain—the cerebral cortex. This thin, extensive
region of wrinkled gray matter is responsible for the higher functions of the
nervous system. A gyrus (plural = gyri) is the ridge of one of those
wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The
pattern of these folds of tissue indicates specific regions of the cerebral
cortex.
The head is limited by the size of the birth canal, and the brain must fit
inside the cranial cavity of the skull. Extensive folding in the cerebral
cortex enables more gray matter to fit into this limited space. If the gray
matter of the cortex were peeled off of the cerebrum and laid out flat, its
surface area would be roughly equal to one square meter.
The folding of the cortex maximizes the amount of gray matter in the
cranial cavity. During embryonic development, as the telencephalon
expands within the skull, the brain goes through a regular course of growth
that results in everyone’s brain having a similar pattern of folds. The surface
of the brain can be mapped on the basis of the locations of large gyri and
sulci. Using these landmarks, the cortex can be separated into four major
regions, or lobes ((link]). The lateral sulcus that separates the temporal
lobe from the other regions is one such landmark. Superior to the lateral
sulcus are the parietal lobe and frontal lobe, which are separated from
each other by the central sulcus. The posterior region of the cortex is the
occipital lobe, which has no obvious anatomical border between it and the
parietal or temporal lobes on the lateral surface of the brain. From the
medial surface, an obvious landmark separating the parietal and occipital
lobes is called the parieto-occipital sulcus. The fact that there is no
obvious anatomical border between these lobes is consistent with the
functions of these regions being interrelated.
Lobes of the Cerebral Cortex
Central sulcus
Precentral gyrus Postcentral gyrus
Frontal lobe Parietal lobe
Parieto-occipital
sulcus
Lateral sulcus
Occipital lobe
Temporal lobe
The cerebral cortex is divided into four
lobes. Extensive folding increases the
surface area available for cerebral
functions.
Different regions of the cerebral cortex can be associated with particular
functions, a concept known as localization of function. In the early 1900s, a
German neuroscientist named Korbinian Brodmann performed an extensive
study of the microscopic anatomy—the cytoarchitecture—of the cerebral
cortex and divided the cortex into 52 separate regions on the basis of the
histology of the cortex. His work resulted in a system of classification
known as Brodmann’s areas, which is still used today to describe the
anatomical distinctions within the cortex ({link]). The results from
Brodmann’s work on the anatomy align very well with the functional
differences within the cortex. Areas 17 and 18 in the occipital lobe are
responsible for primary visual perception. That visual information is
complex, so it is processed in the temporal and parietal lobes as well.
The temporal lobe is associated with primary auditory sensation, known as
Brodmann’s areas 41 and 42 in the superior temporal lobe. Because regions
of the temporal lobe are part of the limbic system, memory is an important
function associated with that lobe. Memory is essentially a sensory
function; memories are recalled sensations such as the smell of Mom’s
baking or the sound of a barking dog. Even memories of movement are
really the memory of sensory feedback from those movements, such as
stretching muscles or the movement of the skin around a joint. Structures in
the temporal lobe are responsible for establishing long-term memory, but
the ultimate location of those memories is usually in the region in which the
sensory perception was processed.
The main sensation associated with the parietal lobe is somatosensation,
meaning the general sensations associated with the body. Posterior to the
central sulcus is the postcentral gyrus, the primary somatosensory cortex,
which is identified as Brodmann’s areas 1, 2, and 3. All of the tactile senses
are processed in this area, including touch, pressure, tickle, pain, itch, and
vibration, as well as more general senses of the body such as
proprioception and kinesthesia, which are the senses of body position and
movement, respectively.
Anterior to the central sulcus is the frontal lobe, which is primarily
associated with motor functions. The precentral gyrus is the primary
motor cortex. Cells from this region of the cerebral cortex are the upper
motor neurons that instruct cells in the spinal cord to move skeletal
muscles. Anterior to this region are a few areas that are associated with
planned movements. The premotor area is responsible for thinking of a
movement to be made. The frontal eye fields are important in eliciting eye
movements and in attending to visual stimuli. Broca’s area is responsible
for the production of language, or controlling movements responsible for
speech; in the vast majority of people, it is located only on the left side.
Anterior to these regions is the prefrontal lobe, which serves cognitive
functions that can be the basis of personality, short-term memory, and
consciousness. The prefrontal lobotomy is an outdated mode of treatment
for personality disorders (psychiatric conditions) that profoundly affected
the personality of the patient.
Brodmann's Areas of the Cerebral Cortex
Areas 1, 2,3
Primary
somatosensory
cortex
Area 4
Primary motor cortex
Areas 44, 45
Broca’s area
Area 4
Primary motor
cortex
Areas 39, 40
Wernicke’s area
d
Area 22 % Area 17
: ; SB oe ee ok A :
Primary auditory “Sel fe 2» Primary visual cortex
cortex
Brodmann’s cytotechtonic map (1909): Brodmann’s cytotechtonic map (1909):
Lateral surface Medial surface
Brodmann mapping of functionally distinct regions of the
cortex was based on its cytoarchitecture at a microscopic level.
Subcortical structures
Beneath the cerebral cortex are sets of nuclei known as subcortical nuclei
that augment cortical processes. The nuclei of the basal forebrain serve as
the primary location for acetylcholine production, which modulates the
overall activity of the cortex, possibly leading to greater attention to sensory
stimuli. Alzheimer’s disease is associated with a loss of neurons in the basal
forebrain. The hippocampus and amygdala are medial-lobe structures that,
along with the adjacent cortex, are involved in long-term memory formation
and emotional responses. The basal nuclei are a set of nuclei in the
cerebrum responsible for comparing cortical processing with the general
state of activity in the nervous system to influence the likelihood of
movement taking place. For example, while a student is sitting in a
classroom listening to a lecture, the basal nuclei will keep the urge to jump
up and scream from actually happening. (The basal nuclei are also referred
to as the basal ganglia, although that is potentially confusing because the
term ganglia is typically used for peripheral structures.)
The major structures of the basal nuclei that control movement are the
caudate, putamen, and globus pallidus, which are located deep in the
cerebrum. The caudate is a long nucleus that follows the basic C-shape of
the cerebrum from the frontal lobe, through the parietal and occipital lobes,
into the temporal lobe. The putamen is mostly deep in the anterior regions
of the frontal and parietal lobes. Together, the caudate and putamen are
called the striatum. The globus pallidus is a layered nucleus that lies just
medial to the putamen; they are called the lenticular nuclei because they
look like curved pieces fitting together like lenses. The globus pallidus has
two subdivisions, the external and internal segments, which are lateral and
medial, respectively. These nuclei are depicted in a frontal section of the
brain in [link].
Frontal Section of Cerebral Cortex and Basal Nuclei
Lateral ventricle
Striatum:
Caudate
Putamen
Corpus
callosum
Globus pallidus
Frontal section
The major components of the basal
nuclei, shown in a frontal section of the
brain, are the caudate (just lateral to the
lateral ventricle), the putamen (inferior to
the caudate and separated by the large
white-matter structure called the internal
capsule), and the globus pallidus (medial
to the putamen).
The basal nuclei in the cerebrum are connected with a few more nuclei in
the brain stem that together act as a functional group that forms a motor
pathway. Two streams of information processing take place in the basal
nuclei. All input to the basal nuclei is from the cortex into the striatum
({link]). The direct pathway is the projection of axons from the striatum to
the globus pallidus internal segment (GPi) and the substantia nigra pars
reticulata (SNr). The GPi/SNr then projects to the thalamus, which projects
back to the cortex. The indirect pathway is the projection of axons from
the striatum to the globus pallidus external segment (GPe), then to the
subthalamic nucleus (STN), and finally to GPi/SNr. The two streams both
target the GPi/SNr, but one has a direct projection and the other goes
through a few intervening nuclei. The direct pathway causes the
disinhibition of the thalamus (inhibition of one cell on a target cell that
then inhibits the first cell), whereas the indirect pathway causes, or
reinforces, the normal inhibition of the thalamus. The thalamus then can
either excite the cortex (as a result of the direct pathway) or fail to excite
the cortex (as a result of the indirect pathway).
Connections of Basal Nuclei
Basal nuclei
} t = Glutamate
= Dopamine
A
i
[Thalarws |<—]
Input to the basal nuclei is from
the cerebral cortex, which is an
excitatory connection releasing
glutamate as a
neurotransmitter. This input is
to the striatum, or the caudate
and putamen. In the direct
pathway, the striatum projects
to the internal segment of the
globus pallidus and the
substantia nigra pars reticulata
(GPi/SNr). This is an inhibitory
pathway, in which GABA is
released at the synapse, and the
target cells are hyperpolarized
and less likely to fire. The
output from the basal nuclei is
to the thalamus, which is an
inhibitory projection using
GABA.
The switch between the two pathways is the substantia nigra pars
compacta, which projects to the striatum and releases the neurotransmitter
dopamine. Dopamine receptors are either excitatory (D1-type receptors) or
inhibitory (D2-type receptors). The direct pathway is activated by
dopamine, and the indirect pathway is inhibited by dopamine. When the
substantia nigra pars compacta is firing, it signals to the basal nuclei that the
body is in an active state, and movement will be more likely. When the
substantia nigra pars compacta is silent, the body is in a passive state, and
movement is inhibited. To illustrate this situation, while a student is sitting
listening to a lecture, the substantia nigra pars compacta would be silent and
the student less likely to get up and walk around. Likewise, while the
professor is lecturing, and walking around at the front of the classroom, the
professor’s substantia nigra pars compacta would be active, in keeping with
his or her activity level.
Watch this video to learn about the basal nuclei (also known as the basal
ganglia), which have two pathways that process information within the
cerebrum. As shown in this video, the direct pathway is the shorter
pathway through the system that results in increased activity in the cerebral
cortex and increased motor activity. The direct pathway is described as
resulting in “disinhibition” of the thalamus. What does disinhibition mean?
What are the two neurons doing individually to cause this?
Note:
[elie
—
mss Openstax COLLEGE
ro-e."
Watch this video to learn about the basal nuclei (also known as the basal
ganglia), which have two pathways that process information within the
cerebrum. As shown in this video, the indirect pathway is the longer
pathway through the system that results in decreased activity in the
cerebral cortex, and therefore less motor activity. The indirect pathway has
an extra couple of connections in it, including disinhibition of the
subthalamic nucleus. What is the end result on the thalamus, and therefore
on movement initiated by the cerebral cortex?
Note:
Everyday Connections
The Myth of Left Brain/Right Brain
There is a persistent myth that people are “right-brained” or “left-brained,”
which is an oversimplification of an important concept about the cerebral
hemispheres. There is some lateralization of function, in which the left side
of the brain is devoted to language function and the right side is devoted to
spatial and nonverbal reasoning. Whereas these functions are
predominantly associated with those sides of the brain, there is no
monopoly by either side on these functions. Many pervasive functions,
such as language, are distributed globally around the cerebrum.
Some of the support for this misconception has come from studies of split
brains. A drastic way to deal with a rare and devastating neurological
condition (intractable epilepsy) is to separate the two hemispheres of the
brain. After sectioning the corpus callosum, a split-brained patient will
have trouble producing verbal responses on the basis of sensory
information processed on the right side of the cerebrum, leading to the idea
that the left side is responsible for language function.
However, there are well-documented cases of language functions lost from
damage to the right side of the brain. The deficits seen in damage to the left
side of the brain are classified as aphasia, a loss of speech function;
damage on the right side can affect the use of language. Right-side damage
can result in a loss of ability to understand figurative aspects of speech,
such as jokes, irony, or metaphors. Nonverbal aspects of speech can be
affected by damage to the right side, such as facial expression or body
language, and right-side damage can lead to a “flat affect” in speech, or a
loss of emotional expression in speech—sounding like a robot when
talking.
The Diencephalon
The diencephalon is the one region of the adult brain that retains its name
from embryologic development. The etymology of the word diencephalon
translates to “through brain.” It is the connection between the cerebrum and
the rest of the nervous system, with one exception. The rest of the brain, the
spinal cord, and the PNS all send information to the cerebrum through the
diencephalon. Output from the cerebrum passes through the diencephalon.
The single exception is the system associated with olfaction, or the sense of
smell, which connects directly with the cerebrum. In the earliest vertebrate
species, the cerebrum was not much more than olfactory bulbs that received
peripheral information about the chemical environment (to call it smell in
these organisms is imprecise because they lived in the ocean).
The diencephalon is deep beneath the cerebrum and constitutes the walls of
the third ventricle. The diencephalon can be described as any region of the
brain with “thalamus” in its name. The two major regions of the
diencephalon are the thalamus itself and the hypothalamus ([link]). There
are other structures, such as the epithalamus, which contains the pineal
gland, or the subthalamus, which includes the subthalamic nucleus that is
part of the basal nuclei.
Thalamus
The thalamus is a collection of nuclei that relay information between the
cerebral cortex and the periphery, spinal cord, or brain stem. All sensory
information, except for the sense of smell, passes through the thalamus
before processing by the cortex. Axons from the peripheral sensory organs,
or intermediate nuclei, synapse in the thalamus, and thalamic neurons
project directly to the cerebrum. It is a requisite synapse in any sensory
pathway, except for olfaction. The thalamus does not just pass the
information on, it also processes that information. For example, the portion
of the thalamus that receives visual information will influence what visual
stimuli are important, or what receives attention.
The cerebrum also sends information down to the thalamus, which usually
communicates motor commands. This involves interactions with the
cerebellum and other nuclei in the brain stem. The cerebrum interacts with
the basal nuclei, which involves connections with the thalamus. The
primary output of the basal nuclei is to the thalamus, which relays that
output to the cerebral cortex. The cortex also sends information to the
thalamus that will then influence the effects of the basal nuclei.
Hypothalamus
Inferior and slightly anterior to the thalamus is the hypothalamus, the other
major region of the diencephalon. The hypothalamus is a collection of
nuclei that are largely involved in regulating homeostasis. The
hypothalamus is the executive region in charge of the autonomic nervous
system and the endocrine system through its regulation of the anterior
pituitary gland. Other parts of the hypothalamus are involved in memory
and emotion as part of the limbic system.
The Diencephalon
Thalamus
Hypothalamus
Pituitary gland
The diencephalon is composed primarily of the
thalamus and hypothalamus, which together define
the walls of the third ventricle. The thalami are
two elongated, ovoid structures on either side of
the midline that make contact in the middle. The
hypothalamus is inferior and anterior to the
thalamus, culminating in a sharp angle to which
the pituitary gland is attached.
Brain Stem
The midbrain and hindbrain (composed of the pons and the medulla) are
collectively referred to as the brain stem ({link]). The structure emerges
from the ventral surface of the forebrain as a tapering cone that connects the
brain to the spinal cord. Attached to the brain stem, but considered a
separate region of the adult brain, is the cerebellum. The midbrain
coordinates sensory representations of the visual, auditory, and
somatosensory perceptual spaces. The pons is the main connection with the
cerebellum. The pons and the medulla regulate several crucial functions,
including the cardiovascular and respiratory systems and rates.
The cranial nerves connect through the brain stem and provide the brain
with the sensory input and motor output associated with the head and neck,
including most of the special senses. The major ascending and descending
pathways between the spinal cord and brain, specifically the cerebrum, pass
through the brain stem.
The Brain Stem
Midbrain
Pons
Medulla
The brain stem comprises three regions: the
midbrain, the pons, and the medulla.
Midbrain
One of the original regions of the embryonic brain, the midbrain is a small
region between the thalamus and pons. It is separated into the tectum and
tegmentum, from the Latin words for roof and floor, respectively. The
cerebral aqueduct passes through the center of the midbrain, such that these
regions are the roof and floor of that canal.
The tectum is composed of four bumps known as the colliculi (singular =
colliculus), which means “little hill” in Latin. The inferior colliculus is the
inferior pair of these enlargements and is part of the auditory brain stem
pathway. Neurons of the inferior colliculus project to the thalamus, which
then sends auditory information to the cerebrum for the conscious
perception of sound. The superior colliculus is the superior pair and
combines sensory information about visual space, auditory space, and
somatosensory space. Activity in the superior colliculus is related to
orienting the eyes to a sound or touch stimulus. If you are walking along the
sidewalk on campus and you hear chirping, the superior colliculus
coordinates that information with your awareness of the visual location of
the tree right above you. That is the correlation of auditory and visual maps.
If you suddenly feel something wet fall on your head, your superior
colliculus integrates that with the auditory and visual maps and you know
that the chirping bird just relieved itself on you. You want to look up to see
the culprit, but do not.
The tegmentum is continuous with the gray matter of the rest of the brain
stem. Throughout the midbrain, pons, and medulla, the tegmentum contains
the nuclei that receive and send information through the cranial nerves, as
well as regions that regulate important functions such as those of the
cardiovascular and respiratory systems.
Pons
The word pons comes from the Latin word for bridge. It is visible on the
anterior surface of the brain stem as the thick bundle of white matter
attached to the cerebellum. The pons is the main connection between the
cerebellum and the brain stem. The bridge-like white matter is only the
anterior surface of the pons; the gray matter beneath that is a continuation
of the tegmentum from the midbrain. Gray matter in the tegmentum region
of the pons contains neurons receiving descending input from the forebrain
that is sent to the cerebellum.
Medulla
The medulla is the region known as the myelencephalon in the embryonic
brain. The initial portion of the name, “myel,” refers to the significant white
matter found in this region—especially on its exterior, which is continuous
with the white matter of the spinal cord. The tegmentum of the midbrain
and pons continues into the medulla because this gray matter is responsible
for processing cranial nerve information. A diffuse region of gray matter
throughout the brain stem, known as the reticular formation, is related to
sleep and wakefulness, such as general brain activity and attention.
The Cerebellum
The cerebellum, as the name suggests, is the “little brain.” It is covered in
gyri and sulci like the cerebrum, and looks like a miniature version of that
part of the brain ([link]). The cerebellum is largely responsible for
comparing information from the cerebrum with sensory feedback from the
periphery through the spinal cord. It accounts for approximately 10 percent
of the mass of the brain.
The Cerebellum
Cerebellum
Deep cerebellar
white matter
(arbor vitae)
Inferior olive
The cerebellum is situated on the posterior
surface of the brain stem. Descending input
from the cerebellum enters through the large
white matter structure of the pons. Ascending
input from the periphery and spinal cord enters
through the fibers of the inferior olive. Output
goes to the midbrain, which sends a
descending signal to the spinal cord.
Descending fibers from the cerebrum have branches that connect to neurons
in the pons. Those neurons project into the cerebellum, providing a copy of
motor commands sent to the spinal cord. Sensory information from the
periphery, which enters through spinal or cranial nerves, is copied to a
nucleus in the medulla known as the inferior olive. Fibers from this nucleus
enter the cerebellum and are compared with the descending commands
from the cerebrum. If the primary motor cortex of the frontal lobe sends a
command down to the spinal cord to initiate walking, a copy of that
instruction is sent to the cerebellum. Sensory feedback from the muscles
and joints, proprioceptive information about the movements of walking, and
sensations of balance are sent to the cerebellum through the inferior olive
and the cerebellum compares them. If walking is not coordinated, perhaps
because the ground is uneven or a strong wind is blowing, then the
cerebellum sends out a corrective command to compensate for the
difference between the original cortical command and the sensory feedback.
The output of the cerebellum is into the midbrain, which then sends a
descending input to the spinal cord to correct the messages going to skeletal
muscles.
The Spinal Cord
The description of the CNS is concentrated on the structures of the brain,
but the spinal cord is another major organ of the system. Whereas the brain
develops out of expansions of the neural tube into primary and then
secondary vesicles, the spinal cord maintains the tube structure and is only
specialized into certain regions. As the spinal cord continues to develop in
the newborn, anatomical features mark its surface. The anterior midline is
marked by the anterior median fissure, and the posterior midline is
marked by the posterior median sulcus. Axons enter the posterior side
through the dorsal (posterior) nerve root, which marks the posterolateral
sulcus on either side. The axons emerging from the anterior side do so
through the ventral (anterior) nerve root. Note that it is common to see
the terms dorsal (dorsal = “back”) and ventral (ventral = “belly”) used
interchangeably with posterior and anterior, particularly in reference to
nerves and the structures of the spinal cord. You should learn to be
comfortable with both.
On the whole, the posterior regions are responsible for sensory functions
and the anterior regions are associated with motor functions. This comes
from the initial development of the spinal cord, which is divided into the
basal plate and the alar plate. The basal plate is closest to the ventral
midline of the neural tube, which will become the anterior face of the spinal
cord and gives rise to motor neurons. The alar plate is on the dorsal side of
the neural tube and gives rise to neurons that will receive sensory input
from the periphery.
The length of the spinal cord is divided into regions that correspond to the
regions of the vertebral column. The name of a spinal cord region
corresponds to the level at which spinal nerves pass through the
intervertebral foramina. Immediately adjacent to the brain stem is the
cervical region, followed by the thoracic, then the lumbar, and finally the
sacral region. The spinal cord is not the full length of the vertebral column
because the spinal cord does not grow significantly longer after the first or
second year, but the skeleton continues to grow. The nerves that emerge
from the spinal cord pass through the intervertebral formina at the
respective levels. As the vertebral column grows, these nerves grow with it
and result in a long bundle of nerves that resembles a horse’s tail and is
named the cauda equina. The sacral spinal cord is at the level of the upper
lumbar vertebral bones. The spinal nerves extend from their various levels
to the proper level of the vertebral column.
Gray Horns
In cross-section, the gray matter of the spinal cord has the appearance of an
ink-blot test, with the spread of the gray matter on one side replicated on the
other—a shape reminiscent of a bulbous capital “H.” As shown in [link],
the gray matter is subdivided into regions that are referred to as horns. The
posterior horn is responsible for sensory processing. The anterior horn
sends out motor signals to the skeletal muscles. The lateral horn, which is
only found in the thoracic, upper lumbar, and sacral regions, is the central
component of the sympathetic division of the autonomic nervous system.
Some of the largest neurons of the spinal cord are the multipolar motor
neurons in the anterior horn. The fibers that cause contraction of skeletal
muscles are the axons of these neurons. The motor neuron that causes
contraction of the big toe, for example, is located in the sacral spinal cord.
The axon that has to reach all the way to the belly of that muscle may be a
meter in length. The neuronal cell body that maintains that long fiber must
be quite large, possibly several hundred micrometers in diameter, making it
one of the largest cells in the body.
Cross-section of Spinal Cord
Posterior (dorsal)
columns
Gray matter:
Posterior (dorsal) ~¢
horn
Lateral columns
Lateral horn
Central canal
Anterior (ventral)
Anterior (ventral) columns
horn
The cross-section of a thoracic spinal cord
segment shows the posterior, anterior, and
lateral horns of gray matter, as well as the
posterior, anterior, and lateral columns of
white matter. LM x 40. (Micrograph
provided by the Regents of University of
Michigan Medical School © 2012)
White Columns
Just as the gray matter is separated into horns, the white matter of the spinal
cord is separated into columns. Ascending tracts of nervous system fibers
in these columns carry sensory information up to the brain, whereas
descending tracts carry motor commands from the brain. Looking at the
spinal cord longitudinally, the columns extend along its length as
continuous bands of white matter. Between the two posterior horns of gray
matter are the posterior columns. Between the two anterior horns, and
bounded by the axons of motor neurons emerging from that gray matter
area, are the anterior columns. The white matter on either side of the
spinal cord, between the posterior horn and the axons of the anterior horn
neurons, are the lateral columns. The posterior columns are composed of
axons of ascending tracts. The anterior and lateral columns are composed of
many different groups of axons of both ascending and descending tracts—
the latter carrying motor commands down from the brain to the spinal cord
to control output to the periphery.
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Watch this video to learn about the gray matter of the spinal cord that
receives input from fibers of the dorsal (posterior) root and sends
information out through the fibers of the ventral (anterior) root. As
discussed in this video, these connections represent the interactions of the
CNS with peripheral structures for both sensory and motor functions. The
cervical and lumbar spinal cords have enlargements as a result of larger
populations of neurons. What are these enlargements responsible for?
Note:
Disorders of the...
Basal Nuclei
Parkinson’s disease is a disorder of the basal nuclei, specifically of the
substantia nigra, that demonstrates the effects of the direct and indirect
pathways. Parkinson’s disease is the result of neurons in the substantia
nigra pars compacta dying. These neurons release dopamine into the
striatum. Without that modulatory influence, the basal nuclei are stuck in
the indirect pathway, without the direct pathway being activated. The direct
pathway is responsible for increasing cortical movement commands. The
increased activity of the indirect pathway results in the hypokinetic
disorder of Parkinson’s disease.
Parkinson’s disease is neurodegenerative, meaning that neurons die that
cannot be replaced, so there is no cure for the disorder. Treatments for
Parkinson’s disease are aimed at increasing dopamine levels in the
striatum. Currently, the most common way of doing that is by providing
the amino acid L-DOPA, which is a precursor to the neurotransmitter
dopamine and can cross the blood-brain barrier. With levels of the
precursor elevated, the remaining cells of the substantia nigra pars
compacta can make more neurotransmitter and have a greater effect.
Unfortunately, the patient will become less responsive to L-DOPA
treatment as time progresses, and it can cause increased dopamine levels
elsewhere in the brain, which are associated with psychosis or
schizophrenia.
Note:
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Visit this site for a thorough explanation of Parkinson’s disease.
Note:
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Compared with the nearest evolutionary relative, the chimpanzee, the
human has a brain that is huge. At a point in the past, a common ancestor
gave rise to the two species of humans and chimpanzees. That evolutionary
history is long and is still an area of intense study. But something happened
to increase the size of the human brain relative to the chimpanzee. Read
this article in which the author explores the current understanding of why
this happened.
According to one hypothesis about the expansion of brain size, what tissue
might have been sacrificed so energy was available to grow our larger
brain? Based on what you know about that tissue and nervous tissue, why
would there be a trade-off between them in terms of energy use?
Chapter Review
The adult brain is separated into four major regions: the cerebrum, the
diencephalon, the brain stem, and the cerebellum. The cerebrum is the
largest portion and contains the cerebral cortex and subcortical nuclei. It is
divided into two halves by the longitudinal fissure.
The cortex is separated into the frontal, parietal, temporal, and occipital
lobes. The frontal lobe is responsible for motor functions, from planning
movements through executing commands to be sent to the spinal cord and
periphery. The most anterior portion of the frontal lobe is the prefrontal
cortex, which is associated with aspects of personality through its influence
on motor responses in decision-making.
The other lobes are responsible for sensory functions. The parietal lobe is
where somatosensation is processed. The occipital lobe is where visual
processing begins, although the other parts of the brain can contribute to
visual function. The temporal lobe contains the cortical area for auditory
processing, but also has regions crucial for memory formation.
Nuclei beneath the cerebral cortex, known as the subcortical nuclei, are
responsible for augmenting cortical functions. The basal nuclei receive
input from cortical areas and compare it with the general state of the
individual through the activity of a dopamine-releasing nucleus. The output
influences the activity of part of the thalamus that can then increase or
decrease cortical activity that often results in changes to motor commands.
The basal forebrain is responsible for modulating cortical activity in
attention and memory. The limbic system includes deep cerebral nuclei that
are responsible for emotion and memory.
The diencephalon includes the thalamus and the hypothalamus, along with
some other structures. The thalamus is a relay between the cerebrum and
the rest of the nervous system. The hypothalamus coordinates homeostatic
functions through the autonomic and endocrine systems.
The brain stem is composed of the midbrain, pons, and medulla. It controls
the head and neck region of the body through the cranial nerves. There are
control centers in the brain stem that regulate the cardiovascular and
respiratory systems.
The cerebellum is connected to the brain stem, primarily at the pons, where
it receives a copy of the descending input from the cerebrum to the spinal
cord. It can compare this with sensory feedback input through the medulla
and send output through the midbrain that can correct motor commands for
coordination.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn about the basal nuclei (also known as the
basal ganglia), which have two pathways that process information
within the cerebrum. As shown in this video, the direct pathway is the
shorter pathway through the system that results in increased activity in
the cerebral cortex and increased motor activity. The direct pathway is
described as resulting in “disinhibition” of the thalamus. What does
disinhibition mean? What are the two neurons doing individually to
cause this?
Solution:
Both cells are inhibitory. The first cell inhibits the second one.
Therefore, the second cell can no longer inhibit its target. This is
disinhibition of that target across two synapses.
Exercise:
Problem:
Watch this video to learn about the basal nuclei (also known as the
basal ganglia), which have two pathways that process information
within the cerebrum. As shown in this video, the indirect pathway is
the longer pathway through the system that results in decreased
activity in the cerebral cortex, and therefore less motor activity. The
indirect pathway has an extra couple of connections in it, including
disinhibition of the subthalamic nucleus. What is the end result on the
thalamus, and therefore on movement initiated by the cerebral cortex?
Solution:
By disinhibiting the subthalamic nucleus, the indirect pathway
increases excitation of the globus pallidus internal segment. That, in
turn, inhibits the thalamus, which is the opposite effect of the direct
pathway that disinhibits the thalamus.
Exercise:
Problem:
Watch this video to learn about the gray matter of the spinal cord that
receives input from fibers of the dorsal (posterior) root and sends
information out through the fibers of the ventral (anterior) root. As
discussed in this video, these connections represent the interactions of
the CNS with peripheral structures for both sensory and motor
functions. The cervical and lumbar spinal cords have enlargements as a
result of larger populations of neurons. What are these enlargements
responsible for?
Solution:
There are more motor neurons in the anterior horns that are responsible
for movement in the limbs. The cervical enlargement is for the arms,
and the lumbar enlargement is for the legs.
Exercise:
Problem:
Compared with the nearest evolutionary relative, the chimpanzee, the
human has a brain that is huge. At a point in the past, a common
ancestor gave rise to the two species of humans and chimpanzees. That
evolutionary history is long and is still an area of intense study. But
something happened to increase the size of the human brain relative to
the chimpanzee. Read this article in which the author explores the
current understanding of why this happened.
According to one hypothesis about the expansion of brain size, what
tissue might have been sacrificed so energy was available to grow our
larger brain? Based on what you know about that tissue and nervous
tissue, why would there be a trade-off between them in terms of energy
use?
Solution:
Energy is needed for the brain to develop and perform higher cognitive
functions. That energy is not available for the muscle tissues to
develop and function. The hypothesis suggests that humans have larger
brains and less muscle mass, and chimpanzees have the smaller brains
but more muscle mass.
Review Questions
Exercise:
Problem:
Which lobe of the cerebral cortex is responsible for generating motor
commands?
a. temporal
b. parietal
c. occipital
d. frontal
Solution:
D
Exercise:
Problem: What region of the diencephalon coordinates homeostasis?
a. thalamus
b. epithalamus
c. hypothalamus
d. subthalamus
Solution:
C
Exercise:
Problem:
What level of the brain stem is the major input to the cerebellum?
a. midbrain
b. pons
c. medulla
d. spinal cord
Solution:
B
Exercise:
Problem:
What region of the spinal cord contains motor neurons that direct the
movement of skeletal muscles?
a. anterior horn
b. posterior horn
c. lateral horn
d. alar plate
Solution:
A
Exercise:
Problem:
Brodmann’s areas map different regions of the to particular
functions.
a. cerebellum
b. cerebral cortex
c. basal forebrain
d. corpus callosum
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Damage to specific regions of the cerebral cortex, such as through a
stroke, can result in specific losses of function. What functions would
likely be lost by a stroke in the temporal lobe?
Solution:
The temporal lobe has sensory functions associated with hearing and
vision, as well as being important for memory. A stroke in the
temporal lobe can result in specific sensory deficits in these systems
(known as agnosias) or losses in memory.
Exercise:
Problem:
Why do the anatomical inputs to the cerebellum suggest that it can
compare motor commands and sensory feedback?
Solution:
A copy of descending input from the cerebrum to the spinal cord,
through the pons, and sensory feedback from the spinal cord and
special senses like balance, through the medulla, both go to the
cerebellum. It can therefore send output through the midbrain that will
correct spinal cord control of skeletal muscle movements.
Glossary
alar plate
developmental region of the spinal cord that gives rise to the posterior
horn of the gray matter
amygdala
nucleus deep in the temporal lobe of the cerebrum that is related to
memory and emotional behavior
anterior column
white matter between the anterior horns of the spinal cord composed of
many different groups of axons of both ascending and descending
tracts
anterior horn
gray matter of the spinal cord containing multipolar motor neurons,
sometimes referred to as the ventral horn
anterior median fissure
deep midline feature of the anterior spinal cord, marking the separation
between the right and left sides of the cord
ascending tract
central nervous system fibers carrying sensory information from the
spinal cord or periphery to the brain
basal forebrain
nuclei of the cerebrum related to modulation of sensory stimuli and
attention through broad projections to the cerebral cortex, loss of
which is related to Alzheimer’s disease
basal nuclei
nuclei of the cerebrum (with a few components in the upper brain stem
and diencephalon) that are responsible for assessing cortical movement
commands and comparing them with the general state of the individual
through broad modulatory activity of dopamine neurons; largely
related to motor functions, as evidenced through the symptoms of
Parkinson’s and Huntington’s diseases
basal plate
developmental region of the spinal cord that gives rise to the lateral
and anterior horns of gray matter
Broca’s area
region of the frontal lobe associated with the motor commands
necessary for speech production and located only in the cerebral
hemisphere responsible for language production, which is the left side
in approximately 95 percent of the population
Brodmann’s areas
mapping of regions of the cerebral cortex based on microscopic
anatomy that relates specific areas to functional differences, as
described by Brodmann in the early 1900s
cauda equina
bundle of spinal nerve roots that descend from the lower spinal cord
below the first lumbar vertebra and lie within the vertebral cavity; has
the appearance of a horse's tail
caudate
nucleus deep in the cerebrum that is part of the basal nuclei; along with
the putamen, it is part of the striatum
central sulcus
surface landmark of the cerebral cortex that marks the boundary
between the frontal and parietal lobes
cerebral cortex
outer gray matter covering the forebrain, marked by wrinkles and folds
known as gyri and sulci
cerebrum
region of the adult brain that develops from the telencephalon and is
responsible for higher neurological functions such as memory,
emotion, and consciousness
cerebellum
region of the adult brain connected primarily to the pons that
developed from the metencephalon (along with the pons) and is largely
responsible for comparing information from the cerebrum with sensory
feedback from the periphery through the spinal cord
cerebral hemisphere
one half of the bilaterally symmetrical cerebrum
corpus callosum
large white matter structure that connects the right and left cerebral
hemispheres
descending tract
central nervous system fibers carrying motor commands from the brain
to the spinal cord or periphery
direct pathway
connections within the basal nuclei from the striatum to the globus
pallidus internal segment and substantia nigra pars reticulata that
disinhibit the thalamus to increase cortical control of movement
disinhibition
disynaptic connection in which the first synapse inhibits the second
cell, which then stops inhibiting the final target
dorsal (posterior) nerve root
axons entering the posterior horn of the spinal cord
epithalamus
region of the diecephalon containing the pineal gland
frontal eye field
region of the frontal lobe associated with motor commands to orient
the eyes toward an object of visual attention
frontal lobe
region of the cerebral cortex directly beneath the frontal bone of the
cranium
globus pallidus
nuclei deep in the cerebrum that are part of the basal nuclei and can be
divided into the internal and external segments
gyrus
ridge formed by convolutions on the surface of the cerebrum or
cerebellum
hippocampus
gray matter deep in the temporal lobe that is very important for long-
term memory formation
hypothalamus
major region of the diencephalon that is responsible for coordinating
autonomic and endocrine control of homeostasis
indirect pathway
connections within the basal nuclei from the striatum through the
globus pallidus external segment and subthalamic nucleus to the
globus pallidus internal segment/substantia nigra pars compacta that
result in inhibition of the thalamus to decrease cortical control of
movement
inferior colliculus
half of the midbrain tectum that is part of the brain stem auditory
pathway
inferior olive
nucleus in the medulla that is involved in processing information
related to motor control
kinesthesia
general sensory perception of movement of the body
lateral column
white matter of the spinal cord between the posterior horn on one side
and the axons from the anterior horn on the same side; composed of
many different groups of axons, of both ascending and descending
tracts, carrying motor commands to and from the brain
lateral hor
region of the spinal cord gray matter in the thoracic, upper lumbar, and
sacral regions that is the central component of the sympathetic division
of the autonomic nervous system
lateral sulcus
surface landmark of the cerebral cortex that marks the boundary
between the temporal lobe and the frontal and parietal lobes
limbic cortex
collection of structures of the cerebral cortex that are involved in
emotion, memory, and behavior and are part of the larger limbic
system
limbic system
structures at the edge (limit) of the boundary between the forebrain and
hindbrain that are most associated with emotional behavior and
memory formation
longitudinal fissure
large separation along the midline between the two cerebral
hemispheres
occipital lobe
region of the cerebral cortex directly beneath the occipital bone of the
cranium
olfaction
special sense responsible for smell, which has a unique, direct
connection to the cerebrum
parietal lobe
region of the cerebral cortex directly beneath the parietal bone of the
cranium
parieto-occipital sulcus
groove in the cerebral cortex representing the border between the
parietal and occipital cortices
postcentral gyrus
primary motor cortex located in the frontal lobe of the cerebral cortex
posterior columns
white matter of the spinal cord that lies between the posterior horns of
the gray matter, sometimes referred to as the dorsal column; composed
of axons of ascending tracts that carry sensory information up to the
brain
posterior hor
gray matter region of the spinal cord in which sensory input arrives,
sometimes referred to as the dorsal horn
posterior median sulcus
midline feature of the posterior spinal cord, marking the separation
between right and left sides of the cord
posterolateral sulcus
feature of the posterior spinal cord marking the entry of posterior nerve
roots and the separation between the posterior and lateral columns of
the white matter
precentral gyrus
ridge just posterior to the central sulcus, in the parietal lobe, where
somatosensory processing initially takes place in the cerebrum
prefrontal lobe
specific region of the frontal lobe anterior to the more specific motor
function areas, which can be related to the early planning of
movements and intentions to the point of being personality-type
functions
premotor area
region of the frontal lobe responsible for planning movements that will
be executed through the primary motor cortex
proprioception
general sensory perceptions providing information about location and
movement of body parts; the “sense of the self”
putamen
nucleus deep in the cerebrum that is part of the basal nuclei; along with
the caudate, it is part of the striatum
reticular formation
diffuse region of gray matter throughout the brain stem that regulates
Sleep, wakefulness, and states of consciousness
somatosensation
general senses related to the body, usually thought of as the senses of
touch, which would include pain, temperature, and proprioception
striatum
the caudate and putamen collectively, as part of the basal nuclei, which
receive input from the cerebral cortex
subcortical nucleus
all the nuclei beneath the cerebral cortex, including the basal nuclei
and the basal forebrain
substantia nigra pars compacta
nuclei within the basal nuclei that release dopamine to modulate the
function of the striatum; part of the motor pathway
substantia nigra pars reticulata
nuclei within the basal nuclei that serve as an output center of the
nuclei; part of the motor pathway
subthalamus
nucleus within the basal nuclei that is part of the indirect pathway
sulcus
groove formed by convolutions in the surface of the cerebral cortex
superior colliculus
half of the midbrain tectum that is responsible for aligning visual,
auditory, and somatosensory spatial perceptions
tectum
region of the midbrain, thought of as the roof of the cerebral aqueduct,
which is subdivided into the inferior and superior colliculi
tegmentum
region of the midbrain, thought of as the floor of the cerebral aqueduct,
which continues into the pons and medulla as the floor of the fourth
ventricle
temporal lobe
region of the cerebral cortex directly beneath the temporal bone of the
cranium
thalamus
major region of the diencephalon that is responsible for relaying
information between the cerebrum and the hindbrain, spinal cord, and
periphery
ventral (anterior) nerve root
axons emerging from the anterior or lateral horns of the spinal cord
Circulation and the Central Nervous System
By the end of this section, you will be able to:
e Describe the vessels that supply the CNS with blood
e Name the components of the ventricular system and the regions of the brain in which each is located
e Explain the production of cerebrospinal fluid and its flow through the ventricles
e Explain how a disruption in circulation would result in a stroke
The CNS is crucial to the operation of the body, and any compromise in the brain and spinal cord can lead to
severe difficulties. The CNS has a privileged blood supply, as suggested by the blood-brain barrier. The function of
the tissue in the CNS is crucial to the survival of the organism, so the contents of the blood cannot simply pass into
the central nervous tissue. To protect this region from the toxins and pathogens that may be traveling through the
blood stream, there is strict control over what can move out of the general systems and into the brain and spinal
cord. Because of this privilege, the CNS needs specialized structures for the maintenance of circulation. This
begins with a unique arrangement of blood vessels carrying fresh blood into the CNS. Beyond the supply of blood,
the CNS filters that blood into cerebrospinal fluid (CSF), which is then circulated through the cavities of the brain
and spinal cord called ventricles.
Blood Supply to the Brain
A lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to
ensure that the blood supply is not interrupted. There are multiple routes for blood to get into the CNS, with
specializations to protect that blood supply and to maximize the ability of the brain to get an uninterrupted
perfusion.
Arterial Supply
The major artery carrying recently oxygenated blood away from the heart is the aorta. The very first branches off
the aorta supply the heart with nutrients and oxygen. The next branches give rise to the common carotid arteries,
which further branch into the internal carotid arteries. The external carotid arteries supply blood to the tissues on
the surface of the cranium. The bases of the common carotids contain stretch receptors that immediately respond to
the drop in blood pressure upon standing. The orthostatic reflex is a reaction to this change in body position, so
that blood pressure is maintained against the increasing effect of gravity (orthostatic means “standing up”). Heart
rate increases—a reflex of the sympathetic division of the autonomic nervous system—and this raises blood
pressure.
The internal carotid artery enters the cranium through the carotid canal in the temporal bone. A second set of
vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region
by the transverse foramina of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen
magnum of the occipital bone. Branches off the left and right vertebral arteries merge into the anterior spinal
artery supplying the anterior aspect of the spinal cord, found along the anterior median fissure. The two vertebral
arteries then merge into the basilar artery, which gives rise to branches to the brain stem and cerebellum. The left
and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of
arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part
({link]).
Circle of Willis
Anterior cerebral
Anterior artery
communicating
artery Ophthalmic
artery
Middle
cerebral Anterior
artery choroidal
Internal artery
carotid Posterior
artery cerebral
arte!
Posterior me
communicating
artery
Superior
cerebellar
artery
Pontine ;
arteries Basilar artery
Anterior Vertebral
inferior artery
cerebellar
artery .
Posterior
inferior
cerebellar
Anterior artely
spinal artery
The blood supply to the brain
enters through the internal carotid
arteries and the vertebral arteries,
eventually giving rise to the circle
of Willis.
Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being
distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant
perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows
the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood
come from if there were a blockage just posterior to the middle cerebral artery on the left?
Venous Return
After passing through the CNS, blood returns to the circulation through a series of dural sinuses and veins
({link]). The superior sagittal sinus runs in the groove of the longitudinal fissure, where it absorbs CSF from the
meninges. The superior sagittal sinus drains to the confluence of sinuses, along with the occipital sinuses and
straight sinus, to then drain into the transverse sinuses. The transverse sinuses connect to the sigmoid sinuses,
which then connect to the jugular veins. From there, the blood continues toward the heart to be pumped to the
lungs for reoxygenation.
Dural Sinuses and Veins
Superior sagittal
sinus
Cranium Inferior sagittal
Dura mater sinus
Straight
sinus
Cerebral veins
Tranverse
sinus
Great cerebral
vein Confluence
of sinuses
Occipital
sinus
Blood returns to
jugular vein via
the sigmoid sinus
Blood drains from the brain through a series of sinuses that
connect to the jugular veins.
Protective Coverings of the Brain and Spinal Cord
The outer surface of the CNS is covered by a series of membranes composed of connective tissue called the
meninges, which protect the brain. The dura mater is a thick fibrous layer and a strong protective sheath over the
entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The arachnoid
mater is a membrane of thin fibrous tissue that forms a loose sac around the CNS. Beneath the arachnoid is a thin,
filamentous mesh called the arachnoid trabeculae, which looks like a spider web, giving this layer its name.
Directly adjacent to the surface of the CNS is the pia mater, a thin fibrous membrane that follows the
convolutions of gyri and sulci in the cerebral cortex and fits into other grooves and indentations ([link]).
Meningeal Layers of Superior Sagittal Sinus
Superior sagittal sinus
Arachnoid mater Dura mater
Subdural space
Shee Arachnoid
Pia mater granulation villi
Arachnoid trabeculae Longitudinal fissure
Cerebral cortex
The layers of the meninges in the longitudinal fissure of
the superior sagittal sinus are shown, with the dura mater
adjacent to the inner surface of the cranium, the pia
mater adjacent to the surface of the brain, and the
arachnoid and subarachnoid space between them. An
arachnoid villus is shown emerging into the dural sinus
to allow CSF to filter back into the blood for drainage.
Dura Mater
Like a thick cap covering the brain, the dura mater is a tough outer covering. The name comes from the Latin for
“tough mother” to represent its physically protective role. It encloses the entire CNS and the major blood vessels
that enter the cranium and vertebral cavity. It is directly attached to the inner surface of the bones of the cranium
and to the very end of the vertebral cavity.
There are infoldings of the dura that fit into large crevasses of the brain. Two infoldings go through the midline
separations of the cerebrum and cerebellum; one forms a shelf-like tent between the occipital lobes of the
cerebrum and the cerebellum, and the other surrounds the pituitary gland. The dura also surrounds and supports the
venous sinuses.
Arachnoid Mater
The middle layer of the meninges is the arachnoid, named for the spider-web-like trabeculae between it and the
pia mater. The arachnoid defines a sac-like enclosure around the CNS. The trabeculae are found in the
subarachnoid space, which is filled with circulating CSF. The arachnoid emerges into the dural sinuses as the
arachnoid granulations, where the CSF is filtered back into the blood for drainage from the nervous system.
The subarachnoid space is filled with circulating CSF, which also provides a liquid cushion to the brain and spinal
cord. Similar to clinical blood work, a sample of CSF can be withdrawn to find chemical evidence of
neuropathology or metabolic traces of the biochemical functions of nervous tissue.
Pia Mater
The outer surface of the CNS is covered in the thin fibrous membrane of the pia mater. It is thought to have a
continuous layer of cells providing a fluid-impermeable membrane. The name pia mater comes from the Latin for
“tender mother,” suggesting the thin membrane is a gentle covering for the brain. The pia extends into every
convolution of the CNS, lining the inside of the sulci in the cerebral and cerebellar cortices. At the end of the
spinal cord, a thin filament extends from the inferior end of CNS at the upper lumbar region of the vertebral
column to the sacral end of the vertebral column. Because the spinal cord does not extend through the lower
lumbar region of the vertebral column, a needle can be inserted through the dura and arachnoid layers to withdraw
CSF. This procedure is called a lumbar puncture and avoids the risk of damaging the central tissue of the spinal
cord. Blood vessels that are nourishing the central nervous tissue are between the pia mater and the nervous tissue.
Note:
Disorders of the...
Meninges
Meningitis is an inflammation of the meninges, the three layers of fibrous membrane that surround the CNS.
Meningitis can be caused by infection by bacteria or viruses. The particular pathogens are not special to
meningitis; it is just an inflammation of that specific set of tissues from what might be a broader infection.
Bacterial meningitis can be caused by Streptococcus, Staphylococcus, or the tuberculosis pathogen, among many
others. Viral meningitis is usually the result of common enteroviruses (such as those that cause intestinal
disorders), but may be the result of the herpes virus or West Nile virus. Bacterial meningitis tends to be more
severe.
The symptoms associated with meningitis can be fever, chills, nausea, vomiting, light sensitivity, soreness of the
neck, or severe headache. More important are the neurological symptoms, such as changes in mental state
(confusion, memory deficits, and other dementia-type symptoms). A serious risk of meningitis can be damage to
peripheral structures because of the nerves that pass through the meninges. Hearing loss is a common result of
meningitis.
The primary test for meningitis is a lumbar puncture. A needle inserted into the lumbar region of the spinal
column through the dura mater and arachnoid membrane into the subarachnoid space can be used to withdraw the
fluid for chemical testing. Fatality occurs in 5 to 40 percent of children and 20 to 50 percent of adults with
bacterial meningitis. Treatment of bacterial meningitis is through antibiotics, but viral meningitis cannot be
treated with antibiotics because viruses do not respond to that type of drug. Fortunately, the viral forms are milder.
Note:
ees
= openstax couse
mite
Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample
the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar
puncture performed in the lower lumbar area of the vertebral column?
The Ventricular System
Cerebrospinal fluid (CSF) circulates throughout and around the CNS. In other tissues, water and small molecules
are filtered through capillaries as the major contributor to the interstitial fluid. In the brain, CSF is produced in
special structures to perfuse through the nervous tissue of the CNS and is continuous with the interstitial fluid.
Specifically, CSF circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return
them to the blood stream. The ventricles are the open spaces within the brain where CSF circulates. In some of
these spaces, CSF is produced by filtering of the blood that is performed by a specialized membrane known as a
choroid plexus. The CSF circulates through all of the ventricles to eventually emerge into the subarachnoid space
where it will be reabsorbed into the blood.
The Ventricles
There are four ventricles within the brain, all of which developed from the original hollow space within the neural
tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These
ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third
ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct
that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the
cerebellum and the pons and upper medulla ([link]).
Cerebrospinal Fluid Circulation
Superior sagittal
sinus
Arachnoid granulation
Subarachnoid space
Choroid plexus Meningeal dura mater
Right lateral ventricle
Interventricular
foramen
Third ventricle
Cerebral aqueduct
AN :
Lateral aperture SS Median aperture
Fourth ventricle
Central canal
The choroid plexus in the four ventricles produce CSF, which
is circulated through the ventricular system and then enters the
subarachnoid space through the median and lateral apertures.
The CSF is then reabsorbed into the blood at the arachnoid
granulations, where the arachnoid membrane emerges into the
dural sinuses.
As the telencephalon enlarges and grows into the cranial cavity, it is limited by the space within the skull. The
telencephalon is the most anterior region of what was the neural tube, but cannot grow past the limit of the frontal
bone of the skull. Because the cerebrum fits into this space, it takes on a C-shaped formation, through the frontal,
parietal, occipital, and finally temporal regions. The space within the telencephalon is stretched into this same C-
shape. The two ventricles are in the left and right sides, and were at one time referred to as the first and second
ventricles. The interventricular foramina connect the frontal region of the lateral ventricles with the third ventricle.
The third ventricle is the space bounded by the medial walls of the hypothalamus and thalamus. The two thalami
touch in the center in most brains as the massa intermedia, which is surrounded by the third ventricle. The cerebral
aqueduct opens just inferior to the epithalamus and passes through the midbrain. The tectum and tegmentum of the
midbrain are the roof and floor of the cerebral aqueduct, respectively. The aqueduct opens up into the fourth
ventricle. The floor of the fourth ventricle is the dorsal surface of the pons and upper medulla (that gray matter
making a continuation of the tegmentum of the midbrain). The fourth ventricle then narrows into the central canal
of the spinal cord.
The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture
and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles
and around the outside of the CNS. Cerebrospinal fluid is produced within the ventricles by a type of specialized
membrane called a choroid plexus. Ependymal cells (one of the types of glial cells described in the introduction to
the nervous system) surround blood capillaries and filter the blood to make CSF. The fluid is a clear solution with
a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes. Oxygen and
carbon dioxide are dissolved into the CSF, as they are in blood, and can diffuse between the fluid and the nervous
tissue.
Cerebrospinal Fluid Circulation
The choroid plexuses are found in all four ventricles. Observed in dissection, they appear as soft, fuzzy structures
that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The
CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of
cardiovascular circulation.
From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through
the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is
filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses
through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the
central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular
system and moves into the subarachnoid space through the median and lateral apertures.
Within the subarachnoid space, the CSF flows around all of the CNS, providing two important functions. As with
elsewhere in its circulation, the CSF picks up metabolic wastes from the nervous tissue and moves it out of the
CNS. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the
subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The
arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be
reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head
and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs
and wastes to be filtered out by the kidneys ((link]).
Note:
Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the
ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses
to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found?
How are the structures indicated in this animation?
Components of CSF Circulation
Lateral Third Cerebral Fourth Central Subarachnoid
ventricles ventricle aqueduct ventricle canal space
Between
: pons/upper :
Location : . 7 Spinal External to
in CNS Cerebrum Diencephalon Midbrain aes cond entire CNS
cerebellum
Blood Choroid Choroid Choroid Arachnoid
vessel None None :
plexus plexus plexus granulations
structure
Note:
Disorders of the...
Central Nervous System
The supply of blood to the brain is crucial to its ability to perform many functions. Without a steady supply of
oxygen, and to a lesser extent glucose, the nervous tissue in the brain cannot keep up its extensive electrical
activity. These nutrients get into the brain through the blood, and if blood flow is interrupted, neurological
function is compromised.
The common name for a disruption of blood supply to the brain is a stroke. It is caused by a blockage to an artery
in the brain. The blockage is from some type of embolus: a blood clot, a fat embolus, or an air bubble. When the
blood cannot travel through the artery, the surrounding tissue that is deprived starves and dies. Strokes will often
result in the loss of very specific functions. A stroke in the lateral medulla, for example, can cause a loss in the
ability to swallow. Sometimes, seemingly unrelated functions will be lost because they are dependent on
structures in the same region. Along with the swallowing in the previous example, a stroke in that region could
affect sensory functions from the face or extremities because important white matter pathways also pass through
the lateral medulla. Loss of blood flow to specific regions of the cortex can lead to the loss of specific higher
functions, from the ability to recognize faces to the ability to move a particular region of the body. Severe or
limited memory loss can be the result of a temporal lobe stroke.
Related to strokes are transient ischemic attacks (TIAs), which can also be called “mini-strokes.” These are events
in which a physical blockage may be temporary, cutting off the blood supply and oxygen to a region, but not to
the extent that it causes cell death in that region. While the neurons in that area are recovering from the event,
neurological function may be lost. Function can return if the area is able to recover from the event.
Recovery from a stroke (or TIA) is strongly dependent on the speed of treatment. Often, the person who is present
and notices something is wrong must then make a decision. The mnemonic FAST helps people remember what to
look for when someone is dealing with sudden losses of neurological function. If someone complains of feeling
“funny,” check these things quickly: Look at the person’s face. Does he or she have problems moving Face
muscles and making regular facial expressions? Ask the person to raise his or her Arms above the head. Can the
person lift one arm but not the other? Has the person’s Speech changed? Is he or she slurring words or having
trouble saying things? If any of these things have happened, then it is ‘Time to call for help.
Sometimes, treatment with blood-thinning drugs can alleviate the problem, and recovery is possible. If the tissue
is damaged, the amazing thing about the nervous system is that it is adaptable. With physical, occupational, and
speech therapy, victims of strokes can recover, or more accurately relearn, functions.
Chapter Review
The CNS has a privileged blood supply established by the blood-brain barrier. Establishing this barrier are
anatomical structures that help to protect and isolate the CNS. The arterial blood to the brain comes from the
internal carotid and vertebral arteries, which both contribute to the unique circle of Willis that provides constant
perfusion of the brain even if one of the blood vessels is blocked or narrowed. That blood is eventually filtered to
make a separate medium, the CSF, that circulates within the spaces of the brain and then into the surrounding
space defined by the meninges, the protective covering of the brain and spinal cord.
The blood that nourishes the brain and spinal cord is behind the glial-cell-enforced blood-brain barrier, which
limits the exchange of material from blood vessels with the interstitial fluid of the nervous tissue. Thus, metabolic
wastes are collected in cerebrospinal fluid that circulates through the CNS. This fluid is produced by filtering
blood at the choroid plexuses in the four ventricles of the brain. It then circulates through the ventricles and into
the subarachnoid space, between the pia mater and the arachnoid mater. From the arachnoid granulations, CSF is
reabsorbed into the blood, removing the waste from the privileged central nervous tissue.
The blood, now with the reabsorbed CSF, drains out of the cranium through the dural sinuses. The dura mater is
the tough outer covering of the CNS, which is anchored to the inner surface of the cranial and vertebral cavities. It
surrounds the venous space known as the dural sinuses, which connect to the jugular veins, where blood drains
from the head and neck.
Interactive Link Questions
Exercise:
Problem:
Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being
distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure
constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The
animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where
would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left?
Solution:
If blood could not get to the middle cerebral artery through the posterior circulation, the blood would flow
around the circle of Willis to reach that artery from an anterior vessel. Blood flow would just reverse within
the circle.
Exercise:
Problem:
Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to
sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is
the lumbar puncture performed in the lower lumbar area of the vertebral column?
Solution:
The spinal cord ends in the upper lumbar area of the vertebral column, so a needle inserted lower than that
will not damage the nervous tissue of the CNS.
Exercise:
Problem:
Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates
from the ventricles and then spreads into the space within the meninges, where the fluids then move into the
venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where
are they found? How are the structures indicated in this animation?
Solution:
The choroid plexuses of the ventricles make CSF. As shown, there is a little of the blue color appearing in
each ventricle that is joined by the color flowing from the other ventricles.
Review Questions
Exercise:
Problem: What blood vessel enters the cranium to supply the brain with fresh, oxygenated blood?
a. common carotid artery
b. jugular vein
c. internal carotid artery
d. aorta
Solution:
C
Exercise:
Problem:
Which layer of the meninges surrounds and supports the sinuses that form the route through which blood
drains from the CNS?
a. dura mater
b. arachnoid mater
c. subarachnoid
d. pia mater
Solution:
A
Exercise:
Problem: What type of glial cell is responsible for filtering blood to produce CSF at the choroid plexus?
a. ependymal cell
b. astrocyte
c. oligodendrocyte
d. Schwann cell
Solution:
A
Exercise:
Problem: Which portion of the ventricular system is found within the diencephalon?
a. lateral ventricles
b. third ventricle
c. cerebral aqueduct
d. fourth ventricle
Solution:
B
Exercise:
Problem:What condition causes a stroke?
a. inflammation of meninges
b. lumbar puncture
c. infection of cerebral spinal fluid
d. disruption of blood to the brain
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Why can the circle of Willis maintain perfusion of the brain even if there is a blockage in one part of the
structure?
Solution:
The structure is a circular connection of blood vessels, so that blood coming up from one of the arteries can
flow in either direction around the circle and avoid any blockage or narrowing of the blood vessels.
Exercise:
Problem:
Meningitis is an inflammation of the meninges that can have severe effects on neurological function. Why is
infection of this structure potentially so dangerous?
Solution:
The nerves that connect the periphery to the CNS pass through these layers of tissue and can be damaged by
that inflammation, causing a loss of important neurological functions.
Glossary
anterior spinal artery
blood vessel from the merged branches of the vertebral arteries that runs along the anterior surface of the
spinal cord
arachnoid granulation
outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood
arachnoid mater
middle layer of the meninges named for the spider-web-like trabeculae that extend between it and the pia
mater
arachnoid trabeculae
filaments between the arachnoid and pia mater within the subarachnoid space
basilar artery
blood vessel from the merged vertebral arteries that runs along the dorsal surface of the brain stem
carotid canal
opening in the temporal bone through which the internal carotid artery enters the cranium
central canal
hollow space within the spinal cord that is the remnant of the center of the neural tube
cerebral aqueduct
connection of the ventricular system between the third and fourth ventricles located in the midbrain
choroid plexus
specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in
the four ventricles of the brain
circle of Willis
unique anatomical arrangement of blood vessels around the base of the brain that maintains perfusion of
blood into the brain even if one component of the structure is blocked or narrowed
common carotid artery
blood vessel that branches off the aorta (or the brachiocephalic artery on the right) and supplies blood to the
head and neck
dura mater
tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral
column and surrounds the entire CNS
dural sinus
any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from
the CNS to the common venous return of the jugular veins
foramen magnum
large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral
arteries enter the cranium
fourth ventricle
the portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid
space through the median and lateral apertures
internal carotid artery
branch from the common carotid artery that enters the cranium and supplies blood to the brain
interventricular foramina
openings between the lateral ventricles and third ventricle allowing for the passage of CSF
jugular veins
blood vessels that return “used” blood from the head and neck
lateral apertures
pair of openings from the fourth ventricle to the subarachnoid space on either side and between the medulla
and cerebellum
lateral ventricles
portions of the ventricular system that are in the region of the cerebrum
lumbar puncture
procedure used to withdraw CSF from the lower lumbar region of the vertebral column that avoids the risk of
damaging CNS tissue because the spinal cord ends at the upper lumbar vertebrae
median aperture
singular opening from the fourth ventricle into the subarachnoid space at the midline between the medulla and
cerebellum
meninges
protective outer coverings of the CNS composed of connective tissue
occipital sinuses
dural sinuses along the edge of the occipital lobes of the cerebrum
orthostatic reflex
sympathetic function that maintains blood pressure when standing to offset the increased effect of gravity
pia mater
thin, innermost membrane of the meninges that directly covers the surface of the CNS
sigmoid sinuses
dural sinuses that drain directly into the jugular veins
straight sinus
dural sinus that drains blood from the deep center of the brain to collect with the other sinuses
subarachnoid space
space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the
arachnoid trabeculae
superior sagittal sinus
dural sinus that runs along the top of the longitudinal fissure and drains blood from the majority of the outer
cerebrum
third ventricle
portion of the ventricular system that is in the region of the diencephalon
transverse sinuses
dural sinuses that drain along either side of the occipital—-cerebellar space
ventricles
remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the
brain
vertebral arteries
arteries that ascend along either side of the vertebral column through the transverse foramina of the cervical
vertebrae and enter the cranium through the foramen magnum
Nerves and ganglia
By the end of this section, you will be able to:
e Describe the structures found in the PNS
e Distinguish between somatic and autonomic structures, including the special peripheral structures of the
enteric nervous system
e Name the twelve cranial nerves and explain the functions associated with each
e Describe the sensory and motor components of spinal nerves and the plexuses that they pass through
The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral
structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary
to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many
of the neural structures that are incorporated into other organs are features of the digestive system; these structures
are known as the enteric nervous system and are a special subset of the PNS.
Ganglia
A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as
either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of
sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons
that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve
root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the
cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root ([link]). The cells of the
dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can
be seen surrounding—as if they were orbiting—the neuron cell bodies.
Dorsal Root Ganglion
Pas. na
The cell bodies of sensory neurons, which are unipolar neurons by
shape, are seen in this photomicrograph. Also, the fibrous region is
composed of the axons of these neurons that are passing through
the ganglion to be part of the dorsal nerve root (tissue source:
canine). LM x 40. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)
Spinal Cord and Root Ganglion
The slide includes both a cross-section of the lumbar spinal cord
and a section of the dorsal root ganglion (see also [link]) (tissue
source: canine). LM x 1600. (Micrograph provided by the Regents
of University of Michigan Medical School © 2012)
Note:
a
=> openstax coLLece
L
= q
ogy ae
View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom in on the
dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory
neurons. From what structure do satellite cells derive during embryologic development?
Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion,
except that it is associated with a cranial nerve instead of a spinal nerve. The roots of cranial nerves are within
the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the
temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of
cranial nerve ganglia are also unipolar in shape with associated satellite cells.
The other major category of ganglia are those of the autonomic nervous system, which is divided into the
sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia
along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal
cord. Superior to the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic
ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the
chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral
column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the
cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and
prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to
regulate the sympathetic aspect of homeostatic mechanisms.
Another group of autonomic ganglia are the terminal ganglia that receive input from cranial nerves or sacral
spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two
sets of ganglia, sympathetic and parasympathetic, often project to the same organs—one input from the chain
ganglia and one input from a terminal ganglion—to regulate the overall function of an organ. For example, the
heart receives two inputs such as these; one increases heart rate, and the other decreases it. The terminal ganglia
that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal
cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities.
Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a plexus. A
plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or
structures containing blood vessels (such as a choroid plexus). For example, the enteric plexus is the extensive
network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the
enteric nervous system, along with the gastric plexuses and the esophageal plexus. Though the enteric nervous
system receives input originating from central neurons of the autonomic nervous system, it does not require CNS
input to function. In fact, it operates independently to regulate the digestive system.
Nerves
Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the
central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective
tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer
surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve,
axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue
called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium
({link]). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the
region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves
connected to the spinal cord.
Nerve Structure
Spinal nerve
Epineurium
Perineurium
Blood vessels
Axion
Perineurium
Endoneurium
Perineurium
Fascicles
(b)
The structure of a nerve is organized by the layers of
connective tissue on the outside, around each fascicle,
and surrounding the individual nerve fibers (tissue
source: simian). LM x 40. (Micrograph provided by
the Regents of University of Michigan Medical School
mana
© 2U12)
Close-Up of Nerve Trunk
; Wp asf
SST
Zoom in on this slide of a nerve trunk to examine the
endoneurium, perineurium, and epineurium in greater detail (tissue
source: simian). LM x 1600. (Micrograph provided by the Regents
of University of Michigan Medical School © 2012)
Note:
— openstax coLLece*
View the University of Michigan WebScope to explore the tissue sample in greater detail. With what structures in
a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?
Cranial Nerves
The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor
functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of
the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII
for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor
nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external
to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor
axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers;
five are strictly motor; and the remaining four are mixed nerves.
Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to
remember the nerve names. A traditional mnemonic is the rhyming couplet, “On Old Olympus’ Towering Tops/A
Finn And German Viewed Some Hops,” in which the initial letter of each word corresponds to the initial letter in
the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more
accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that
have personal significance. The names of the cranial nerves are listed in [link] along with a brief description of
their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal
muscle). They are listed here with a brief explanation of each nerve ((Link]).
The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The
oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also
responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve
and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular
muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of
mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the
sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing
and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper
throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for
contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal
accessory nerve is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The
hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue.
The Cranial Nerves
Oculomotor
nerve III Optic nerve II
Trochlear
IV
nerve Trigeminal nerve V
Abducens
nerve VI
. Facial nerve VII
Vestibulocochlear
nerve VIII
Glossopharyngeal
nerve IX
Hypoglossal
nerve XII
Vagus nerve X
The anatomical arrangement of the roots of the cranial
nerves observed from an inferior view of the brain.
Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the
autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic
ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both
initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal
cavities.
Note:
CRS i]
*
=> Snensae COLLEGE"
Os Ba
Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to
an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately
sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause
cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally.
How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented
as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?
Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve
plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see [link]). The sentence,
“Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of
each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and
vestibulocochlear (CNVIID nerves. The three eye-movement nerves are all motor: the oculomotor (CNHI),
trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also
strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV),
facial (CNVID, glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related
to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other
concems the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying
gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral
responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but
there is a thread of relation between them.
Cranial Nerves
Function
Mnemonic # Name (S/M/B) Central connection (nuclei)
On I Olfactory Smell (S) Olfactory bulb
Old II Optic Vision (S) Hypothalamus/thalamus/midbrain
Eye movements
Olympus’ Il Oculomotor (M) Oculomotor nucleus
Towering IV Trochlear Re movements Trochlear nucleus
. F Sensory/motor Trigeminal nuclei in the
Ops ” oo — face (B) midbrain, pons, and medulla
A VI Abducens oe pvcnese Abducens nucleus
(M)
Cranial Nerves
Function
Mnemonic # Name (S/M/B) Central connection (nuclei)
Finn VII Facial Motor — face, Facial nucleus, solitary nucleus,
Taste (B) superior salivatory nucleus
aa VII Auditory Hearing/balance Cochlear nucleus, Vestibular
(Vestibulocochlear) (S) nucleus/cerebellum
Motor teat Solitary nucleus, inferior
German Ix Glossopharyngeal salivatory nucleus, nucleus
Taste (B) :
ambiguus
Motor/sensory —
Viewed Xx Vagus viscera Medulla
(autonomic) (B)
. Motor — head :
Some XI Spinal Accessory and neck (M) Spinal accessory nucleus
Hops XI Hypoglossal Motor lower Hypoglossal nucleus
throat (M)
Spinal Nerves
The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more
regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate
into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both
somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an
enlargement of the spinal nerve.
There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs
of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves
designated L1 to LS, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves
are numbered from the superior to inferior positions, and each emerges from the vertebral column through the
intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital
bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to
C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and
lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the
column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra.
Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are
not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow
different courses. Axons from different spinal nerves will come together into a systemic nerve. This occurs at four
places along the length of the vertebral column, each identified as a nerve plexus, whereas the other spinal nerves
directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe
networks of nerve fibers with no associated cell bodies.
Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level
({link]). The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in
the posterior neck and head, as well as the phrenic nerve, which connects to the diaphragm at the base of the
thoracic cavity. The other plexus from the cervical level is the brachial plexus. Spinal nerves C4 through T1
reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve
from this plexus is the radial nerve from which the axillary nerve branches to go to the armpit region. The radial
nerve continues through the arm and is paralleled by the ulnar nerve and the median nerve. The lumbar plexus
arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg.
The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve as a
branch that extends through the anterior lower leg. The sacral plexus comes from the lower lumbar nerves L4 and
L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the sciatic
nerve, which is a combination of the tibial nerve and the fibular nerve. The sciatic nerve extends across the hip
joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation
of the nerve or any of the spinal nerves giving rise to it.
These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they
contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend
from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of
sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are
the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send
action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains
fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm.
Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to
the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.
Nerve Plexuses of the Body
Cervical plexus
Phrenic nerve
Brachial plexus
Axillary nerve
Median nerve ———_—
Radial nerve — b
Ulnar nerve ; @)
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Lumbar plexus
Femoral nerve
Obturator nerve
Sacral plexus
Common
fibular nerve
There are four main nerve plexuses in the human
body. The cervical plexus supplies nerves to the
posterior head and neck, as well as to the
diaphragm. The brachial plexus supplies nerves
to the arm. The lumbar plexus supplies nerves to
the anterior leg. The sacral plexus supplies nerves
to the posterior leg.
Note:
Aging and the...
Nervous System
Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually
because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan
of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their
axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be
replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss
of the sensory neurons with no new neurons born to replace them.
Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue,
and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of
food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore
passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the
posterior nasal cavity. Anosmia results in a loss of the enjoyment of food.
As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many
sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed
on the tongue, which usually means adding table salt. The problem with this solution, however, is that this
increases sodium intake, which can lead to cardiovascular problems through water retention and the associated
increase in blood pressure.
Chapter Review
The PNS is composed of the groups of neurons (ganglia) and bundles of axons (nerves) that are outside of the
brain and spinal cord. Ganglia are of two types, sensory or autonomic. Sensory ganglia contain unipolar sensory
neurons and are found on the dorsal root of all spinal nerves as well as associated with many of the cranial nerves.
Autonomic ganglia are in the sympathetic chain, the associated paravertebral or prevertebral ganglia, or in terminal
ganglia near or within the organs controlled by the autonomic nervous system.
Nerves are classified as cranial nerves or spinal nerves on the basis of their connection to the brain or spinal cord,
respectively. The twelve cranial nerves can be strictly sensory in function, strictly motor in function, or a
combination of the two functions. Sensory fibers are axons of sensory ganglia that carry sensory information into
the brain and target sensory nuclei. Motor fibers are axons of motor neurons in motor nuclei of the brain stem and
target skeletal muscles of the head and neck. Spinal nerves are all mixed nerves with both sensory and motor
fibers. Spinal nerves emerge from the spinal cord and reorganize through plexuses, which then give rise to
systemic nerves. Thoracic spinal nerves are not part of any plexus, but give rise to the intercostal nerves directly.
Interactive Link Questions
Exercise:
Problem:
[link] If you zoom in on the DRG, you can see smaller satellite glial cells surrounding the large cell bodies of
the sensory neurons. From what structure do satellite cells derive during embryologic development?
Solution:
[link] They derive from the neural crest.
Exercise:
Problem:
[link] To what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?
Solution:
[link] The endoneurium surrounding individual nerve fibers is comparable to the endomysium surrounding
myofibrils, the perineurium bundling axons into fascicles is comparable to the perimysium bundling muscle
fibers into fascicles, and the epineurium surrounding the whole nerve is comparable to the epimysium
surrounding the muscle.
Exercise:
Problem:
Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent
him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and
immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but
a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what
caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve,
which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to
push on the optic nerve?
Solution:
The optic nerve enters the CNS in its projection from the eyes in the periphery, which means that it crosses
through the meninges. Meningitis will include swelling of those protective layers of the CNS, resulting in
pressure on the optic nerve, which can compromise vision.
Review Questions
Exercise:
Problem: What type of ganglion contains neurons that control homeostatic mechanisms of the body?
a. sensory ganglion
b. dorsal root ganglion
c. autonomic ganglion
d. cranial nerve ganglion
Solution:
GC
Exercise:
Problem:Which ganglion is responsible for cutaneous sensations of the face?
a. otic ganglion
b. vestibular ganglion
c. geniculate ganglion
d. trigeminal ganglion
Solution:
D
Exercise:
Problem:What is the name for a bundle of axons within a nerve?
a. fascicle
b. tract
c. nerve root
d. epineurium
Solution:
A
Exercise:
Problem:Which cranial nerve does not control functions in the head and neck?
a. olfactory
b. trochlear
c. glossopharyngeal
d. vagus
Solution:
D
Exercise:
Problem: Which of these structures is not under direct control of the peripheral nervous system?
a. trigeminal ganglion
b. gastric plexus
c. sympathetic chain ganglia
d. cervical plexus
Solution:
B
Critical Thinking Questions
Exercise:
Problem:Why are ganglia and nerves not surrounded by protective structures like the meninges of the CNS?
Solution:
The peripheral nervous tissues are out in the body, sometimes part of other organ systems. There is not a
privileged blood supply like there is to the brain and spinal cord, so peripheral nervous tissues do not need the
same sort of protections.
Exercise:
Problem:
Testing for neurological function involves a series of tests of functions associated with the cranial nerves.
What functions, and therefore which nerves, are being tested by asking a patient to follow the tip of a pen
with their eyes?
Solution:
The contraction of extraocular muscles is being tested, which is the function of the oculomotor, trochlear, and
abducens nerves.
Glossary
abducens nerve
sixth cranial nerve; responsible for contraction of one of the extraocular muscles
axillary nerve
systemic nerve of the arm that arises from the brachial plexus
brachial plexus
nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve
cervical plexus
nerve plexus associated with the upper cervical spinal nerves
cranial nerve
one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head
and neck
cranial nerve ganglion
sensory ganglion of cranial nerves
dorsal (posterior) root ganglion
sensory ganglion attached to the posterior nerve root of a spinal nerve
endoneurium
innermost layer of connective tissue that surrounds individual axons within a nerve
enteric nervous system
peripheral structures, namely ganglia and nerves, that are incorporated into the digestive system organs
enteric plexus
neuronal plexus in the wall of the intestines, which is part of the enteric nervous system
epineurium
outermost layer of connective tissue that surrounds an entire nerve
esophageal plexus
neuronal plexus in the wall of the esophagus that is part of the enteric nervous system
extraocular muscles
six skeletal muscles that control eye movement within the orbit
facial nerve
seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as
well as causing saliva production
fascicle
small bundles of nerve or muscle fibers enclosed by connective tissue
femoral nerve
systemic nerve of the anterior leg that arises from the lumbar plexus
fibular nerve
systemic nerve of the posterior leg that begins as part of the sciatic nerve
gastric plexuses
neuronal networks in the wall of the stomach that are part of the enteric nervous system
glossopharyngeal nerve
ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of
taste, as well as causing saliva production
hypoglossal nerve
twelfth cranial nerve; responsible for contraction of muscles of the tongue
intercostal nerve
systemic nerve in the thoracic cavity that is found between two ribs
lumbar plexus
nerve plexus associated with the lumbar spinal nerves
median nerve
systemic nerve of the arm, located between the ulnar and radial nerves
nerve plexus
network of nerves without neuronal cell bodies included
oculomotor nerve
third cranial nerve; responsible for contraction of four of the extraocular muscles, the muscle in the upper
eyelid, and pupillary constriction
olfactory nerve
first cranial nerve; responsible for the sense of smell
optic nerve
second cranial nerve; responsible for visual sensation
paravertebral ganglia
autonomic ganglia superior to the sympathetic chain ganglia
perineurium
layer of connective tissue surrounding fascicles within a nerve
phrenic nerve
systemic nerve from the cervical plexus that enervates the diaphragm
plexus
network of nerves or nervous tissue
prevertebral ganglia
autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain
ganglia
radial nerve
systemic nerve of the arm, the distal component of which is located near the radial bone
sacral plexus
nerve plexus associated with the lower lumbar and sacral spinal nerves
saphenous nerve
systemic nerve of the lower anterior leg that is a branch from the femoral nerve
sciatic nerve
systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across
the hip joint and gluteal region into the upper posterior leg
sciatica
painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves
that contribute to it
spinal accessory nerve
eleventh cranial nerve; responsible for contraction of neck muscles
spinal nerve
one of 31 nerves connected to the spinal cord
sympathetic chain ganglia
autonomic ganglia in a chain along the anterolateral aspect of the vertebral column that are responsible for
contributing to homeostatic mechanisms of the autonomic nervous system
systemic nerve
nerve in the periphery distal to a nerve plexus or spinal nerve
terminal ganglion
autonomic ganglia that are near or within the walls of organs that are responsible for contributing to
homeostatic mechanisms of the autonomic nervous system
tibial nerve
systemic nerve of the posterior leg that begins as part of the sciatic nerve
trigeminal ganglion
sensory ganglion that contributes sensory fibers to the trigeminal nerve
trigeminal nerve
fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of
mastication
trochlear nerve
fourth cranial nerve; responsible for contraction of one of the extraocular muscles
ulnar nerve
systemic nerve of the arm located close to the ulna, a bone of the forearm
vagus nerve
tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal
cavities
vestibulocochlear nerve
eighth cranial nerve; responsible for the sensations of hearing and balance
Sensory Perception
By the end of this section, you will be able to:
e Describe different types of sensory receptors
e Describe the structures responsible for the special senses of taste, smell,
hearing, balance, and vision
e Distinguish how different tastes are transduced
e Describe the means of mechanoreception for hearing and balance
e List the supporting structures around the eye and describe the structure of
the eyeball
e Describe the processes of phototransduction
A major role of sensory receptors is to help us learn about the environment
around us, or about the state of our internal environment. Stimuli from varying
sources, and of different types, are received and changed into the
electrochemical signals of the nervous system. This occurs when a stimulus
changes the cell membrane potential of a sensory neuron. The stimulus causes
the sensory cell to produce an action potential that is relayed into the central
nervous system (CNS), where it is integrated with other sensory information—
or sometimes higher cognitive functions—to become a conscious perception of
that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a
deliberate distinction. Sensation is the activation of sensory receptor cells at the
level of the stimulus. Perception is the central processing of sensory stimuli
into a meaningful pattern. Perception is dependent on sensation, but not all
sensations are perceived. Receptors are the cells or structures that detect
sensations. A receptor cell is changed directly by a stimulus. A transmembrane
protein receptor is a protein in the cell membrane that mediates a physiological
change in a neuron, most often through the opening of ion channels or changes
in the cell signaling processes. Transmembrane receptors are activated by
chemicals called ligands. For example, a molecule in food can serve as a ligand
for taste receptors. Other transmembrane proteins, which are not accurately
called receptors, are sensitive to mechanical or thermal changes. Physical
changes in these proteins increase ion flow across the membrane, and can
generate an action potential or a graded potential in the sensory neurons.
Sensory Receptors
Stimuli in the environment activate specialized receptor cells in the peripheral
nervous system. Different types of stimuli are sensed by different types of
receptor cells. Receptor cells can be classified into types on the basis of three
different criteria: cell type, position, and function. Receptors can be classified
structurally on the basis of cell type and their position in relation to stimuli
they sense. They can also be classified functionally on the basis of the
transduction of stimuli, or how the mechanical stimulus, light, or chemical
changed the cell membrane potential.
Structural Receptor Types
The cells that interpret information about the environment can be either (1) a
neuron that has a free nerve ending, with dendrites embedded in tissue that
would receive a sensation; (2) a neuron that has an encapsulated ending in
which the sensory nerve endings are encapsulated in connective tissue that
enhances their sensitivity; or (3) a specialized receptor cell, which has distinct
structural components that interpret a specific type of stimulus ((link]). The
pain and temperature receptors in the dermis of the skin are examples of
neurons that have free nerve endings. Also located in the dermis of the skin are
lamellated corpuscles, neurons with encapsulated nerve endings that respond to
pressure and touch. The cells in the retina that respond to light stimuli are an
example of a specialized receptor, a photoreceptor.
Receptor Classification by Cell Type
Free nerve endings
(dendrites)
Axon Cell body
L esaseaacats a a a a
(a) Neuron (receptor) with free nerve endings
Dendrite
Axon
Dee keeeedreernrsts
(b) Neuron (receptor) with encapsulated nerve endings
Encapsulated
nerve ending Bipolar cell
(c) Specialized receptor cell
Receptor cell types can be classified on the basis of
their structure. Sensory neurons can have either (a) free
nerve endings or (b) encapsulated endings.
Photoreceptors in the eyes, such as rod cells, are
examples of (c) specialized receptor cells. These cells
release neurotransmitters onto a bipolar cell, which then
synapses with the optic nerve neurons.
Another way that receptors can be classified is based on their location relative
to the stimuli. An exteroceptor is a receptor that is located near a stimulus in
the external environment, such as the somatosensory receptors that are located
in the skin. An interoceptor is one that interprets stimuli from internal organs
and tissues, such as the receptors that sense the increase in blood pressure in
the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a
moving part of the body, such as a muscle, that interprets the positions of the
tissues as they move.
Functional Receptor Types
A third classification of receptors is by how the receptor transduces stimuli into
membrane potential changes. Stimuli are of three general types. Some stimuli
are ions and macromolecules that affect transmembrane receptor proteins when
these chemicals diffuse across the cell membrane. Some stimuli are physical
variations in the environment that affect receptor cell membrane potentials.
Other stimuli include the electromagnetic radiation from visible light. For
humans, the only electromagnetic energy that is perceived by our eyes is
visible light. Some other organisms have receptors that humans lack, such as
the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic
receptors in migratory birds.
Receptor cells can be further categorized on the basis of the type of stimuli
they transduce. Chemical stimuli can be interpreted by a chemoreceptor that
interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors
respond to solute concentrations of body fluids. Additionally, pain is primarily
a chemical sense that interprets the presence of chemicals from tissue damage,
or similar intense stimuli, through a nociceptor. Physical stimuli, such as
pressure and vibration, as well as the sensation of sound and body position
(balance), are interpreted through a mechanoreceptor. Another physical
stimulus that has its own type of receptor is temperature, which is sensed
through a thermoreceptor that is either sensitive to temperatures above (heat)
or below (cold) normal body temperature.
Sensory Modalities
Ask anyone what the senses are, and they are likely to list the five major senses
—taste, smell, touch, hearing, and sight. However, these are not all of the
senses. The most obvious omission from this list is balance. Also, what is
referred to simply as touch can be further subdivided into pressure, vibration,
stretch, and hair-follicle position, on the basis of the type of mechanoreceptors
that perceive these touch sensations. Other overlooked senses include
temperature perception by thermoreceptors and pain perception by nociceptors.
Within the realm of physiology, senses can be classified as either general or
specific. A general sense is one that is distributed throughout the body and has
receptor cells within the structures of other organs. Mechanoreceptors in the
skin, muscles, or the walls of blood vessels are examples of this type. General
senses often contribute to the sense of touch, as described above, or to
proprioception (body movement) and kinesthesia (body movement), or to a
visceral sense, which is most important to autonomic functions. A special
sense is one that has a specific organ devoted to it, namely the eye, inner ear,
tongue, or nose.
Each of the senses is referred to as a sensory modality. Modality refers to the
way that information is encoded, which is similar to the idea of transduction.
The main sensory modalities can be described on the basis of how each is
transduced. The chemical senses are taste and smell. The general sense that is
usually referred to as touch includes chemical sensation in the form of
nociception, or pain. Pressure, vibration, muscle stretch, and the movement of
hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and
balance are also sensed by mechanoreceptors. Finally, vision involves the
activation of photoreceptors.
Listing all the different sensory modalities, which can number as many as 17,
involves separating the five major senses into more specific categories, or
submodalities, of the larger sense. An individual sensory modality represents
the sensation of a specific type of stimulus. For example, the general sense of
touch, which is known as somatosensation, can be separated into light
pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.
Gustation (Taste)
Only a few recognized submodalities exist within the sense of taste, or
gustation. Until recently, only four tastes were recognized: sweet, salty, sour,
and bitter. Research at the turn of the 20th century led to recognition of the
fifth taste, umami, during the mid-1980s. Umamii is a Japanese word that
means “delicious taste,” and is often translated to mean savory. Very recent
research has suggested that there may also be a sixth taste for fats, or lipids.
Gustation is the special sense associated with the tongue. The surface of the
tongue, along with the rest of the oral cavity, is lined by a stratified squamous
epithelium. Raised bumps called papillae (singular = papilla) contain the
structures for gustatory transduction. There are four types of papillae, based on
their appearance ([link]): circumvallate, foliate, filiform, and fungiform.
Within the structure of the papillae are taste buds that contain specialized
gustatory receptor cells for the transduction of taste stimuli. These receptor
cells are sensitive to the chemicals contained within foods that are ingested,
and they release neurotransmitters based on the amount of the chemical in the
food. Neurotransmitters from the gustatory cells can activate sensory neurons
in the facial, glossopharyngeal, and vagus cranial nerves.
The Tongue
Taste buds
Circumvallate papilla
Taste hairs Taste pore
Fungiform papilla Filiform papilla Foliate papilla
Basal cell Transitional cell
Gustatory cell
The tongue is covered with small bumps, called papillae,
which contain taste buds that are sensitive to chemicals in
ingested food or drink. Different types of papillae are found
in different regions of the tongue. The taste buds contain
specialized gustatory receptor cells that respond to chemical
stimuli dissolved in the saliva. These receptor cells activate
sensory neurons that are part of the facial and
glossopharyngeal nerves. LM x 1600. (Micrograph
provided by the Regents of University of Michigan Medical
School © 2012)
Salty taste is simply the perception of sodium ions (Na’) in the saliva. When
you eat something salty, the salt crystals dissociate into the component ions
Na’ and CI, which dissolve into the saliva in your mouth. The Na*
concentration becomes high outside the gustatory cells, creating a strong
concentration gradient that drives the diffusion of the ion into the cells. The
entry of Na* into these cells results in the depolarization of the cell membrane
and the generation of a receptor potential.
Sour taste is the perception of H* concentration. Just as with sodium ions in
salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour
flavors are, essentially, the perception of acids in our food. Increasing
hydrogen ion concentrations in the saliva (lowering saliva pH) triggers
progressively stronger graded potentials in the gustatory cells. For example,
orange juice—which contains citric acid—will taste sour because it has a pH
value of approximately 3. Of course, it is often sweetened so that the sour taste
is masked.
The first two tastes (salty and sour) are triggered by the cations Na” and H’.
The other tastes result from food molecules binding to a G protein-coupled
receptor. A G protein signal transduction system ultimately leads to
depolarization of the gustatory cell. The sweet taste is the sensitivity of
gustatory cells to the presence of glucose dissolved in the saliva. Other
monosaccharides such as fructose, or artificial sweeteners such as aspartame
(NutraSweet™), saccharine, or sucralose (Splenda™) also activate the sweet
receptors. The affinity for each of these molecules varies, and some will taste
sweeter than glucose because they bind to the G protein-coupled receptor
differently.
Bitter taste is similar to sweet in that food molecules bind to G protein—coupled
receptors. However, there are a number of different ways in which this can
happen because there are a large diversity of bitter-tasting molecules. Some
bitter molecules depolarize gustatory cells, whereas others hyperpolarize
gustatory cells. Likewise, some bitter molecules increase G protein activation
within the gustatory cells, whereas other bitter molecules decrease G protein
activation. The specific response depends on which molecule is binding to the
receptor.
One major group of bitter-tasting molecules are alkaloids. Alkaloids are
nitrogen containing molecules that are commonly found in bitter-tasting plant
products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By
containing toxic alkaloids, the plant is less susceptible to microbe infection and
less attractive to herbivores.
Therefore, the function of bitter taste may primarily be related to stimulating
the gag reflex to avoid ingesting poisons. Because of this, many bitter foods
that are normally ingested are often combined with a sweet component to make
them more palatable (cream and sugar in coffee, for example). The highest
concentration of bitter receptors appear to be in the posterior tongue, where a
gag reflex could still spit out poisonous food.
The taste known as umami is often referred to as the savory taste. Like sweet
and bitter, it is based on the activation of G protein-coupled receptors by a
specific molecule. The molecule that activates this receptor is the amino acid
L-glutamate. Therefore, the umami flavor is often perceived while eating
protein-rich foods. Not surprisingly, dishes that contain meat are often
described as savory.
Once the gustatory cells are activated by the taste molecules, they release
neurotransmitters onto the dendrites of sensory neurons. These neurons are part
of the facial and glossopharyngeal cranial nerves, as well as a component
within the vagus nerve dedicated to the gag reflex. The facial nerve connects to
taste buds in the anterior third of the tongue. The glossopharyngeal nerve
connects to taste buds in the posterior two thirds of the tongue. The vagus
nerve connects to taste buds in the extreme posterior of the tongue, verging on
the pharynx, which are more sensitive to noxious stimuli such as bitterness.
Note:
openstax COLLEGE
3
Watch this video to learn about Dr. Danielle Reed of the Monell Chemical
Senses Center in Philadelphia, Pennsylvania, who became interested in
science at an early age because of her sensory experiences. She recognized
that her sense of taste was unique compared with other people she knew. Now,
she studies the genetic differences between people and their sensitivities to
taste stimuli. In the video, there is a brief image of a person sticking out their
tongue, which has been covered with a colored dye. This is how Dr. Reed is
able to visualize and count papillae on the surface of the tongue. People fall
into two groups known as “tasters” and “non-tasters” based on the density of
papillae on their tongue, which also indicates the number of taste buds. Non-
tasters can taste food, but they are not as sensitive to certain tastes, such as
bitterness. Dr. Reed discovered that she is a non-taster, which explains why
she perceived bitterness differently than other people she knew. Are you very
sensitive to tastes? Can you see any similarities among the members of your
family?
Olfaction (Smell)
Like taste, the sense of smell, or olfaction, is also responsive to chemical
stimuli. The olfactory receptor neurons are located in a small region within the
superior nasal cavity ({link]). This region is referred to as the olfactory
epithelium and contains bipolar sensory neurons. Each olfactory sensory
neuron has dendrites that extend from the apical surface of the epithelium into
the mucus lining the cavity. As airborne molecules are inhaled through the
nose, they pass over the olfactory epithelial region and dissolve into the mucus.
These odorant molecules bind to proteins that keep them dissolved in the
mucus and help transport them to the olfactory dendrites. The odorant—protein
complex binds to a receptor protein within the cell membrane of an olfactory
dendrite. These receptors are G protein—coupled, and will produce a graded
membrane potential in the olfactory neurons.
The axon of an olfactory neuron extends from the basal surface of the
epithelium, through an olfactory foramen in the cribriform plate of the ethmoid
bone, and into the brain. The group of axons called the olfactory tract connect
to the olfactory bulb on the ventral surface of the frontal lobe. From there, the
axons split to travel to several brain regions. Some travel to the cerebrum,
specifically to the primary olfactory cortex that is located in the inferior and
medial areas of the temporal lobe. Others project to structures within the limbic
system and hypothalamus, where smells become associated with long-term
memory and emotional responses. This is how certain smells trigger emotional
memories, such as the smell of food associated with one’s birthplace. Smell is
the one sensory modality that does not synapse in the thalamus before
connecting to the cerebral cortex. This intimate connection between the
olfactory system and the cerebral cortex is one reason why smell can be a
potent trigger of memories and emotion.
The nasal epithelium, including the olfactory cells, can be harmed by airborne
toxic chemicals. Therefore, the olfactory neurons are regularly replaced within
the nasal epithelium, after which the axons of the new neurons must find their
appropriate connections in the olfactory bulb. These new axons grow along the
axons that are already in place in the cranial nerve.
The Olfactory System
Olfactory tract
Olfactory bulb Olfactory
tract
Mitral cells
Olfactory Olfactory neurons
epithelium
Nasalconchae = :
Ethmoid bone
Path of
inhaled air
Filaments of
olfactory nerve
Connective tissue
Olfactory gland
Olfactory receptor
Dendrite
Olfactory
cilia Mucus
Path of inhaled air containing
odorant molecules
(a) Nasal cavity (b) Olfactory system
(c) Olfactory epithelium
(a) The olfactory system begins in the peripheral
structures of the nasal cavity. (b) The olfactory receptor
neurons are within the olfactory epithelium. (c) Axons of
the olfactory receptor neurons project through the
cribriform plate of the ethmoid bone and synapse with
the neurons of the olfactory bulb (tissue source: simian).
LM x 812. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)
Note:
Disorders of the...
Olfactory System: Anosmia
Blunt force trauma to the face, such as that common in many car accidents,
can lead to the loss of the olfactory nerve, and subsequently, loss of the sense
of smell. This condition is known as anosmia. When the frontal lobe of the
brain moves relative to the ethmoid bone, the olfactory tract axons may be
sheared apart. Professional fighters often experience anosmia because of
repeated trauma to face and head. In addition, certain pharmaceuticals, such as
antibiotics, can cause anosmia by killing all the olfactory neurons at once. If
no axons are in place within the olfactory nerve, then the axons from newly
formed olfactory neurons have no guide to lead them to their connections
within the olfactory bulb. There are temporary causes of anosmia, as well,
such as those caused by inflammatory responses related to respiratory
infections or allergies.
Loss of the sense of smell can result in food tasting bland. A person with an
impaired sense of smell may require additional spice and seasoning levels for
food to be tasted. Anosmia may also be related to some presentations of mild
depression, because the loss of enjoyment of food may lead to a general sense
of despair.
The ability of olfactory neurons to replace themselves decreases with age,
leading to age-related anosmia. This explains why some elderly people salt
their food more than younger people do. However, this increased sodium
intake can increase blood volume and blood pressure, increasing the risk of
cardiovascular diseases in the elderly.
Audition (Hearing)
Hearing, or audition, is the transduction of sound waves into a neural signal
that is made possible by the structures of the ear ([link]). The large, fleshy
structure on the lateral aspect of the head is known as the auricle. Some
sources will also refer to this structure as the pinna, though that term is more
appropriate for a structure that can be moved, such as the external ear of a cat.
The C-shaped curves of the auricle direct sound waves toward the auditory
canal. The canal enters the skull through the external auditory meatus of the
temporal bone. At the end of the auditory canal is the tympanic membrane, or
ear drum, which vibrates after it is struck by sound waves. The auricle, ear
canal, and tympanic membrane are often referred to as the external ear. The
middle ear consists of a space spanned by three small bones called the
ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin
names that roughly translate to hammer, anvil, and stirrup. The malleus is
attached to the tympanic membrane and articulates with the incus. The incus,
in turn, articulates with the stapes. The stapes is then attached to the inner ear,
where the sound waves will be transduced into a neural signal. The middle ear
is connected to the pharynx through the Eustachian tube, which helps
equilibrate air pressure across the tympanic membrane. The tube is normally
closed but will pop open when the muscles of the pharynx contract during
swallowing or yawning.
Structures of the Ear
Malleus —_Incus Stapes (attached to
oval window)
Vestibule
Auricle
Vestibular nerve
Cochlear nerve
Round window
Ear canal
Cochlea
: Eustachian tube
Tympanic
membrane cavity
External ear Middle ear Inner ear
Tympanic
The external ear contains the auricle, ear canal,
and tympanic membrane. The middle ear
contains the ossicles and is connected to the
pharynx by the Eustachian tube. The inner ear
contains the cochlea and vestibule, which are
responsible for audition and equilibrium,
respectively.
The inner ear is often described as a bony labyrinth, as it is composed of a
series of canals embedded within the temporal bone. It has two separate
regions, the cochlea and the vestibule, which are responsible for hearing and
balance, respectively. The neural signals from these two regions are relayed to
the brain stem through separate fiber bundles. However, these two distinct
bundles travel together from the inner ear to the brain stem as the
vestibulocochlear nerve. Sound is transduced into neural signals within the
cochlear region of the inner ear, which contains the sensory neurons of the
spiral ganglia. These ganglia are located within the spiral-shaped cochlea of
the inner ear. The cochlea is attached to the stapes through the oval window.
The oval window is located at the beginning of a fluid-filled tube within the
cochlea called the scala vestibuli. The scala vestibuli extends from the oval
window, travelling above the cochlear duct, which is the central cavity of the
cochlea that contains the sound-transducing neurons. At the uppermost tip of
the cochlea, the scala vestibuli curves over the top of the cochlear duct. The
fluid-filled tube, now called the scala tympani, returns to the base of the
cochlea, this time travelling under the cochlear duct. The scala tympani ends at
the round window, which is covered by a membrane that contains the fluid
within the scala. As vibrations of the ossicles travel through the oval window,
the fluid of the scala vestibuli and scala tympani moves in a wave-like motion.
The frequency of the fluid waves match the frequencies of the sound waves
({link]). The membrane covering the round window will bulge out or pucker in
with the movement of the fluid within the scala tympani.
Transmission of Sound Waves to Cochlea
@ Tympanic membrane _—_@) Vibrations are
vibrates in response amplified across
to sound wave. ossicles.
(0) Sound wave represents alternating
areas of high and low pressure.
139d)
}
\
\\
| |
Wavelength
wy ,
Ne Z
oe Frequency of sound wave : , , - ;
measured in Hz (cycles ® Vibrations against oval window set up standing
per second) wave in fluid of vestibuli.
Scala
’ vestibuli
ima ; Cochlear
nf duct
Organ of Corti
Basilar
membrane
Scala tympani
© Pressure bends the membrane of the
cochlear duct at a point of maximum WENEVAVAVAAVE
vibration for a given frequency, causing
hair cells in the basilar membrane to Frequency of standing
vibrate. wave is the same as
sound wave
A sound wave causes the tympanic membrane to vibrate.
This vibration is amplified as it moves across the malleus,
incus, and stapes. The amplified vibration is picked up by
the oval window causing pressure waves in the fluid of the
scala vestibuli and scala tympani. The complexity of the
pressure waves is determined by the changes in amplitude
and frequency of the sound waves entering the ear.
A cross-sectional view of the cochlea shows that the scala vestibuli and scala
tympani run along both sides of the cochlear duct ([link]). The cochlear duct
contains several organs of Corti, which tranduce the wave motion of the two
scala into neural signals. The organs of Corti lie on top of the basilar
membrane, which is the side of the cochlear duct located between the organs
of Corti and the scala tympani. As the fluid waves move through the scala
vestibuli and scala tympani, the basilar membrane moves at a specific spot,
depending on the frequency of the waves. Higher frequency waves move the
region of the basilar membrane that is close to the base of the cochlea. Lower
frequency waves move the region of the basilar membrane that is near the tip
of the cochlea.
Cross Section of the Cochlea
Bony cochlear wall
Scala vestibuli
Cochlear duct
Tectorial membrane
Basilar membrane
Scala tympani
— ———— Cochlear branch
of N VIII
The three major spaces within the cochlea are
highlighted. The scala tympani and scala vestibuli lie
on either side of the cochlear duct. The organ of Corti,
containing the mechanoreceptor hair cells, is adjacent
to the scala tympani, where it sits atop the basilar
membrane.
The organs of Corti contain hair cells, which are named for the hair-like
stereocilia extending from the cell’s apical surfaces ([{link]). The stereocilia are
an array of microvilli-like structures arranged from tallest to shortest. Protein
fibers tether adjacent hairs together within each array, such that the array will
bend in response to movements of the basilar membrane. The stereocilia
extend up from the hair cells to the overlying tectorial membrane, which is
attached medially to the organ of Corti. When the pressure waves from the
scala move the basilar membrane, the tectorial membrane slides across the
stereocilia. This bends the stereocilia either toward or away from the tallest
member of each array. When the stereocilia bend toward the tallest member of
their array, tension in the protein tethers opens ion channels in the hair cell
membrane. This will depolarize the hair cell membrane, triggering nerve
impulses that travel down the afferent nerve fibers attached to the hair cells.
When the stereocilia bend toward the shortest member of their array, the
tension on the tethers slackens and the ion channels close. When no sound is
present, and the stereocilia are standing straight, a small amount of tension still
exists on the tethers, keeping the membrane potential of the hair cell slightly
depolarized.
Hair Cell
ge tia
Tether
Stereocilia
Hair cell
The hair cell is a mechanoreceptor with an array of
stereocilia emerging from its apical surface. The
stereocilia are tethered together by proteins that open ion
channels when the array is bent toward the tallest
member of their array, and closed when the array is bent
toward the shortest member of their array.
Cochlea and Organ of Corti
LM x 412. (Micrograph provided by the Regents
of University of Michigan Medical School ©
2012)
Note:
ae
— openstax COLLEGE
View the University of Michigan WebScope to explore the tissue sample in
greater detail. The basilar membrane is the thin membrane that extends from
the central core of the cochlea to the edge. What is anchored to this membrane
so that they can be activated by movement of the fluids within the cochlea?
As stated above, a given region of the basilar membrane will only move if the
incoming sound is at a specific frequency. Because the tectorial membrane
only moves where the basilar membrane moves, the hair cells in this region
will also only respond to sounds of this specific frequency. Therefore, as the
frequency of a sound changes, different hair cells are activated all along the
basilar membrane. The cochlea encodes auditory stimuli for frequencies
between 20 and 20,000 Hz, which is the range of sound that human ears can
detect. The unit of Hertz measures the frequency of sound waves in terms of
cycles produced per second. Frequencies as low as 20 Hz are detected by hair
cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20
KHz are encoded by hair cells at the base of the cochlea, close to the round and
oval windows ((link]). Most auditory stimuli contain a mixture of sounds at a
variety of frequencies and intensities (represented by the amplitude of the
sound wave). The hair cells along the length of the cochlear duct, which are
each sensitive to a particular frequency, allow the cochlea to separate auditory
stimuli by frequency, just as a prism separates visible light into its component
colors.
Frequency Coding in the Cochlea
Oval window base
Tectorial
membrane
20,000 Hz 1500 Hz 20 Hz
(high frequency) = (medium frequency) (low frequency)
_—
Relative length of fibers in basilar membrane
Basilar
membrane
Round window
The standing sound wave generated in the cochlea by the
movement of the oval window deflects the basilar
membrane on the basis of the frequency of sound.
Therefore, hair cells at the base of the cochlea are
activated only by high frequencies, whereas those at the
apex of the cochlea are activated only by low
frequencies.
Note:
cerae
pene Ea)
_ openstax COLLEGE”
. 7,
1 oe rat
Watch this video to learn more about how the structures of the ear convert
sound waves into a neural signal by moving the “hairs,” or stereocilia, of the
cochlear duct. Specific locations along the length of the duct encode specific
frequencies, or pitches. The brain interprets the meaning of the sounds we
hear as music, speech, noise, etc. Which ear structures are responsible for the
amplification and transfer of sound from the external ear to the inner ear?
Note:
mms Openstax COLLEGE
| Te on Fa tae
Watch this animation to learn more about the inner ear and to see the cochlea
unroll, with the base at the back of the image and the apex at the front.
Specific wavelengths of sound cause specific regions of the basilar membrane
to vibrate, much like the keys of a piano produce sound at different
frequencies. Based on the animation, where do frequencies—from high to low
pitches—cause activity in the hair cells within the cochlear duct?
Equilibrium (Balance)
Along with audition, the inner ear is responsible for encoding information
about equilibrium, the sense of balance. A similar mechanoreceptor—a hair
cell with stereocilia—senses head position, head movement, and whether our
bodies are in motion. These cells are located within the vestibule of the inner
ear. Head position is sensed by the utricle and saccule, whereas head
movement is sensed by the semicircular canals. The neural signals generated
in the vestibular ganglion are transmitted through the vestibulocochlear nerve
to the brain stem and cerebellum.
The utricle and saccule are both largely composed of macula tissue (plural =
maculae). The macula is composed of hair cells surrounded by support cells.
The stereocilia of the hair cells extend into a viscous gel called the otolithic
membrane ((link]). On top of the otolithic membrane is a layer of calcium
carbonate crystals, called otoliths. The otoliths essentially make the otolithic
membrane top-heavy. The otolithic membrane moves separately from the
macula in response to head movements. Tilting the head causes the otolithic
membrane to slide over the macula in the direction of gravity. The moving
otolithic membrane, in turn, bends the sterocilia, causing some hair cells to
depolarize as others hyperpolarize. The exact position of the head is interpreted
by the brain based on the pattern of hair-cell depolarization.
Linear Acceleration Coding by Maculae
Otolithic
Otoliths
membrane i
2 a Teh = 2-5
Head upright
Endolymph
Macula
Otoliths
Otolithic
membrane
Hair cells
Vestibular division of ‘ -
vestibulocochlear nerve Head tilted forward
aC
\
The maculae are specialized for sensing linear acceleration,
such as when gravity acts on the tilting head, or if the head
starts moving in a straight line. The difference in inertia
between the hair cell stereocilia and the otolithic membrane
in which they are embedded leads to a shearing force that
causes the stereocilia to bend in the direction of that linear
acceleration.
The semicircular canals are three ring-like extensions of the vestibule. One is
oriented in the horizontal plane, whereas the other two are oriented in the
vertical plane. The anterior and posterior vertical canals are oriented at
approximately 45 degrees relative to the sagittal plane ([link]). The base of
each semicircular canal, where it meets with the vestibule, connects to an
enlarged region known as the ampulla. The ampulla contains the hair cells that
respond to rotational movement, such as turning the head while saying “no.”
The stereocilia of these hair cells extend into the cupula, a membrane that
attaches to the top of the ampulla. As the head rotates in a plane parallel to the
semicircular canal, the fluid lags, deflecting the cupula in the direction
opposite to the head movement. The semicircular canals contain several
ampullae, with some oriented horizontally and others oriented vertically. By
comparing the relative movements of both the horizontal and vertical
ampullae, the vestibular system can detect the direction of most head
movements within three-dimensional (3-D) space.
Rotational Coding by Semicircular Canals
h As the head rotates,
Ampullary nerve \\\\\) cupula bends in opposite
direction of the rotation
Rotational movement of the head is encoded by the hair
cells in the base of the semicircular canals. As one of the
canals moves in an arc with the head, the internal fluid
moves in the opposite direction, causing the cupula and
stereocilia to bend. The movement of two canals within a
plane results in information about the direction in which the
head is moving, and activation of all six canals can give a
very precise indication of head movement in three
dimensions.
Somatosensation (Touch)
Somatosensation is considered a general sense, as opposed to the special senses
discussed in this section. Somatosensation is the group of sensory modalities
that are associated with touch, proprioception, and interoception. These
modalities include pressure, vibration, light touch, tickle, itch, temperature,
pain, proprioception, and kinesthesia. This means that its receptors are not
associated with a specialized organ, but are instead spread throughout the body
in a variety of organs. Many of the somatosensory receptors are located in the
skin, but receptors are also found in muscles, tendons, joint capsules,
ligaments, and in the walls of visceral organs.
Two types of somatosensory signals that are transduced by free nerve endings
are pain and temperature. These two modalities use thermoreceptors and
nociceptors to transduce temperature and pain stimuli, respectively.
Temperature receptors are stimulated when local temperatures differ from body
temperature. Some thermoreceptors are sensitive to just cold and others to just
heat. Nociception is the sensation of potentially damaging stimuli. Mechanical,
chemical, or thermal stimuli beyond a set threshold will elicit painful
sensations. Stressed or damaged tissues release chemicals that activate receptor
proteins in the nociceptors. For example, the sensation of heat associated with
spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin
molecules bind to a transmembrane ion channel in nociceptors that is sensitive
to temperatures above 37°C. The dynamics of capsaicin binding with this
transmembrane ion channel is unusual in that the molecule remains bound for a
long time. Because of this, it will decrease the ability of other stimuli to elicit
pain sensations through the activated nociceptor. For this reason, capsaicin can
be used as a topical analgesic, such as in products such as Icy Hot™.
If you drag your finger across a textured surface, the skin of your finger will
vibrate. Such low frequency vibrations are sensed by mechanoreceptors called
Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells
are located in the stratum basale of the epidermis. Deep pressure and vibration
is transduced by lamellated (Pacinian) corpuscles, which are receptors with
encapsulated endings found deep in the dermis, or subcutaneous tissue. Light
touch is transduced by the encapsulated endings known as tactile (Meissner)
corpuscles. Follicles are also wrapped in a plexus of nerve endings known as
the hair follicle plexus. These nerve endings detect the movement of hair at the
surface of the skin, such as when an insect may be walking along the skin.
Stretching of the skin is transduced by stretch receptors known as bulbous
corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II
cutaneous mechanoreceptors.
Other somatosensory receptors are found in the joints and muscles. Stretch
receptors monitor the stretching of tendons, muscles, and the components of
joints. For example, have you ever stretched your muscles before or after
exercise and noticed that you can only stretch so far before your muscles
spasm back to a less stretched state? This spasm is a reflex that is initiated by
stretch receptors to avoid muscle tearing. Such stretch receptors can also
prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch
receptors are called muscle spindles. Golgi tendon organs similarly transduce
the stretch levels of tendons. Bulbous corpuscles are also present in joint
capsules, where they measure stretch in the components of the skeletal system
within the joint. The types of nerve endings, their locations, and the stimuli
they transduce are presented in [link].
Mechanoreceptors of Somatosensation
Name
Free nerve
endings
Mechanoreceptors
Bulbous
corpuscle
Historical
(eponymous)
name
Merkel’s
discs
Ruffini’s
corpuscle
Location(s)
Dermis,
cornea,
tongue, joint
capsules,
visceral
organs
Epidermal—
dermal
junction,
mucosal
membranes
Dermis, joint
capsules
Stimuli
Pain,
temperature,
mechanical
deformation
Low
frequency
vibration
(5-15 Hz)
Stretch
Mechanoreceptors of Somatosensation
Name
Tactile corpuscle
Lamellated
corpuscle
Hair follicle
plexus
Muscle spindle
Tendon stretch
organ
Historical
(eponymous)
name
Meissner’s
corpuscle
Pacinian
corpuscle
Golgi tendon
organ
*No corresponding eponymous name.
Vision
Location(s)
Papillary
dermis,
especially in
the fingertips
and lips
Deep dermis,
subcutaneous
tissue
Wrapped
around hair
follicles in
the dermis
In line with
skeletal
muscle fibers
In line with
tendons
Stimuli
Light touch,
vibrations
below 50
Hz
Deep
pressure,
high-
frequency
vibration
(around 250
Hz)
Movement
of hair
Muscle
contraction
and stretch
Stretch of
tendons
Vision is the special sense of sight that is based on the transduction of light
stimuli received through the eyes. The eyes are located within either orbit in
the skull. The bony orbits surround the eyeballs, protecting them and anchoring
the soft tissues of the eye ({link]). The eyelids, with lashes at their leading
edges, help to protect the eye from abrasions by blocking particles that may
land on the surface of the eye. The inner surface of each lid is a thin membrane
known as the palpebral conjunctiva. The conjunctiva extends over the white
areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are
produced by the lacrimal gland, located beneath the lateral edges of the nose.
Tears produced by this gland flow through the lacrimal duct to the medial
corner of the eye, where the tears flow over the conjunctiva, washing away
foreign particles.
The Eye in the Orbit
ign _— ; Eyebrow
i
we Aas Orbicularis oculi
muscle
Levator palpebrae
superioris muscle
Palpebral conjunctiva
Eyelashes
Cornea
Conjunctiva
The eye is located within the orbit and
surrounded by soft tissues that protect and
support its function. The orbit is surrounded by
cranial bones of the skull.
Movement of the eye within the orbit is accomplished by the contraction of six
extraocular muscles that originate from the bones of the orbit and insert into
the surface of the eyeball ([link]). Four of the muscles are arranged at the
cardinal points around the eye and are named for those locations. They are the
superior rectus, medial rectus, inferior rectus, and lateral rectus. When
each of these muscles contract, the eye to moves toward the contracting
muscle. For example, when the superior rectus contracts, the eye rotates to
look up. The superior oblique originates at the posterior orbit, near the origin
of the four rectus muscles. However, the tendon of the oblique muscles threads
through a pulley-like piece of cartilage known as the trochlea. The tendon
inserts obliquely into the superior surface of the eye. The angle of the tendon
through the trochlea means that contraction of the superior oblique rotates the
eye medially. The inferior oblique muscle originates from the floor of the
orbit and inserts into the inferolateral surface of the eye. When it contracts, it
laterally rotates the eye, in opposition to the superior oblique. Rotation of the
eye by the two oblique muscles is necessary because the eye is not perfectly
aligned on the sagittal plane. When the eye looks up or down, the eye must
also rotate slightly to compensate for the superior rectus pulling at
approximately a 20-degree angle, rather than straight up. The same is true for
the inferior rectus, which is compensated by contraction of the inferior oblique.
A seventh muscle in the orbit is the levator palpebrae superioris, which is
responsible for elevating and retracting the upper eyelid, a movement that
usually occurs in concert with elevation of the eye by the superior rectus (see
[link]).
The extraocular muscles are innervated by three cranial nerves. The lateral
rectus, which causes abduction of the eye, is innervated by the abducens nerve.
The superior oblique is innervated by the trochlear nerve. All of the other
muscles are innervated by the oculomotor nerve, as is the levator palpebrae
superioris. The motor nuclei of these cranial nerves connect to the brain stem,
which coordinates eye movements.
Extraocular Muscles
Superior oblique
muscle
Trochlea Superior x Trochlea
; ‘ rectus ae
Superior oblique
tendon Superior
Superior rectus oblique
muscle
Lateral rectus
muscle r =;
Lateral rectus a Medial rectus
Inferior Inferior
oblique rectus
Common Inferior rectus Inferior oblique SS
tendinous ring muscle muscle
Lateral view of the right eye Anterior view of the right eye
The extraocular muscles move the eye within the orbit.
The eye itself is a hollow sphere composed of three layers of tissue. The
outermost layer is the fibrous tunic, which includes the white sclera and clear
cornea. The sclera accounts for five sixths of the surface of the eye, most of
which is not visible, though humans are unique compared with many other
species in having so much of the “white of the eye” visible ({link]). The
transparent cornea covers the anterior tip of the eye and allows light to enter
the eye. The middle layer of the eye is the vascular tunic, which is mostly
composed of the choroid, ciliary body, and iris. The choroid is a layer of
highly vascularized connective tissue that provides a blood supply to the
eyeball. The choroid is posterior to the ciliary body, a muscular structure that
is attached to the lens by suspensory ligaments, or zonule fibers. These two
structures bend the lens, allowing it to focus light on the back of the eye.
Overlaying the ciliary body, and visible in the anterior eye, is the iris—the
colored part of the eye. The iris is a smooth muscle that opens or closes the
pupil, which is the hole at the center of the eye that allows light to enter. The
iris constricts the pupil in response to bright light and dilates the pupil in
response to dim light. The innermost layer of the eye is the neural tunic, or
retina, which contains the nervous tissue responsible for photoreception.
The eye is also divided into two cavities: the anterior cavity and the posterior
cavity. The anterior cavity is the space between the cornea and lens, including
the iris and ciliary body. It is filled with a watery fluid called the aqueous
humor. The posterior cavity is the space behind the lens that extends to the
posterior side of the interior eyeball, where the retina is located. The posterior
cavity is filled with a more viscous fluid called the vitreous humor.
The retina is composed of several layers and contains specialized cells for the
initial processing of visual stimuli. The photoreceptors (rods and cones) change
their membrane potential when stimulated by light energy. The change in
membrane potential alters the amount of neurotransmitter that the
photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is
the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion
cell (RGC) in the inner synaptic layer. There, amacrine cells additionally
contribute to retinal processing before an action potential is produced by the
RGC. The axons of RGCs, which lie at the innermost layer of the retina,
collect at the optic disc and leave the eye as the optic nerve (see [link]).
Because these axons pass through the retina, there are no photoreceptors at the
very back of the eye, where the optic nerve begins. This creates a “blind spot”
in the retina, and a corresponding blind spot in our visual field.
Structure of the Eye
Lateral
Posterior cavity
Vitreous chamber
; N
Scleral venous sinus
= . (canal of Schlemm)
|
/ A \ Suspensory ligaments
Lens
Lateral rectus
muscle
Sclera
Choroid
; ———— Cornea
Retina
Iris
] Pupil
Anterior cavity
(contains aqueous humor):
. Posterior chamber
| J Anterior chamber
@
| r > Suspensory ligaments
ff [ la Ciliary body:
' Ciliary process
Ciliary muscle
Fovea centralis
Optic (II) nerve —
Central retinal
artery and vein
Optic disc
(blind spot)
Medial rectus
muscle
Medial
The sphere of the eye can be divided into anterior and
posterior chambers. The wall of the eye is composed of
three layers: the fibrous tunic, vascular tunic, and neural
tunic. Within the neural tunic is the retina, with three layers
of cells and two synaptic layers in between. The center of
the retina has a small indentation known as the fovea.
Note that the photoreceptors in the retina (rods and cones) are located behind
the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount
of light is absorbed by these structures before the light reaches the
photoreceptor cells. However, at the exact center of the retina is a small area
known as the fovea. At the fovea, the retina lacks the supporting cells and
blood vessels, and only contains photoreceptors. Therefore, visual acuity, or
the sharpness of vision, is greatest at the fovea. This is because the fovea is
where the least amount of incoming light is absorbed by other retinal structures
(see [link]). As one moves in either direction from this central point of the
retina, visual acuity drops significantly. In addition, each photoreceptor cell of
the fovea is connected to a single RGC. Therefore, this RGC does not have to
integrate inputs from multiple photoreceptors, which reduces the accuracy of
visual transduction. Toward the edges of the retina, several photoreceptors
converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The
difference in visual acuity between the fovea and peripheral retina is easily
evidenced by looking directly at a word in the middle of this paragraph. The
visual stimulus in the middle of the field of view falls on the fovea and is in the
sharpest focus. Without moving your eyes off that word, notice that words at
the beginning or end of the paragraph are not in focus. The images in your
peripheral vision are focused by the peripheral retina, and have vague, blurry
edges and words that are not as clearly identified. As a result, a large part of
the neural function of the eyes is concerned with moving the eyes and head so
that important visual stimuli are centered on the fovea.
Light falling on the retina causes chemical changes to pigment molecules in the
photoreceptors, ultimately leading to a change in the activity of the RGCs.
Photoreceptor cells have two parts, the inner segment and the outer segment
([link]). The inner segment contains the nucleus and other common organelles
of a cell, whereas the outer segment is a specialized region in which
photoreception takes place. There are two types of photoreceptors—rods and
cones—which differ in the shape of their outer segment. The rod-shaped outer
segments of the rod photoreceptor contain a stack of membrane-bound discs
that contain the photosensitive pigment rhodopsin. The cone-shaped outer
segments of the cone photoreceptor contain their photosensitive pigments in
infoldings of the cell membrane. There are three cone photopigments, called
opsins, which are each sensitive to a particular wavelength of light. The
wavelength of visible light determines its color. The pigments in human eyes
are specialized in perceiving three different primary colors: red, green, and
blue.
Photoreceptor
y Pigment
epithelium
Melanin granules
Discs
Connecting stalks Mitochondria
Rods
Golgi apparatus
Cone
Nuclei
Bipolar cell
Ganglion cell
(a)
Choroid
Pigment epithelium
Rods and cones
Bipolar cells
Ganglion cells
Optic nerve axons
(b)
(a) All photoreceptors have inner segments
containing the nucleus and other important
organelles and outer segments with membrane
arrays containing the photosensitive opsin
molecules. Rod outer segments are long columnar
shapes with stacks of membrane-bound discs that
contain the rhodopsin pigment. Cone outer
segments are short, tapered shapes with folds of
membrane in place of the discs in the rods. (b)
Tissue of the retina shows a dense layer of nuclei
of the rods and cones. LM x 800. (Micrograph
provided by the Regents of University of Michigan
Medical School © 2012)
At the molecular level, visual stimuli cause changes in the photopigment
molecule that lead to changes in membrane potential of the photoreceptor cell.
A single unit of light is called a photon, which is described in physics as a
packet of energy with properties of both a particle and a wave. The energy of a
photon is represented by its wavelength, with each wavelength of visible light
corresponding to a particular color. Visible light is electromagnetic radiation
with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic
radiation longer than 720 nm fall into the infrared range, whereas wavelengths
shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of
380 nm is blue whereas light with a wavelength of 720 nm is dark red. All
other colors fall between red and blue at various points along the wavelength
scale.
Opsin pigments are actually transmembrane proteins that contain a cofactor
known as retinal. Retinal is a hydrocarbon molecule related to vitamin A.
When a photon hits retinal, the long hydrocarbon chain of the molecule is
biochemically altered. Specifically, photons cause some of the double-bonded
carbons within the chain to switch from a cis to a trans conformation. This
process is called photoisomerization. Before interacting with a photon,
retinal’s flexible double-bonded carbons are in the cis conformation. This
molecule is referred to as 11-cis-retinal. A photon interacting with the
molecule causes the flexible double-bonded carbons to change to the trans-
conformation, forming all-trans-retinal, which has a straight hydrocarbon chain
((link]).
The shape change of retinal in the photoreceptors initiates visual transduction
in the retina. Activation of retinal and the opsin proteins result in activation of
a G protein. The G protein changes the membrane potential of the
photoreceptor cell, which then releases less neurotransmitter into the outer
synaptic layer of the retina. Until the retinal molecule is changed back to the
11-cis-retinal shape, the opsin cannot respond to light energy, which is called
bleaching. When a large group of photopigments is bleached, the retina will
send information as if opposing visual information is being perceived. After a
bright flash of light, afterimages are usually seen in negative. The
photoisomerization is reversed by a series of enzymatic changes so that the
retinal responds to more light energy.
Retinal Isomers
Photon
11-trans-retinal
11-cis-retinal Fai “eA
and opsin are
reassembled
to form
rhodopsin
Regeneration
Rhodopsin
molecules
Rod bar of
11-cis- tra,
(a) 11-cis-retinal (b) all-trans-retinal
The retinal molecule has two isomers, (a) one before a
photon interacts with it and (b) one that is altered through
photoisomerization.
The opsins are sensitive to limited wavelengths of light. Rhodopsin, the
photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The
three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm
corresponding roughly to the primary colors of red, green, and blue ({link]).
The absorbance of rhodopsin in the rods is much more sensitive than in the
cone opsins; specifically, rods are sensitive to vision in low light conditions,
and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin
will be constantly bleached while the cones are active. In a darkened room,
there is not enough light to activate cone opsins, and vision is entirely
dependent on rods. Rods are so sensitive to light that a single photon can result
in an action potential from a rod’s corresponding RGC.
The three types of cone opsins, being sensitive to different wavelengths of
light, provide us with color vision. By comparing the activity of the three
different cones, the brain can extract color information from visual stimuli. For
example, a bright blue light that has a wavelength of approximately 450 nm
would activate the “red” cones minimally, the “green” cones marginally, and
the “blue” cones predominantly. The relative activation of the three different
cones is calculated by the brain, which perceives the color as blue. However,
cones cannot react to low-intensity light, and rods do not sense the color of
light. Therefore, our low-light vision is—in essence—in grayscale. In other
words, in a dark room, everything appears as a shade of gray. If you think that
you can see colors in the dark, it is most likely because your brain knows what
color something is and is relying on that memory.
Comparison of Color Sensitivity of Photopigments
420 nm 498nm 534nm 564nm
Green Red
Blue cones Rods cones cones
Normalized absorbance
400 500 600 700
Violet Blue Cyan Green Yellow Red
Wavelength (nm)
Comparing the peak sensitivity and absorbance
spectra of the four photopigments suggests that
they are most sensitive to particular
wavelengths.
OR page |
Watch this video to learn more about a transverse section through the brain
that depicts the visual pathway from the eye to the occipital cortex. The first
half of the pathway is the projection from the RGCs through the optic nerve to
the lateral geniculate nucleus in the thalamus on either side. This first fiber in
the pathway synapses on a thalamic cell that then projects to the visual cortex
in the occipital lobe where “seeing,” or visual perception, takes place. This
video gives an abbreviated overview of the visual system by concentrating on
the pathway from the eyes to the occipital lobe. The video makes the
statement (at 0:45) that “specialized cells in the retina called ganglion cells
convert the light rays into electrical signals.” What aspect of retinal processing
is simplified by that statement? Explain your answer.
Sensory Nerves
Once any sensory cell transduces a stimulus into a nerve impulse, that impulse
has to travel along axons to reach the CNS. In many of the special senses, the
axons leaving the sensory receptors have a topographical arrangement,
meaning that the location of the sensory receptor relates to the location of the
axon in the nerve. For example, in the retina, axons from RGCs in the fovea
are located at the center of the optic nerve, where they are surrounded by axons
from the more peripheral RGCs.
Spinal Nerves
Generally, spinal nerves contain afferent axons from sensory receptors in the
periphery, such as from the skin, mixed with efferent axons travelling to the
muscles or other effector organs. As the spinal nerve nears the spinal cord, it
splits into dorsal and ventral roots. The dorsal root contains only the axons of
sensory neurons, whereas the ventral roots contain only the axons of the motor
neurons. Some of the branches will synapse with local neurons in the dorsal
root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the
level of the spinal cord where they enter. Other branches will travel a short
distance up or down the spine to interact with neurons at other levels of the
spinal cord. A branch may also turn into the posterior (dorsal) column of the
white matter to connect with the brain. For the sake of convenience, we will
use the terms ventral and dorsal in reference to structures within the spinal cord
that are part of these pathways. This will help to underscore the relationships
between the different components. Typically, spinal nerve systems that connect
to the brain are contralateral, in that the right side of the body is connected to
the left side of the brain and the left side of the body to the right side of the
brain.
Cranial Nerves
Cranial nerves convey specific sensory information from the head and neck
directly to the brain. For sensations below the neck, the right side of the body
is connected to the left side of the brain and the left side of the body to the right
side of the brain. Whereas spinal information is contralateral, cranial nerve
systems are mostly ipsilateral, meaning that a cranial nerve on the right side of
the head is connected to the right side of the brain. Some cranial nerves contain
only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves.
Other cranial nerves contain both sensory and motor axons, including the
trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus
nerve is not associated with the somatic nervous system). The general senses of
somatosensation for the face travel through the trigeminal system.
Chapter Review
The senses are olfaction (smell), gustation (taste), somatosensation (sensations
associated with the skin and body), audition (hearing), equilibrium (balance),
and vision. With the exception of somatosensation, this list represents the
special senses, or those systems of the body that are associated with specific
organs such as the tongue or eye. Somatosensation belongs to the general
senses, which are those sensory structures that are distributed throughout the
body and in the walls of various organs. The special senses are all primarily
part of the somatic nervous system in that they are consciously perceived
through cerebral processes, though some special senses contribute to
autonomic function. The general senses can be divided into somatosensation,
which is commonly considered touch, but includes tactile, pressure, vibration,
temperature, and pain perception. The general senses also include the visceral
senses, which are separate from the somatic nervous system function in that
they do not normally rise to the level of conscious perception.
The cells that transduce sensory stimuli into the electrochemical signals of the
nervous system are classified on the basis of structural or functional aspects of
the cells. The structural classifications are either based on the anatomy of the
cell that is interacting with the stimulus (free nerve endings, encapsulated
endings, or specialized receptor cell), or where the cell is located relative to the
stimulus (interoceptor, exteroceptor, proprioceptor). Thirdly, the functional
classification is based on how the cell transduces the stimulus into a neural
signal. Chemoreceptors respond to chemical stimuli and are the basis for
olfaction and gustation. Related to chemoreceptors are osmoreceptors and
nociceptors for fluid balance and pain reception, respectively.
Mechanoreceptors respond to mechanical stimuli and are the basis for most
aspects of somatosensation, as well as being the basis of audition and
equilibrium in the inner ear. Thermoreceptors are sensitive to temperature
changes, and photoreceptors are sensitive to light energy.
The nerves that convey sensory information from the periphery to the CNS are
either spinal nerves, connected to the spinal cord, or cranial nerves, connected
to the brain. Spinal nerves have mixed populations of fibers; some are motor
fibers and some are sensory. The sensory fibers connect to the spinal cord
through the dorsal root, which is attached to the dorsal root ganglion. Sensory
information from the body that is conveyed through spinal nerves will project
to the opposite side of the brain to be processed by the cerebral cortex. The
cranial nerves can be strictly sensory fibers, such as the olfactory, optic, and
vestibulocochlear nerves, or mixed sensory and motor nerves, such as the
trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves are
connected to the same side of the brain from which the sensory information
originates.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn about Dr. Danielle Reed of the Monell Chemical
Senses Center in Philadelphia, PA, who became interested in science at an
early age because of her sensory experiences. She recognized that her
sense of taste was unique compared with other people she knew. Now, she
studies the genetic differences between people and their sensitivities to
taste stimuli. In the video, there is a brief image of a person sticking out
their tongue, which has been covered with a colored dye. This is how Dr.
Reed is able to visualize and count papillae on the surface of the tongue.
People fall into two large groups known as “tasters” and “non-tasters” on
the basis of the density of papillae on their tongue, which also indicates
the number of taste buds. Non-tasters can taste food, but they are not as
sensitive to certain tastes, such as bitterness. Dr. Reed discovered that she
is anon-taster, which explains why she perceived bitterness differently
than other people she knew. Are you very sensitive to tastes? Can you see
any similarities among the members of your family?
Solution:
Answers will vary, but a typical answer might be: I can eat most anything
(except mushrooms! ), so I don’t think that I’m that sensitive to tastes. My
whole family likes eating a variety of foods, so it seems that we all have
the same level of sensitivity.
Exercise:
Problem:
[link] The basilar membrane is the thin membrane that extends from the
central core of the cochlea to the edge. What is anchored to this
membrane so that they can be activated by movement of the fluids within
the cochlea?
Solution:
[link] The hair cells are located in the organ of Corti, which is located on
the basilar membrane. The stereocilia of those cells would normally be
attached to the tectorial membrane (though they are detached in the
micrograph because of processing of the tissue).
Exercise:
Problem:
Watch this video to learn more about how the structures of the ear convert
sound waves into a neural signal by moving the “hairs,” or stereocilia, of
the cochlear duct. Specific locations along the length of the duct encode
specific frequencies, or pitches. The brain interprets the meaning of the
sounds we hear as music, speech, noise, etc. Which ear structures are
responsible for the amplification and transfer of sound from the external
ear to the inner ear?
Solution:
The small bones in the middle ear, the ossicles, amplify and transfer
sound between the tympanic membrane of the external ear and the oval
window of the inner ear.
Exercise:
Problem:
Watch this animation to learn more about the inner ear and to see the
cochlea unroll, with the base at the back of the image and the apex at the
front. Specific wavelengths of sound cause specific regions of the basilar
membrane to vibrate, much like the keys of a piano produce sound at
different frequencies. Based on the animation, where do frequencies—
from high to low pitches—cause activity in the hair cells within the
cochlear duct?
Solution:
High frequencies activate hair cells toward the base of the cochlea, and
low frequencies activate hair cells toward the apex of the cochlea.
Exercise:
Problem:
Watch this video to learn more about a transverse section through the
brain that depicts the visual pathway from the eye to the occipital cortex.
The first half of the pathway is the projection from the RGCs through the
optic nerve to the lateral geniculate nucleus in the thalamus on either side.
This first fiber in the pathway synapses on a thalamic cell that then
projects to the visual cortex in the occipital lobe where “seeing,” or visual
perception, takes place. This video gives an abbreviated overview of the
visual system by concentrating on the pathway from the eyes to the
occipital lobe. The video makes the statement (at 0:45) that “specialized
cells in the retina called ganglion cells convert the light rays into electrical
signals.” What aspect of retinal processing is simplified by that statement?
Explain your answer.
Solution:
Photoreceptors convert light energy, or photons, into an electrochemical
signal. The retina contains bipolar cells and the RGCs that finally convert
it into action potentials that are sent from the retina to the CNS. It is
important to recognize when popular media and online sources
oversimplify complex physiological processes so that misunderstandings
are not generated. This video was created by a medical device
manufacturer who might be trying to highlight other aspects of the visual
system than retinal processing. The statement they make is not incorrect,
it just bundles together several steps, which makes it sound like RGCs are
the transducers, rather than photoreceptors.
Review Questions
Exercise:
Problem:
What type of receptor cell is responsible for transducing pain stimuli?
a. mechanoreceptor
b. nociceptor
c. osmoreceptor
d. photoreceptor
Solution:
B
Exercise:
Problem: Which of these cranial nerves is part of the gustatory system?
a. olfactory
b. trochlear
c. trigeminal
d. facial
Solution:
D
Exercise:
Problem:Which submodality of taste is sensitive to the pH of saliva?
a. umMami
b. sour
c. bitter
d. sweet
Solution:
B
Exercise:
Problem: Axons from which neuron in the retina make up the optic nerve?
a. amacrine cells
b. photoreceptors
c. bipolar cells
d. retinal ganglion cells
Solution:
D
Exercise:
Problem:
What type of receptor cell is involved in the sensations of sound and
balance?
a. photoreceptor
b. chemoreceptor
c. mechanoreceptor
d. nociceptor
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
The sweetener known as stevia can replace glucose in food. What does
the molecular similarity of stevia to glucose mean for the gustatory sense?
Solution:
The stevia molecule is similar to glucose such that it will bind to the
glucose receptor in sweet-sensitive taste buds. However, it is not a
substrate for the ATP-generating metabolism within cells.
Exercise:
Problem:
Why does the blind spot from the optic disc in either eye not result in a
blind spot in the visual field?
Solution:
The visual field for each eye is projected onto the retina as light is focused
by the lens. The visual information from the right visual field falls on the
left side of the retina and vice versa. The optic disc in the right eye is on
the medial side of the fovea, which would be the left side of the retina.
However, the optic disc in the left eye would be on the right side of that
fovea, so the right visual field falls on the side of the retina in the left field
where there is no blind spot.
Glossary
alkaloid
substance, usually from a plant source, that is chemically basic with
respect to pH and will stimulate bitter receptors
amacrine cell
type of cell in the retina that connects to the bipolar cells near the outer
synaptic layer and provides the basis for early image processing within
the retina
ampulla
in the ear, the structure at the base of a semicircular canal that contains the
hair cells and cupula for transduction of rotational movement of the head
anosmia
loss of the sense of smell; usually the result of physical disruption of the
first cranial nerve
aqueous humor
watery fluid that fills the anterior chamber containing the cornea, iris,
ciliary body, and lens of the eye
audition
sense of hearing
auricle
fleshy external structure of the ear
basilar membrane
in the ear, the floor of the cochlear duct on which the organ of Corti sits
bipolar cell
cell type in the retina that connects the photoreceptors to the RGCs
capsaicin
molecule that activates nociceptors by interacting with a temperature-
sensitive ion channel and is the basis for “hot” sensations in spicy food
chemoreceptor
sensory receptor cell that is sensitive to chemical stimuli, such as in taste,
smell, or pain
choroid
highly vascular tissue in the wall of the eye that supplies the outer retina
with blood
ciliary body
smooth muscle structure on the interior surface of the iris that controls the
shape of the lens through the zonule fibers
cochlea
auditory portion of the inner ear containing structures to transduce sound
stimuli
cochlear duct
space within the auditory portion of the inner ear that contains the organ
of Corti and is adjacent to the scala tympani and scala vestibuli on either
side
cone photoreceptor
one of the two types of retinal receptor cell that is specialized for color
vision through the use of three photopigments distributed through three
separate populations of cells
contralateral
word meaning “on the opposite side,” as in axons that cross the midline in
a fiber tract
cornea
fibrous covering of the anterior region of the eye that is transparent so that
light can pass through it
cupula
specialized structure within the base of a semicircular canal that bends the
stereocilia of hair cells when the head rotates by way of the relative
movement of the enclosed fluid
encapsulated ending
configuration of a sensory receptor neuron with dendrites surrounded by
specialized structures to aid in transduction of a particular type of
sensation, such as the lamellated corpuscles in the deep dermis and
subcutaneous tissue
equilibrium
sense of balance that includes sensations of position and movement of the
head
external ear
structures on the lateral surface of the head, including the auricle and the
ear canal back to the tympanic membrane
exteroceptor
sensory receptor that is positioned to interpret stimuli from the external
environment, such as photoreceptors in the eye or somatosensory
receptors in the skin
extraocular muscle
one of six muscles originating out of the bones of the orbit and inserting
into the surface of the eye which are responsible for moving the eye
fibrous tunic
outer layer of the eye primarily composed of connective tissue known as
the sclera and cornea
fovea
exact center of the retina at which visual stimuli are focused for maximal
acuity, where the retina is thinnest, at which there is nothing but
photoreceptors
free nerve ending
configuration of a sensory receptor neuron with dendrites in the
connective tissue of the organ, such as in the dermis of the skin, that are
most often sensitive to chemical, thermal, and mechanical stimuli
general sense
any sensory system that is distributed throughout the body and
incorporated into organs of multiple other systems, such as the walls of
the digestive organs or the skin
gustation
sense of taste
gustatory receptor cells
sensory cells in the taste bud that transduce the chemical stimuli of
gustation
hair cells
mechanoreceptor cells found in the inner ear that transduce stimuli for the
senses of hearing and balance
incus
(also, anvil) ossicle of the middle ear that connects the malleus to the
stapes
inferior oblique
extraocular muscle responsible for lateral rotation of the eye
inferior rectus
extraocular muscle responsible for looking down
inner ear
structure within the temporal bone that contains the sensory apparati of
hearing and balance
inner segment
in the eye, the section of a photoreceptor that contains the nucleus and
other major organelles for normal cellular functions
inner synaptic layer
layer in the retina where bipolar cells connect to RGCs
interoceptor
sensory receptor that is positioned to interpret stimuli from internal
organs, such as stretch receptors in the wall of blood vessels
ipsilateral
word meaning on the same side, as in axons that do not cross the midline
in a fiber tract
iris
colored portion of the anterior eye that surrounds the pupil
kinesthesia
sense of body movement based on sensations in skeletal muscles, tendons,
joints, and the skin
lacrimal duct
duct in the medial corner of the orbit that drains tears into the nasal cavity
lacrimal gland
gland lateral to the orbit that produces tears to wash across the surface of
the eye
lateral rectus
extraocular muscle responsible for abduction of the eye
lens
component of the eye that focuses light on the retina
levator palpebrae superioris
muscle that causes elevation of the upper eyelid, controlled by fibers in
the oculomotor nerve
macula
enlargement at the base of a semicircular canal at which transduction of
equilibrium stimuli takes place within the ampulla
malleus
(also, hammer) ossicle that is directly attached to the tympanic membrane
mechanoreceptor
receptor cell that transduces mechanical stimuli into an electrochemical
signal
medial rectus
extraocular muscle responsible for adduction of the eye
middle ear
space within the temporal bone between the ear canal and bony labyrinth
where the ossicles amplify sound waves from the tympanic membrane to
the oval window
neural tunic
layer of the eye that contains nervous tissue, namely the retina
nociceptor
receptor cell that senses pain stimuli
odorant molecules
volatile chemicals that bind to receptor proteins in olfactory neurons to
stimulate the sense of smell
olfaction
sense of smell
olfactory bulb
central target of the first cranial nerve; located on the ventral surface of
the frontal lobe in the cerebrum
olfactory epithelium
region of the nasal epithelium where olfactory neurons are located
olfactory sensory neuron
receptor cell of the olfactory system, sensitive to the chemical stimuli of
smell, the axons of which compose the first cranial nerve
opsin
protein that contains the photosensitive cofactor retinal for
phototransduction
optic disc
spot on the retina at which RGC axons leave the eye and blood vessels of
the inner retina pass
optic nerve
second cranial nerve, which is responsible visual sensation
organ of Corti
structure in the cochlea in which hair cells transduce movements from
sound waves into electrochemical signals
osmoreceptor
receptor cell that senses differences in the concentrations of bodily fluids
on the basis of osmotic pressure
ossicles
three small bones in the middle ear
otolith
layer of calcium carbonate crystals located on top of the otolithic
membrane
otolithic membrane
gelatinous substance in the utricle and saccule of the inner ear that
contains calcium carbonate crystals and into which the stereocilia of hair
cells are embedded
outer segment
in the eye, the section of a photoreceptor that contains opsin molecules
that transduce light stimuli
outer synaptic layer
layer in the retina at which photoreceptors connect to bipolar cells
oval window
membrane at the base of the cochlea where the stapes attaches, marking
the beginning of the scala vestibuli
palpebral conjunctiva
membrane attached to the inner surface of the eyelids that covers the
anterior surface of the cornea
papilla
for gustation, a bump-like projection on the surface of the tongue that
contains taste buds
photoisomerization
chemical change in the retinal molecule that alters the bonding so that it
switches from the 11-cis-retinal isomer to the all-trans-retinal isomer
photon
individual “packet” of light
photoreceptor
receptor cell specialized to respond to light stimuli
proprioception
sense of position and movement of the body
proprioceptor
receptor cell that senses changes in the position and kinesthetic aspects of
the body
pupil
open hole at the center of the iris that light passes through into the eye
receptor cell
cell that transduces environmental stimuli into neural signals
retina
nervous tissue of the eye at which phototransduction takes place
retinal
cofactor in an opsin molecule that undergoes a biochemical change when
struck by a photon (pronounced with a stress on the last syllable)
retinal ganglion cell (RGC)
neuron of the retina that projects along the second cranial nerve
rhodopsin
photopigment molecule found in the rod photoreceptors
rod photoreceptor
one of the two types of retinal receptor cell that is specialized for low-
light vision
round window
membrane that marks the end of the scala tympani
saccule
structure of the inner ear responsible for transducing linear acceleration in
the vertical plane
scala tympani
portion of the cochlea that extends from the apex to the round window
scala vestibuli
portion of the cochlea that extends from the oval window to the apex
sclera
white of the eye
semicircular canals
structures within the inner ear responsible for transducing rotational
movement information
sensory modality
a particular system for interpreting and perceiving environmental stimuli
by the nervous system
somatosensation
general sense associated with modalities lumped together as touch
special sense
any sensory system associated with a specific organ structure, namely
smell, taste, sight, hearing, and balance
spiral ganglion
location of neuronal cell bodies that transmit auditory information along
the eighth cranial nerve
stapes
(also, stirrup) ossicle of the middle ear that is attached to the inner ear
stereocilia
array of apical membrane extensions in a hair cell that transduce
movements when they are bent
submodality
specific sense within a broader major sense such as sweet as a part of the
sense of taste, or color as a part of vision
superior oblique
extraocular muscle responsible for medial rotation of the eye
superior rectus
extraocular muscle responsible for looking up
taste buds
structures within a papilla on the tongue that contain gustatory receptor
cells
tectorial membrane
component of the organ of Corti that lays over the hair cells, into which
the stereocilia are embedded
thermoreceptor
sensory receptor specialized for temperature stimuli
topographical
relating to positional information
transduction
process of changing an environmental stimulus into the electrochemical
signals of the nervous system
trochlea
cartilaginous structure that acts like a pulley for the superior oblique
muscle
tympanic membrane
ear drum
umami
taste submodality for sensitivity to the concentration of amino acids; also
called the savory sense
utricle
structure of the inner ear responsible for transducing linear acceleration in
the horizontal plane
vascular tunic
middle layer of the eye primarily composed of connective tissue with a
rich blood supply
vestibular ganglion
location of neuronal cell bodies that transmit equilibrium information
along the eighth cranial nerve
vestibule
in the ear, the portion of the inner ear responsible for the sense of
equilibrium
visceral sense
sense associated with the internal organs
vision
special sense of sight based on transduction of light stimuli
visual acuity
property of vision related to the sharpness of focus, which varies in
relation to retinal position
vitreous humor
viscous fluid that fills the posterior chamber of the eye
zonule fibers
fibrous connections between the ciliary body and the lens
Divisions of the ANS
By the end of this section, you will be able to:
e Name the components that generate the sympathetic and
parasympathetic responses of the autonomic nervous system
e Explain the differences in output connections within the two divisions
of the autonomic nervous system
e Describe the signaling molecules and receptor proteins involved in
communication within the two divisions of the autonomic nervous
system
The nervous system can be divided into two functional parts: the somatic
nervous system and the autonomic nervous system. The major differences
between the two systems are evident in the responses that each produces.
The somatic nervous system causes contraction of skeletal muscles. The
autonomic nervous system controls cardiac and smooth muscle, as well as
glandular tissue. The somatic nervous system is associated with voluntary
responses (though many can happen without conscious awareness, like
breathing), and the autonomic nervous system is associated with
involuntary responses, such as those related to homeostasis.
The autonomic nervous system regulates many of the internal organs
through a balance of two aspects, or divisions. In addition to the endocrine
system, the autonomic nervous system is instrumental in homeostatic
mechanisms in the body. The two divisions of the autonomic nervous
system are the sympathetic division and the parasympathetic division.
The sympathetic system is associated with the fight-or-flight response, and
parasympathetic activity is referred to by the epithet of rest and digest.
Homeostasis is the balance between the two systems. At each target
effector, dual innervation determines activity. For example, the heart
receives connections from both the sympathetic and parasympathetic
divisions. One causes heart rate to increase, whereas the other causes heart
rate to decrease.
Note:
—
wees Openstax COLLEGE
ie Batt
Watch this video to learn more about adrenaline and the fight-or-flight
response. When someone is said to have a rush of adrenaline, the image of
bungee jumpers or skydivers usually comes to mind. But adrenaline, also
known as epinephrine, is an important chemical in coordinating the body’s
fight-or-flight response. In this video, you look inside the physiology of the
fight-or-flight response, as envisioned for a firefighter. His body’s reaction
is the result of the sympathetic division of the autonomic nervous system
causing system-wide changes as it prepares for extreme responses. What
two changes does adrenaline bring about to help the skeletal muscle
response?
Sympathetic Division of the Autonomic Nervous System
To respond to a threat—to fight or to run away—the sympathetic system
causes divergent effects as many different effector organs are activated
together for a common purpose. More oxygen needs to be inhaled and
delivered to skeletal muscle. The respiratory, cardiovascular, and
musculoskeletal systems are all activated together. Additionally, sweating
keeps the excess heat that comes from muscle contraction from causing the
body to overheat. The digestive system shuts down so that blood is not
absorbing nutrients when it should be delivering oxygen to skeletal
muscles. To coordinate all these responses, the connections in the
sympathetic system diverge from a limited region of the central nervous
system (CNS) to a wide array of ganglia that project to the many effector
organs simultaneously. The complex set of structures that compose the
output of the sympathetic system make it possible for these disparate
effectors to come together in a coordinated, systemic change.
The sympathetic division of the autonomic nervous system influences the
various organ systems of the body through connections emerging from the
thoracic and upper lumbar spinal cord. It is referred to as the
thoracolumbar system to reflect this anatomical basis. A central neuron
in the lateral horn of any of these spinal regions projects to ganglia adjacent
to the vertebral column through the ventral spinal roots. The majority of
ganglia of the sympathetic system belong to a network of sympathetic
chain ganglia that runs alongside the vertebral column. The ganglia appear
as a series of clusters of neurons linked by axonal bridges. There are
typically 23 ganglia in the chain on either side of the spinal column. Three
correspond to the cervical region, 12 are in the thoracic region, four are in
the lumbar region, and four correspond to the sacral region. The cervical
and sacral levels are not connected to the spinal cord directly through the
spinal roots, but through ascending or descending connections through the
bridges within the chain.
A diagram that shows the connections of the sympathetic system is
somewhat like a circuit diagram that shows the electrical connections
between different receptacles and devices. In [link], the “circuits” of the
sympathetic system are intentionally simplified.
Connections of Sympathetic Division of the Autonomic Nervous System
Region of
spinal cord
Left chain
ganglia
Medulla
ee
‘ # Ze
Right chain ‘ « .o
ganglia gee gi
WX 7° 1-7 cet
2 ex\0
¢} ey?
mM
Spinal cord a
Coccygeal ganglia
fused together
(ganglion impar)
}
Associated nerves
and prevertebral ganglia
eee eee cress
oc
Superior
mesenteric
ganglion
i
Inferior
ganglion
3 Celiac
ganglion
Cs) .
%
.
“ x
mesenteric N14 “S
Target organs (effectors)
Lacrinal gland
-“‘G@eepEp Mucous membrane-
Siete nose and palate
Submaxillary gland
~~~ Gaex Sublingual gland
= Mucous membrane-
mouth
Parotid gland
Heart
Larynx
Trachea
Bronchi
Esophagus
Stomach
Abdominal
blood vessels
Large
intestine
Rectum
Kidney
Bladder
Gonads
External genitalia
Neurons from the lateral horn of the spinal cord
(preganglionic nerve fibers - solid lines)) project to the
chain ganglia on either side of the vertebral column or to
collateral (prevertebral) ganglia that are anterior to the
vertebral column in the abdominal cavity. Axons from
these ganglionic neurons (postganglionic nerve fibers -
dotted lines) then project to target effectors throughout the
body.
To continue with the analogy of the circuit diagram, there are three different
types of “junctions” that operate within the sympathetic system ([link]). The
first type is most direct: the sympathetic nerve projects to the chain
ganglion at the same level as the target effector (the organ, tissue, or gland
to be innervated). An example of this type is spinal nerve T1 that synapses
with the T1 chain ganglion to innervate the trachea. The fibers of this
branch are called white rami communicantes (singular = ramus
communicans); they are myelinated and therefore referred to as white (see
[link]a). The axon from the central neuron (the preganglionic fiber shown
as a Solid line) synapses with the ganglionic neuron (with the
postganglionic fiber shown as a dashed line). This neuron then projects to a
target effector—in this case, the trachea—via gray rami communicantes,
which are unmyelinated axons.
In some cases, the target effectors are located superior or inferior to the
spinal segment at which the preganglionic fiber emerges. With respect to
the “wiring” involved, the synapse with the ganglionic neuron occurs at
chain ganglia superior or inferior to the location of the central neuron. An
example of this is spinal nerve T1 that innervates the eye. The spinal nerve
tracks up through the chain until it reaches the superior cervical ganglion,
where it synapses with the postganglionic neuron (see [link |b). The cervical
ganglia are referred to as paravertebral ganglia, given their location
adjacent to prevertebral ganglia in the sympathetic chain.
Not all axons from the central neurons terminate in the chain ganglia.
Additional branches from the ventral nerve root continue through the chain
and on to one of the collateral ganglia as the greater splanchnic nerve or
lesser splanchnic nerve. For example, the greater splanchnic nerve at the
level of TS synapses with a collateral ganglion outside the chain before
making the connection to the postganglionic nerves that innervate the
stomach (see [link]c).
Collateral ganglia, also called prevertebral ganglia, are situated anterior
to the vertebral column and receive inputs from splanchnic nerves as well
as central sympathetic neurons. They are associated with controlling organs
in the abdominal cavity, and are also considered part of the enteric nervous
system. The three collateral ganglia are the celiac ganglion, the superior
mesenteric ganglion, and the inferior mesenteric ganglion (see [link]).
The word celiac is derived from the Latin word “coelom,” which refers to a
body cavity (in this case, the abdominal cavity), and the word mesenteric
refers to the digestive system.
Sympathetic Connections and Chain Ganglia
(a) A central neuron synapses
with a ganglion at the same z
level within the chain ganglia. a ) Sympathetic
chain ganglion
; White ramus
Spinal | communicans
Spinal cord —_ nerve ql
---= To target effector
Dorsal root
ganglion
\ To target effector
| BZ Gray ramus
(b) A central neuron N 4 ; : communicans
synapses within a : } AN
more superior or inferior Ww
Spinal
ganglion in the chain. Seb
Spinal cord nerve
(c) Acentral neuron projects
through the white ramus but
does not synapse in a chain
ganglion. Instead, it continues
through one of the splanchnic
nerves to synapse within a
prevertebral ganglion.
Sympathetic
chain ganglion
2 No synapse in
Spinal . spinal ganglion
inal cor ner ;
apineheere ne Prevertebral
: ~Sx. ganglion
/~
To target effector
Splanchnic nerve
Axon of central neuron
---- Axon of ganglionic neuron
e Central neuron body
© Ganglionic neuron body
—© Synapse
The axon from a central sympathetic neuron in the spinal
cord can project to the periphery in a number of different
ways. (a) The fiber can project out to the ganglion at the
same level and synapse on a ganglionic neuron. (b) A
branch can project to more superior or inferior ganglion in
the chain. (c) A branch can project through the white ramus
communicans, but not terminate on a ganglionic neuron in
the chain. Instead, it projects through one of the splanchnic
nerves to a collateral ganglion or the adrenal medulla (not
pictured).
An axon from the central neuron that projects to a sympathetic ganglion is
referred to as a preganglionic fiber or neuron, and represents the output
from the CNS to the ganglion. Because the sympathetic ganglia are adjacent
to the vertebral column, preganglionic sympathetic fibers are relatively
short, and they are myelinated. A postganglionic fiber—the axon from a
ganglionic neuron that projects to the target effector—represents the output
of a ganglion that directly influences the organ. Compared with the
preganglionic fibers, postganglionic sympathetic fibers are long because of
the relatively greater distance from the ganglion to the target effector. These
fibers are unmyelinated. (Note that the term “postganglionic neuron” may
be used to describe the projection from a ganglion to the target. The
problem with that usage is that the cell body is in the ganglion, and only the
fiber is postganglionic. Typically, the term neuron applies to the entire cell.)
One type of preganglionic sympathetic fiber does not terminate in a
ganglion. These are the axons from central sympathetic neurons that project
to the adrenal medulla, the interior portion of the adrenal gland. These
axons are still referred to as preganglionic fibers, but the target is not a
ganglion. The adrenal medulla releases signaling molecules into the
bloodstream, rather than using axons to communicate with target structures.
The cells in the adrenal medulla that are contacted by the preganglionic
fibers are called chromaffin cells. These cells are neurosecretory cells that
develop from the neural crest along with the sympathetic ganglia,
reinforcing the idea that the gland is, functionally, a sympathetic ganglion.
The projections of the sympathetic division of the autonomic nervous
system diverge widely, resulting in a broad influence of the system
throughout the body. As a response to a threat, the sympathetic system
would increase heart rate and breathing rate and cause blood flow to the
skeletal muscle to increase and blood flow to the digestive system to
decrease. Sweat gland secretion should also increase as part of an integrated
response. All of those physiological changes are going to be required to
occur together to run away from the hunting lioness, or the modern
equivalent. This divergence is seen in the branching patterns of
preganglionic sympathetic neurons—a single preganglionic sympathetic
neuron may have 10—20 targets. An axon that leaves a central neuron of the
lateral horn in the thoracolumbar spinal cord will pass through the white
ramus communicans and enter the sympathetic chain, where it will branch
toward a variety of targets. At the level of the spinal cord at which the
preganglionic sympathetic fiber exits the spinal cord, a branch will synapse
on a neuron in the adjacent chain ganglion. Some branches will extend up
or down to a different level of the chain ganglia. Other branches will pass
through the chain ganglia and project through one of the splanchnic nerves
to a collateral ganglion. Finally, some branches may project through the
splanchnic nerves to the adrenal medulla. All of these branches mean that
one preganglionic neuron can influence different regions of the sympathetic
system very broadly, by acting on widely distributed organs.
Parasympathetic Division of the Autonomic Nervous System
The parasympathetic division of the autonomic nervous system is named
because its central neurons are located on either side of the thoracolumbar
region of the spinal cord (para- = “beside” or “near”). The parasympathetic
system can also be referred to as the craniosacral system (or outflow)
because the preganglionic neurons are located in nuclei of the brain stem
and the lateral horn of the sacral spinal cord.
The connections, or “circuits,” of the parasympathetic division are similar
to the general layout of the sympathetic division with a few specific
differences ([{link]). The preganglionic fibers from the cranial region travel
in cranial nerves, whereas preganglionic fibers from the sacral region travel
in spinal nerves. The targets of these fibers are terminal ganglia, which are
located near—or even within—the target effector. These ganglia are often
referred to as intramural ganglia when they are found within the walls of
the target organ. The postganglionic fiber projects from the terminal ganglia
a short distance to the target effector, or to the specific target tissue within
the organ. Comparing the relative lengths of axons in the parasympathetic
system, the preganglionic fibers are long and the postganglionic fibers are
short because the ganglia are close to—and sometimes within—the target
effectors.
The cranial component of the parasympathetic system is based in particular
nuclei of the brain stem. In the midbrain, the Edinger—Westphal nucleus is
part of the oculomotor complex, and axons from those neurons travel with
the fibers in the oculomotor nerve (cranial nerve III) that innervate the
extraocular muscles. The preganglionic parasympathetic fibers within
cranial nerve III terminate in the ciliary ganglion, which is located in the
posterior orbit. The postganglionic parasympathetic fibers then project to
the smooth muscle of the iris to control pupillary size. In the upper medulla,
the salivatory nuclei contain neurons with axons that project through the
facial and glossopharyngeal nerves to ganglia that control salivary glands.
Tear production is influenced by parasympathetic fibers in the facial nerve,
which activate a ganglion, and ultimately the lacrimal (tear) gland. Neurons
in the dorsal nucleus of the vagus nerve and the nucleus ambiguus
project through the vagus nerve (cranial nerve X) to the terminal ganglia of
the thoracic and abdominal cavities. Parasympathetic preganglionic fibers
primarily influence the heart, bronchi, and esophagus in the thoracic cavity
and the stomach, liver, pancreas, gall bladder, and small intestine of the
abdominal cavity. The postganglionic fibers from the ganglia activated by
the vagus nerve are often incorporated into the structure of the organ, such
as the mesenteric plexus of the digestive tract organs and the intramural
ganglia.
Connections of Parasympathetic Division of the Autonomic Nervous
System
Eddinger—Westpha
nucleus
Super salivatory
nucleus
Inferior salivatory
nucleus
Dorsal nucleus
of the vagus and
nucleus ambiguus
Oe ee ee es
Spinal cord
Region of Associated nerves Target organs (effectors)
spinal cord and terminal ganglia
Ciliary ganglion a é ,
A __\ Cranial nerve Ill eS #)
SS Pterygopalatine
|
Cranial nerve VII ganglion G2) =)
\
—} Submandibular
ganglion
S5
0 \
AJ Coccygeal v
ganglia
fused together DreoyG@
(ganglion impar)
a
Parasympathetic fibers
—— Sympathetic fibers
Eye
Lacrinal gland
Mucous membrane
(nose and palate)
Submaxillary gland
Sublingual gland
Mucous membrane
(mouth)
Parotid gland
Heart
Larynx
Trachea
Bronchi
Esophagus
Stomach
Abdominal blood
vessels
Liver and bile duct
Pancreas
Adrenal gland
Small intestine
Large intestine
Rectum
Kidney
Bladder
Gonads
External genitalia
Neurons from brain-stem nuclei, or from the lateral horn of
the sacral spinal cord, project to terminal ganglia near or
within the various organs of the body. Axons from these
ganglionic neurons then project the short distance to those
target effectors.
Chemical Signaling in the Autonomic Nervous System
Where an autonomic neuron connects with a target, there is a synapse. The
electrical signal of the action potential causes the release of a signaling
molecule, which will bind to receptor proteins on the target cell. Synapses
of the autonomic system are classified as either cholinergic, meaning that
acetylcholine (ACh) is released, or adrenergic, meaning that
norepinephrine is released. The terms cholinergic and adrenergic refer not
only to the signaling molecule that is released but also to the class of
receptors that each binds.
The cholinergic system includes two classes of receptor: the nicotinic
receptor and the muscarinic receptor. Both receptor types bind to ACh
and cause changes in the target cell. The nicotinic receptor is a ligand-
gated cation channel and the muscarinic receptor is a G protein—coupled
receptor. The receptors are named for, and differentiated by, other
molecules that bind to them. Whereas nicotine will bind to the nicotinic
receptor, and muscarine will bind to the muscarinic receptor, there is no
cross-reactivity between the receptors. The situation is similar to locks and
keys. Imagine two locks—one for a classroom and the other for an office—
that are opened by two separate keys. The classroom key will not open the
office door and the office key will not open the classroom door. This is
similar to the specificity of nicotine and muscarine for their receptors.
However, a master key can open multiple locks, such as a master key for
the Biology Department that opens both the classroom and the office doors.
This is similar to ACh that binds to both types of receptors. The molecules
that define these receptors are not crucial—they are simply tools for
researchers to use in the laboratory. These molecules are exogenous,
meaning that they are made outside of the human body, so a researcher can
use them without any confounding endogenous results (results caused by
the molecules produced in the body).
The adrenergic system also has two types of receptors, named the alpha
(a)-adrenergic receptor and beta (f)-adrenergic receptor. Unlike
cholinergic receptors, these receptor types are not classified by which drugs
can bind to them. All of them are G protein-coupled receptors. There are
three types of a-adrenergic receptors, termed a1, Q, and a3, and there are
two types of f-adrenergic receptors, termed 8, and B5. An additional aspect
of the adrenergic system is that there is a second signaling molecule called
epinephrine. The chemical difference between norepinephrine and
epinephrine is the addition of a methyl group (CH3) in epinephrine. The
prefix “nor-” actually refers to this chemical difference, in which a methyl
group is missing.
The term adrenergic should remind you of the word adrenaline, which is
associated with the fight-or-flight response described at the beginning of the
chapter. Adrenaline and epinephrine are two names for the same molecule.
The adrenal gland (in Latin, ad- = “on top of”; renal = “kidney”) secretes
adrenaline. The ending “-ine” refers to the chemical being derived, or
extracted, from the adrenal gland. A similar construction from Greek
instead of Latin results in the word epinephrine (epi- = “above”; nephr- =
“kidney”). In scientific usage, epinephrine is preferred in the United States,
whereas adrenaline is preferred in Great Britain, because “adrenalin” was
once a registered, proprietary drug name in the United States. Though the
drug is no longer sold, the convention of referring to this molecule by the
two different names persists. Similarly, norepinephrine and noradrenaline
are two names for the same molecule.
Having understood the cholinergic and adrenergic systems, their role in the
autonomic system is relatively simple to understand. All preganglionic
fibers, both sympathetic and parasympathetic, release ACh. All ganglionic
neurons—the targets of these preganglionic fibers—have nicotinic receptors
in their cell membranes. The nicotinic receptor is a ligand-gated cation
channel that results in depolarization of the postsynaptic membrane. The
postganglionic parasympathetic fibers also release ACh, but the receptors
on their targets are muscarinic receptors, which are G protein—coupled
receptors and do not exclusively cause depolarization of the postsynaptic
membrane. Postganglionic sympathetic fibers release norepinephrine,
except for fibers that project to sweat glands and to blood vessels associated
with skeletal muscles, which release ACh ((link]).
Autonomic System Signaling Molecules
Sympathetic Parasympathetic
hs Acetylcholine > Acetylcholine >
Preganglionic ete tay een
nicotinic receptor nicotinic receptor
Norepinephrine — a- or
B-adrenergic receptors
Acetylcholine >
secon Acetylcholine >
a muscarinic receptor a
Postganglionic muscarinic
(associated with sweat
receptor
glands and the blood
vessels associated with
skeletal muscles only
Signaling molecules can belong to two broad groups. Neurotransmitters are
released at synapses, whereas hormones are released into the bloodstream.
These are simplistic definitions, but they can help to clarify this point.
Acetylcholine can be considered a neurotransmitter because it is released by
axons at synapses. The adrenergic system, however, presents a challenge.
Postganglionic sympathetic fibers release norepinephrine, which can be
considered a neurotransmitter. But the adrenal medulla releases epinephrine
and norepinephrine into circulation, so they should be considered
hormones.
What are referred to here as synapses may not fit the strictest definition of
synapse. Some sources will refer to the connection between a
postganglionic fiber and a target effector as neuroeffector junctions;
neurotransmitters, as defined above, would be called neuromodulators. The
structure of postganglionic connections are not the typical synaptic end bulb
that is found at the neuromuscular junction, but rather are chains of
swellings along the length of a postganglionic fiber called a varicosity
((link]).
Autonomic Varicosities
Synaptic vesicles
> Postganglionic
varicosities
Sarcolemma
The connection between autonomic fibers and target
effectors is not the same as the typical synapse, such as the
neuromuscular junction. Instead of a synaptic end bulb, a
neurotransmitter is released from swellings along the length
of a fiber that makes an extended network of connections in
the target effector.
Note:
Everyday Connections
Fight or Flight? What About Fright and Freeze?
The original usage of the epithet “fight or flight” comes from a scientist
named Walter Cannon who worked at Harvard in 1915. The concept of
homeostasis and the functioning of the sympathetic system had been
introduced in France in the previous century. Cannon expanded the idea,
and introduced the idea that an animal responds to a threat by preparing to
stand and fight or run away. The nature of this response was thoroughly
explained in a book on the physiology of pain, hunger, fear, and rage.
When students learn about the sympathetic system and the fight-or-flight
response, they often stop and wonder about other responses. If you were
faced with a lioness running toward you as pictured at the beginning of this
chapter, would you run or would you stand your ground? Some people
would say that they would freeze and not know what to do. So isn’t there
really more to what the autonomic system does than fight, flight, rest, or
digest. What about fear and paralysis in the face of a threat?
The common epithet of “fight or flight” is being enlarged to be “fight,
flight, or fright” or even “fight, flight, fright, or freeze.” Cannon’s original
contribution was a catchy phrase to express some of what the nervous
system does in response to a threat, but it is incomplete. The sympathetic
system is responsible for the physiological responses to emotional states.
The name “sympathetic” can be said to mean that (sym- = “together”; -
pathos = “pain,” “suffering,” or “emotion”).
Note:
Oia)
apr
= cpenstax couece”
ony ‘
io re
Watch this video to learn more about the nervous system. As described in
this video, the nervous system has a way to deal with threats and stress that
is separate from the conscious control of the somatic nervous system. The
system comes from a time when threats were about survival, but in the
modern age, these responses become part of stress and anxiety. This video
describes how the autonomic system is only part of the response to threats,
or stressors. What other organ system gets involved, and what part of the
brain coordinates the two systems for the entire response, including
epinephrine (adrenaline) and cortisol?
Chapter Review
The primary responsibilities of the autonomic nervous system are to
regulate homeostatic mechanisms in the body, which is also part of what the
endocrine system does. The key to understanding the autonomic system is
to explore the response pathways—the output of the nervous system. The
way we respond to the world around us, to manage the internal environment
on the basis of the external environment, is divided between two parts of
the autonomic nervous system. The sympathetic division responds to threats
and produces a readiness to confront the threat or to run away: the fight-or-
flight response. The parasympathetic division plays the opposite role. When
the external environment does not present any immediate danger, a restful
mode descends on the body, and the digestive system is more active.
The sympathetic output of the nervous system originates out of the lateral
horn of the thoracolumbar spinal cord. An axon from one of these central
neurons projects by way of the ventral spinal nerve root and spinal nerve to
a sympathetic ganglion, either in the sympathetic chain ganglia or one of
the collateral locations, where it synapses on a ganglionic neuron. These
preganglionic fibers release ACh, which excites the ganglionic neuron
through the nicotinic receptor. The axon from the ganglionic neuron—the
postganglionic fiber—then projects to a target effector where it will release
norepinephrine to bind to an adrenergic receptor, causing a change in the
physiology of that organ in keeping with the broad, divergent sympathetic
response. The postganglionic connections to sweat glands in the skin and
blood vessels supplying skeletal muscle are, however, exceptions; those
fibers release ACh onto muscarinic receptors. The sympathetic system has a
specialized preganglionic connection to the adrenal medulla that causes
epinephrine and norepinephrine to be released into the bloodstream rather
than exciting a neuron that contacts an organ directly. This hormonal
component means that the sympathetic chemical signal can spread
throughout the body very quickly and affect many organ systems at once.
The parasympathetic output is based in the brain stem and sacral spinal
cord. Neurons from particular nuclei in the brain stem or from the lateral
horn of the sacral spinal cord (preganglionic neurons) project to terminal
(intramural) ganglia located close to or within the wall of target effectors.
These preganglionic fibers also release ACh onto nicotinic receptors to
excite the ganglionic neurons. The postganglionic fibers then contact the
target tissues within the organ to release ACh, which binds to muscarinic
receptors to induce rest-and-digest responses.
Signaling molecules utilized by the autonomic nervous system are released
from axons and can be considered as either neurotransmitters (when they
directly interact with the effector) or as hormones (when they are released
into the bloodstream). The same molecule, such as norepinephrine, could be
considered either a neurotransmitter or a hormone on the basis of whether it
is released from a postganglionic sympathetic axon or from the adrenal
gland. The synapses in the autonomic system are not always the typical type
of connection first described in the neuromuscular junction. Instead of
having synaptic end bulbs at the very end of an axonal fiber, they may have
swellings—called varicosities—along the length of a fiber so that it makes a
network of connections within the target tissue.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn more about adrenaline and the fight-or-flight
response. When someone is said to have a rush of adrenaline, the
image of bungee jumpers or skydivers usually comes to mind. But
adrenaline, also known as epinephrine, is an important chemical in
coordinating the body’s fight-or-flight response. In this video, you look
inside the physiology of the fight-or-flight response, as envisioned for
a firefighter. His body’s reaction is the result of the sympathetic
division of the autonomic nervous system causing system-wide
changes as it prepares for extreme responses. What two changes does
adrenaline bring about to help the skeletal muscle response?
Solution:
The heart rate increases to send more blood to the muscles, and the
liver releases stored glucose to fuel the muscles.
Exercise:
Problem:
Watch this video to learn more about the nervous system. As described
in this video, the nervous system has a way to deal with threats and
stress that is separate from the conscious control of the somatic
nervous system. The system comes from a time when threats were
about survival, but in the modern age, these responses become part of
stress and anxiety. This video describes how the autonomic system is
only part of the response to threats, or stressors. What other organ
system gets involved, and what part of the brain coordinates the two
systems for the entire response, including epinephrine (adrenaline) and
cortisol?
Solution:
The endocrine system is also responsible for responses to stress in our
lives. The hypothalamus coordinates the autonomic response through
projections into the spinal cord and through influence over the
pituitary gland, the effective center of the endocrine system.
Review Questions
Exercise:
Problem:
Which of these physiological changes would not be considered part of
the sympathetic fight-or-flight response?
a. increased heart rate
b. increased sweating
c. dilated pupils
d. increased stomach motility
Solution:
D
Exercise:
Problem: Which type of fiber could be considered the longest?
a. preganglionic parasympathetic
b. preganglionic sympathetic
c. postganglionic parasympathetic
d. postganglionic sympathetic
Solution:
A
Exercise:
Problem:
Which signaling molecule is most likely responsible for an increase in
digestive activity?
a. epinephrine
b. norepinephrine
c. acetylcholine
d. adrenaline
Solution:
C
Exercise:
Problem:
Which of these cranial nerves contains preganglionic parasympathetic
fibers?
a. optic, CN II
b. facial, CN VII
c. trigeminal, CN V
d. hypoglossal, CN XII
Solution:
B
Exercise:
Problem:
Which of the following is not a target of a sympathetic preganglionic
fiber?
a. intermural ganglion
b. collateral ganglion
c. adrenal gland
d. chain ganglion
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
In the context of a lioness hunting on the savannah, why would the
sympathetic system not activate the digestive system?
Solution:
Whereas energy is needed for running away from the threat, blood
needs to be sent to the skeletal muscles for oxygen supply. The
additional fuel, in the form of carbohydrates, probably wouldn’t
improve the ability to escape the threat as much as the diversion of
oxygen-rich blood would hinder it.
Exercise:
Problem:
A target effector, such as the heart, receives input from the sympathetic
and parasympathetic systems. What is the actual difference between
the sympathetic and parasympathetic divisions at the level of those
connections (i.e., at the synapse)?
Solution:
The postganglionic sympathetic fiber releases norepinephrine, whereas
the postganglionic parasympathetic fiber releases acetylcholine.
Specific locations in the heart have adrenergic receptors and
muscarinic receptors. Which receptors are bound is the signal that
determines how the heart responds.
Glossary
alpha (a)-adrenergic receptor
one of the receptors to which epinephrine and norepinephrine bind,
which comes in three subtypes: a, Q>, and a3
acetylcholine (ACh)
neurotransmitter that binds at a motor end-plate to trigger
depolarization
adrenal medulla
interior portion of the adrenal (or suprarenal) gland that releases
epinephrine and norepinephrine into the bloodstream as hormones
adrenergic
synapse where norepinephrine is released, which binds to a- or B-
adrenergic receptors
beta (B)-adrenergic receptor
one of the receptors to which epinephrine and norepinephrine bind,
which comes in two subtypes: 8, and B»5
celiac ganglion
one of the collateral ganglia of the sympathetic system that projects to
the digestive system
central neuron
specifically referring to the cell body of a neuron in the autonomic
system that is located in the central nervous system, specifically the
lateral horn of the spinal cord or a brain stem nucleus
cholinergic
synapse at which acetylcholine is released and binds to the nicotinic or
muscarinic receptor
chromaffin cells
neuroendocrine cells of the adrenal medulla that release epinephrine
and norepinephrine into the bloodstream as part of sympathetic system
activity
ciliary ganglion
one of the terminal ganglia of the parasympathetic system, located in
the posterior orbit, axons from which project to the iris
collateral ganglia
ganglia outside of the sympathetic chain that are targets of sympathetic
preganglionic fibers, which are the celiac, inferior mesenteric, and
superior mesenteric ganglia
craniosacral system
alternate name for the parasympathetic division of the autonomic
nervous system that is based on the anatomical location of central
neurons in brain-stem nuclei and the lateral horn of the sacral spinal
cord; also referred to as craniosacral outflow
dorsal nucleus of the vagus nerve
location of parasympathetic neurons that project through the vagus
nerve to terminal ganglia in the thoracic and abdominal cavities
Eddinger—Westphal nucleus
location of parasympathetic neurons that project to the ciliary ganglion
endogenous
describes substance made in the human body
epinephrine
signaling molecule released from the adrenal medulla into the
bloodstream as part of the sympathetic response
exogenous
describes substance made outside of the human body
fight-or-flight response
set of responses induced by sympathetic activity that lead to either
fleeing a threat or standing up to it, which in the modem world is often
associated with anxious feelings
G protein—coupled receptor
membrane protein complex that consists of a receptor protein that
binds to a signaling molecule—a G protein—that is activated by that
binding and in tur activates an effector protein (enzyme) that creates a
second-messenger molecule in the cytoplasm of the target cell
ganglionic neuron
specifically refers to the cell body of a neuron in the autonomic system
that is located in a ganglion
gray rami Communicantes
(singular = ramus communicans) unmyelinated structures that provide
a short connection from a sympathetic chain ganglion to the spinal
nerve that contains the postganglionic sympathetic fiber
greater splanchnic nerve
nerve that contains fibers of the central sympathetic neurons that do
not synapse in the chain ganglia but project onto the celiac ganglion
inferior mesenteric ganglion
one of the collateral ganglia of the sympathetic system that projects to
the digestive system
intramural ganglia
terminal ganglia of the parasympathetic system that are found within
the walls of the target effector
lesser splanchnic nerve
nerve that contains fibers of the central sympathetic neurons that do
not synapse in the chain ganglia but project onto the inferior
mesenteric ganglion
ligand-gated cation channel
ion channel, such as the nicotinic receptor, that is specific to positively
charged ions and opens when a molecule such as a neurotransmitter
binds to it
mesenteric plexus
nervous tissue within the wall of the digestive tract that contains
neurons that are the targets of autonomic preganglionic fibers and that
project to the smooth muscle and glandular tissues in the digestive
organ
muscarinic receptor
type of acetylcholine receptor protein that is characterized by also
binding to muscarine and is a metabotropic receptor
nicotinic receptor
type of acetylcholine receptor protein that is characterized by also
binding to nicotine and is an ionotropic receptor
norepinephrine
signaling molecule released as a neurotransmitter by most
postganglionic sympathetic fibers as part of the sympathetic response,
or as a hormone into the bloodstream from the adrenal medulla
nucleus ambiguus
brain-stem nucleus that contains neurons that project through the vagus
nerve to terminal ganglia in the thoracic cavity; specifically associated
with the heart
parasympathetic division
division of the autonomic nervous system responsible for restful and
digestive functions
paravertebral ganglia
autonomic ganglia superior to the sympathetic chain ganglia
postganglionic fiber
axon from a ganglionic neuron in the autonomic nervous system that
projects to and synapses with the target effector; sometimes referred to
as a postganglionic neuron
preganglionic fiber
axon from a central neuron in the autonomic nervous system that
projects to and synapses with a ganglionic neuron; sometimes referred
to as a preganglionic neuron
prevertebral ganglia
autonomic ganglia that are anterior to the vertebral column and
functionally related to the sympathetic chain ganglia
rest and digest
set of functions associated with the parasympathetic system that lead
to restful actions and digestion
superior cervical ganglion
one of the paravertebral ganglia of the sympathetic system that
projects to the head
superior mesenteric ganglion
one of the collateral ganglia of the sympathetic system that projects to
the digestive system
sympathetic chain ganglia
series of ganglia adjacent to the vertebral column that receive input
from central sympathetic neurons
sympathetic division
division of the autonomic nervous system associated with the fight-or-
flight response
target effector
organ, tissue, or gland that will respond to the control of an autonomic
or somatic or endocrine signal
terminal ganglia
ganglia of the parasympathetic division of the autonomic system,
which are located near or within the target effector, the latter also
known as intramural ganglia
thoracolumbar system
alternate name for the sympathetic division of the autonomic nervous
system that is based on the anatomical location of central neurons in
the lateral horn of the thoracic and upper lumbar spinal cord
varicosity
structure of some autonomic connections that is not a typical synaptic
end bulb, but a string of swellings along the length of a fiber that
makes a network of connections with the target effector
white rami communicantes
(singular = ramus communicans) myelinated structures that provide a
short connection from a sympathetic chain ganglion to the spinal nerve
that contains the preganglionic sympathetic fiber
Central Control
By the end of this section, you will be able to:
¢ Describe the role of higher centers of the brain in autonomic regulation
e Explain the connection of the hypothalamus to homeostasis
¢ Describe the regions of the CNS that link the autonomic system with
emotion
e Describe the pathways important to descending control of the
autonomic system
The pupillary light reflex ((link]) begins when light hits the retina and
causes a signal to travel along the optic nerve. This is visual sensation,
because the afferent branch of this reflex is simply sharing the special sense
pathway. Bright light hitting the retina leads to the parasympathetic
response, through the oculomotor nerve, followed by the postganglionic
fiber from the ciliary ganglion, which stimulates the circular fibers of the
iris to contract and constrict the pupil. When light hits the retina in one eye,
both pupils contract. When that light is removed, both pupils dilate again
back to the resting position. When the stimulus is unilateral (presented to
only one eye), the response is bilateral (both eyes). The same is not true for
somatic reflexes. If you touch a hot radiator, you only pull that arm back,
not both. Central control of autonomic reflexes is different than for somatic
reflexes. The hypothalamus, along with other CNS locations, controls the
autonomic system.
Pupillary Reflex Pathways
Pretectal
Action potentials
from right eye reach
both right and left
pretectal nuclei.
Oculomotor SY, VA
nerves (III)
Ciliary
ganglia
The pretectal
nuclei stimulate
both sides of the
Eddinger—Westphal
nucleus even
though the light was
perceived only in
() The right and left sides the right eye.
of the Eddinger—Westphal
nuclei generate action
@) Light is shined on
potentials through the
right and left oculomotor
right eye only.
_<<?
a
Cay
e, nerves, causing both pupils
Y to constrict.
A
The pupil is under competing autonomic control in response
to light levels hitting the retina. The sympathetic system will
dilate the pupil when the retina is not receiving enough light,
and the parasympathetic system will constrict the pupil when
too much light hits the retina.
Forebrain Structures
Autonomic control is based on the visceral reflexes, composed of the
afferent and efferent branches. These homeostatic mechanisms are based on
the balance between the two divisions of the autonomic system, which
results in tone for various organs that is based on the predominant input
from the sympathetic or parasympathetic systems. Coordinating that
balance requires integration that begins with forebrain structures like the
hypothalamus and continues into the brain stem and spinal cord.
The Hypothalamus
The hypothalamus is the control center for many homeostatic mechanisms.
It regulates both autonomic function and endocrine function. The roles it
plays in the pupillary reflexes demonstrates the importance of this control
center. The optic nerve projects primarily to the thalamus, which is the
necessary relay to the occipital cortex for conscious visual perception.
Another projection of the optic nerve, however, goes to the hypothalamus.
The hypothalamus then uses this visual system input to drive the pupillary
reflexes. If the retina is activated by high levels of light, the hypothalamus
stimulates the parasympathetic response. If the optic nerve message shows
that low levels of light are falling on the retina, the hypothalamus activates
the sympathetic response. Output from the hypothalamus follows two main
tracts, the dorsal longitudinal fasciculus and the medial forebrain bundle
({link]). Along these two tracts, the hypothalamus can influence the
Eddinger—Westphal nucleus of the oculomotor complex or the lateral horns
of the thoracic spinal cord.
Fiber Tracts of the Central Autonomic System
Hypothalamus
Oculomotor cortex
(includes Eddinger—
Westphal nucleus)
Dorsal motor nucleus
of the vagus
Nucleus ambiguus
—— Medial forebrain bundle
—— Dorsal longitudinal fasciculus
To spinal cord
The hypothalamus is the source of most of the central
control of autonomic function. It receives input from
cerebral structures and projects to brain stem and
spinal cord structures to regulate the balance of
sympathetic and parasympathetic input to the organ
systems of the body. The main pathways for this are
the medial forebrain bundle and the dorsal
longitudinal fasciculus.
These two tracts connect the hypothalamus with the major parasympathetic
nuclei in the brain stem and the preganglionic (central) neurons of the
thoracolumbar spinal cord. The hypothalamus also receives input from
other areas of the forebrain through the medial forebrain bundle. The
olfactory cortex, the septal nuclei of the basal forebrain, and the amygdala
project into the hypothalamus through the medial forebrain bundle. These
forebrain structures inform the hypothalamus about the state of the nervous
system and can influence the regulatory processes of homeostasis. A good
example of this is found in the amygdala, which is found beneath the
cerebral cortex of the temporal lobe and plays a role in our ability to
remember and feel emotions.
The Amygdala
The amygdala is a group of nuclei in the medial region of the temporal lobe
that is part of the limbic lobe ({link]). The limbic lobe includes structures
that are involved in emotional responses, as well as structures that
contribute to memory function. The limbic lobe has strong connections with
the hypothalamus and influences the state of its activity on the basis of
emotional state. For example, when you are anxious or scared, the
amygdala will send signals to the hypothalamus along the medial forebrain
bundle that will stimulate the sympathetic fight-or-flight response. The
hypothalamus will also stimulate the release of stress hormones through its
control of the endocrine system in response to amygdala input.
The Limbic Lobe
Cingulate
gyrus
Hypothalamic nuclei
Corpus
callosum
Amygdala
) : ' SS Et >) bb Ree Thalamus
Hippocampus . .
Structures arranged around the edge of the cerebrum
constitute the limbic lobe, which includes the amygdala,
hippocampus, and cingulate gyrus, and connects to the
hypothalamus.
The Medulla
The medulla contains nuclei referred to as the cardiovascular center,
which controls the smooth and cardiac muscle of the cardiovascular system
through autonomic connections. When the homeostasis of the
cardiovascular system shifts, such as when blood pressure changes, the
coordination of the autonomic system can be accomplished within this
region. Furthermore, when descending inputs from the hypothalamus
stimulate this area, the sympathetic system can increase activity in the
cardiovascular system, such as in response to anxiety or stress. The
preganglionic sympathetic fibers that are responsible for increasing heart
rate are referred to as the cardiac accelerator nerves, whereas the
preganglionic sympathetic fibers responsible for constricting blood vessels
compose the vasomotor nerves.
Several brain stem nuclei are important for the visceral control of major
organ systems. One brain stem nucleus involved in cardiovascular function
is the solitary nucleus. It receives sensory input about blood pressure and
cardiac function from the glossopharyngeal and vagus nerves, and its output
will activate sympathetic stimulation of the heart or blood vessels through
the upper thoracic lateral horn. Another brain stem nucleus important for
visceral control is the dorsal motor nucleus of the vagus nerve, which is the
motor nucleus for the parasympathetic functions ascribed to the vagus
nerve, including decreasing the heart rate, relaxing bronchial tubes in the
lungs, and activating digestive function through the enteric nervous system.
The nucleus ambiguus, which is named for its ambiguous histology, also
contributes to the parasympathetic output of the vagus nerve and targets
muscles in the pharynx and larynx for swallowing and speech, as well as
contributing to the parasympathetic tone of the heart along with the dorsal
motor nucleus of the vagus.
Note:
Everyday Connections
Exercise and the Autonomic System
In addition to its association with the fight-or-flight response and rest-and-
digest functions, the autonomic system is responsible for certain everyday
functions. For example, it comes into play when homeostatic mechanisms
dynamically change, such as the physiological changes that accompany
exercise. Getting on the treadmill and putting in a good workout will cause
the heart rate to increase, breathing to be stronger and deeper, sweat glands
to activate, and the digestive system to suspend activity. These are the
same physiological changes associated with the fight-or-flight response,
but there is nothing chasing you on that treadmill.
This is not a simple homeostatic mechanism at work because “maintaining
the internal environment” would mean getting all those changes back to
their set points. Instead, the sympathetic system has become active during
exercise so that your body can cope with what is happening. A homeostatic
mechanism is dealing with the conscious decision to push the body away
from a resting state. The heart, actually, is moving away from its
homeostatic set point. Without any input from the autonomic system, the
heart would beat at approximately 100 bpm, and the parasympathetic
system slows that down to the resting rate of approximately 70 bpm. But in
the middle of a good workout, you should see your heart rate at 120-140
bpm. You could say that the body is stressed because of what you are doing
to it. Homeostatic mechanisms are trying to keep blood pH in the normal
range, or to keep body temperature under control, but those are in response
to the choice to exercise.
Note:
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mss" OPENStax COLLEGE
A “a
r r.
io ane
Watch this video to learn about physical responses to emotion. The
autonomic system, which is important for regulating the homeostasis of the
organ systems, is also responsible for our physiological responses to
emotions such as fear. The video summarizes the extent of the body’s
reactions and describes several effects of the autonomic system in response
to fear. On the basis of what you have already studied about autonomic
function, which effect would you expect to be associated with
parasympathetic, rather than sympathetic, activity?
Chapter Review
The autonomic system integrates sensory information and higher cognitive
processes to generate output, which balances homeostatic mechanisms. The
central autonomic structure is the hypothalamus, which coordinates
sympathetic and parasympathetic efferent pathways to regulate activities of
the organ systems of the body. The majority of hypothalamic output travels
through the medial forebrain bundle and the dorsal longitudinal fasciculus
to influence brain stem and spinal components of the autonomic nervous
system. The medial forebrain bundle also connects the hypothalamus with
higher centers of the limbic system where emotion can influence visceral
responses. The amygdala is a structure within the limbic system that
influences the hypothalamus in the regulation of the autonomic system, as
well as the endocrine system.
These higher centers have descending control of the autonomic system
through brain stem centers, primarily in the medulla, such as the
cardiovascular center. This collection of medullary nuclei regulates cardiac
function, as well as blood pressure. Sensory input from the heart, aorta, and
carotid sinuses project to these regions of the medulla. The solitary nucleus
increases sympathetic tone of the cardiovascular system through the cardiac
accelerator and vasomotor nerves. The nucleus ambiguus and the dorsal
motor nucleus both contribute fibers to the vagus nerve, which exerts
parasympathetic control of the heart by decreasing heart rate.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn about physical responses to emotion. The
autonomic system, which is important for regulating the homeostasis
of the organ systems, is also responsible for our physiological
responses to emotions such as fear. The video summarizes the extent
of the body’s reactions and describes several effects of the autonomic
system in response to fear. On the basis of what you have already
studied about autonomic function, which effect would you expect to be
associated with parasympathetic, rather than sympathetic, activity?
Solution:
The release of urine in extreme fear. The sympathetic system normally
constricts sphincters such as that of the urethra.
Review Questions
Exercise:
Problem:
Which of these locations in the forebrain is the master control center
for homeostasis through the autonomic and endocrine systems?
a. hypothalamus
b. thalamus
c. amygdala
d. cerebral cortex
Solution:
A
Exercise:
Problem:
Which nerve projects to the hypothalamus to indicate the level of light
stimuli in the retina?
a. glossopharyngeal
b. oculomotor
c. optic
d. vagus
Solution:
C
Exercise:
Problem:
What region of the limbic lobe is responsible for generating stress
responses via the hypothalamus?
a. hippocampus
b. amygdala
c. mammillary bodies
d. prefrontal cortex
Solution:
B
Exercise:
Problem:
What is another name for the preganglionic sympathetic fibers that
project to the heart?
a. solitary tract
b. vasomotor nerve
c. vagus nerve
d. cardiac accelerator nerve
Solution:
D
Exercise:
Problem:
What central fiber tract connects forebrain and brain stem structures
with the hypothalamus?
a. cardiac accelerator nerve
b. medial forebrain bundle
c. dorsal longitudinal fasciculus
d. corticospinal tract
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Horner’s syndrome is a condition that presents with changes in one
eye, such as pupillary constriction and dropping of eyelids, as well as
decreased sweating in the face. Why could a tumor in the thoracic
cavity have an effect on these autonomic functions?
Solution:
Pupillary dilation and sweating, two functions lost in Horner’s
syndrome, are caused by the sympathetic system. A tumor in the
thoracic cavity may interrupt the output of the thoracic ganglia that
project to the head and face.
Exercise:
Problem:
The cardiovascular center is responsible for regulating the heart and
blood vessels through homeostatic mechanisms. What tone does each
component of the cardiovascular system have? What connections does
the cardiovascular center invoke to keep these two systems in their
resting tone?
Solution:
The heart—based on the resting heart rate—is under parasympathetic
tone, and the blood vessels—based on the lack of parasympathetic
input—are under sympathetic tone. The vagus nerve contributes to the
lowered resting heart rate, whereas the vasomotor nerves maintain the
slight constriction of systemic blood vessels.
Glossary
cardiac accelerator nerves
preganglionic sympathetic fibers that cause the heart rate to increase
when the cardiovascular center in the medulla initiates a signal
cardiovascular center
region in the medulla that controls the cardiovascular system through
cardiac accelerator nerves and vasomotor nerves, which are
components of the sympathetic division of the autonomic nervous
system
dorsal longitudinal fasciculus
major output pathway of the hypothalamus that descends through the
gray matter of the brain stem and into the spinal cord
limbic lobe
structures arranged around the edges of the cerebrum that are involved
in memory and emotion
medial forebrain bundle
fiber pathway that extends anteriorly into the basal forebrain, passes
through the hypothalamus, and extends into the brain stem and spinal
cord
vasomotor nerves
preganglionic sympathetic fibers that cause the constriction of blood
vessels in response to signals from the cardiovascular center
Organs and Structures of the Respiratory System
By the end of this section, you will be able to:
e List the structures that make up the respiratory system
e Describe how the respiratory system processes oxygen and CO»
e Compare and contrast the functions of upper respiratory tract with the
lower respiratory tract
The major organs of the respiratory system function primarily to provide
oxygen to body tissues for cellular respiration, remove the waste product
carbon dioxide, and help to maintain acid-base balance. Portions of the
respiratory system are also used for non-vital functions, such as sensing
odors, speech production, and for straining, such as during childbirth or
coughing ([link]).
Major Respiratory Structures
Nasal cavity
Nostril
Oral cavit
J Pharynx
Larynx
Trachea
Left main
bronchus
Right main
bronchus
Right lun
: Left lung
Diaphragm
The major respiratory structures span the nasal
cavity to the diaphragm.
Functionally, the respiratory system can be divided into a conducting zone
and a respiratory zone. The conducting zone of the respiratory system
includes the organs and structures not directly involved in gas exchange.
The gas exchange occurs in the respiratory zone.
Conducting Zone
The major functions of the conducting zone are to provide a route for
incoming and outgoing air, remove debris and pathogens from the incoming
air, and warm and humidify the incoming air. Several structures within the
conducting zone perform other functions as well. The epithelium of the
nasal passages, for example, is essential to sensing odors, and the bronchial
epithelium that lines the lungs can metabolize some airborne carcinogens.
The Nose and its Adjacent Structures
The major entrance and exit for the respiratory system is through the nose.
When discussing the nose, it is helpful to divide it into two major sections:
the external nose, and the nasal cavity or internal nose.
The external nose consists of the surface and skeletal structures that result
in the outward appearance of the nose and contribute to its numerous
functions ([link]). The root is the region of the nose located between the
eyebrows. The bridge is the part of the nose that connects the root to the
rest of the nose. The dorsum nasi is the length of the nose. The apex is the
tip of the nose. On either side of the apex, the nostrils are formed by the
alae (singular = ala). An ala is a cartilaginous structure that forms the
lateral side of each naris (plural = nares), or nostril opening. The philtrum
is the concave surface that connects the apex of the nose to the upper lip.
Nose
i ee
Dorsum nasi
a a as
———— es _—_ 7"
i ea Philtrum ew 4
Frontal bone
Nasal bone
Maxillary bone
Septal cartilage
Major alar cartilage
Septal cartilage
This illustration shows features of the external
nose (top) and skeletal features of the nose
(bottom).
Underneath the thin skin of the nose are its skeletal features (see [link],
lower illustration). While the root and bridge of the nose consist of bone,
the protruding portion of the nose is composed of cartilage. As a result,
when looking at a skull, the nose is missing. The nasal bone is one of a pair
of bones that lies under the root and bridge of the nose. The nasal bone
articulates superiorly with the frontal bone and laterally with the maxillary
bones. Septal cartilage is flexible hyaline cartilage connected to the nasal
bone, forming the dorsum nasi. The alar cartilage consists of the apex of
the nose; it surrounds the naris.
The nares open into the nasal cavity, which is separated into left and right
sections by the nasal septum ([link]). The nasal septum is formed
anteriorly by a portion of the septal cartilage (the flexible portion you can
touch with your fingers) and posteriorly by the perpendicular plate of the
ethmoid bone (a cranial bone located just posterior to the nasal bones) and
the thin vomer bones (whose name refers to its plough shape). Each lateral
wall of the nasal cavity has three bony projections, called the superior,
middle, and inferior nasal conchae. The inferior conchae are separate bones,
whereas the superior and middle conchae are portions of the ethmoid bone.
Conchae serve to increase the surface area of the nasal cavity and to disrupt
the flow of air as it enters the nose, causing air to bounce along the
epithelium, where it is cleaned and warmed. The conchae and meatuses
also conserve water and prevent dehydration of the nasal epithelium by
trapping water during exhalation. The floor of the nasal cavity is composed
of the palate. The hard palate at the anterior region of the nasal cavity is
composed of bone. The soft palate at the posterior portion of the nasal
cavity consists of muscle tissue. Air exits the nasal cavities via the internal
nares and moves into the pharynx.
Upper Airway
Sphenoidal sinus ee
Ethmoid bone
Nasal meatuses Olfactory epithelium
(superior, middle,
and inferior) Nasal conchae
Pharyngeal tonsil Nasal vestibule
Opening of ase
auditory tube
Nasopharynx Hard palate
Uvula Soft palate
Tounge
Palatine tonsil ;
Lingual tonsil
se Epiglottis
Oropharynx
Hyoid bone
Laryngopharynx
Vestibular fold
Vocal fold
Thyroid cartilage
Cricoid cartilage
Esophagus Thyroid gland
Trachea
Several bones that help form the walls of the nasal cavity have air-
containing spaces called the paranasal sinuses, which serve to warm and
humidify incoming air. Sinuses are lined with a mucosa. Each paranasal
sinus is named for its associated bone: frontal sinus, maxillary sinus,
sphenoidal sinus, and ethmoidal sinus. The sinuses produce mucus and
lighten the weight of the skull.
The nares and anterior portion of the nasal cavities are lined with mucous
membranes, containing sebaceous glands and hair follicles that serve to
prevent the passage of large debris, such as dirt, through the nasal cavity.
An olfactory epithelium used to detect odors is found deeper in the nasal
cavity.
The conchae, meatuses, and paranasal sinuses are lined by respiratory
epithelium composed of pseudostratified ciliated columnar epithelium
({link]). The epithelium contains goblet cells, one of the specialized,
columnar epithelial cells that produce mucus to trap debris. The cilia of the
respiratory epithelium help remove the mucus and debris from the nasal
cavity with a constant beating motion, sweeping materials towards the
throat to be swallowed. Interestingly, cold air slows the movement of the
cilia, resulting in accumulation of mucus that may in turn lead to a runny
nose during cold weather. This moist epithelium functions to warm and
humidify incoming air. Capillaries located just beneath the nasal epithelium
warm the air by convection. Serous and mucus-producing cells also secrete
the lysozyme enzyme and proteins called defensins, which have
antibacterial properties. Immune cells that patrol the connective tissue deep
to the respiratory epithelium provide additional protection.
Pseudostratified Ciliated Columnar Epithelium
Lumen of
Goblet cell trachea Cilia
Pseudostratified
columnar epithelia
Seromucous gland
in submucosa
Respiratory epithelium is pseudostratified ciliated
columnar epithelium. Seromucous glands provide
lubricating mucus. LM x 680. (Micrograph provided by
the Regents of University of Michigan Medical School ©
2012)
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
Pharynx
The pharynx is a tube formed by skeletal muscle and lined by mucous
membrane that is continuous with that of the nasal cavities (see [link]). The
pharynx is divided into three major regions: the nasopharynx, the
oropharynx, and the laryngopharynx ([link]).
Divisions of the Pharynx
Nasal cavity
Hard palate Soft palate
Tongue
Epiglottis
Larynx (voice box)
Esophagus
Trachea
The pharynx is divided into three regions: the
nasopharynx, the oropharynx, and the
laryngopharynx.
The nasopharynx is flanked by the conchae of the nasal cavity, and it
serves only as an airway. At the top of the nasopharynx are the pharyngeal
tonsils. A pharyngeal tonsil, also called an adenoid, is an aggregate of
lymphoid reticular tissue similar to a lymph node that lies at the superior
portion of the nasopharynx. The function of the pharyngeal tonsil is not
well understood, but it contains a rich supply of lymphocytes and is covered
with ciliated epithelium that traps and destroys invading pathogens that
enter during inhalation. The pharyngeal tonsils are large in children, but
interestingly, tend to regress with age and may even disappear. The uvula is
a small bulbous, teardrop-shaped structure located at the apex of the soft
palate. Both the uvula and soft palate move like a pendulum during
swallowing, swinging upward to close off the nasopharynx to prevent
ingested materials from entering the nasal cavity. In addition, auditory
(Eustachian) tubes that connect to each middle ear cavity open into the
nasopharynx. This connection is why colds often lead to ear infections.
The oropharynx is a passageway for both air and food. The oropharynx is
bordered superiorly by the nasopharynx and anteriorly by the oral cavity.
The fauces is the opening at the connection between the oral cavity and the
oropharynx. As the nasopharynx becomes the oropharynx, the epithelium
changes from pseudostratified ciliated columnar epithelium to stratified
squamous epithelium. The oropharynx contains two distinct sets of tonsils,
the palatine and lingual tonsils. A palatine tonsil is one of a pair of
structures located laterally in the oropharynx in the area of the fauces. The
lingual tonsil is located at the base of the tongue. Similar to the pharyngeal
tonsil, the palatine and lingual tonsils are composed of lymphoid tissue, and
trap and destroy pathogens entering the body through the oral or nasal
Cavities.
The laryngopharynx is inferior to the oropharynx and posterior to the
larynx. It continues the route for ingested material and air until its inferior
end, where the digestive and respiratory systems diverge. The stratified
squamous epithelium of the oropharynx is continuous with the
laryngopharynx. Anteriorly, the laryngopharynx opens into the larynx,
whereas posteriorly, it enters the esophagus.
Larynx
The larynx is a cartilaginous structure inferior to the laryngopharynx that
connects the pharynx to the trachea and helps regulate the volume of air that
enters and leaves the lungs ({link]). The structure of the larynx is formed by
several pieces of cartilage. Three large cartilage pieces—the thyroid
cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—
form the major structure of the larynx. The thyroid cartilage is the largest
piece of cartilage that makes up the larynx. The thyroid cartilage consists of
the laryngeal prominence, or “Adam’s apple,” which tends to be more
prominent in males. The thick cricoid cartilage forms a ring, with a wide
posterior region and a thinner anterior region. Three smaller, paired
cartilages—the arytenoids, corniculates, and cuneiforms—attach to the
epiglottis and the vocal cords and muscle that help move the vocal cords to
produce speech.
Larynx
Epiglottis
Body of hyoid bone
Thyrohyoid membrane
Thyroid cartilage
Laryngeal prominence
Cricothyroid ligament
- 9 Cricoid cartilage
Cricotracheal ligament
Tracheal cartilages
Epiglottis
Thyrohyoid membrane
Body of hyoid bone
Fatty pad
Thyrohyoid membrane
Vestibular fold
Vocal fold
Thyroid cartilage
Cricothyroid ligament
Cuneiform cartilage
Corniculate cartilage
Arytenoid cartilage
Cricoid cartilage
Cricotracheal ligament
Tracheal cartilages
Right lateral view
The larynx extends from the laryngopharynx and
the hyoid bone to the trachea.
The epiglottis, attached to the thyroid cartilage, is a very flexible piece of
elastic cartilage that covers the opening of the trachea (see [link]). When in
the “closed” position, the unattached end of the epiglottis rests on the
glottis. The glottis is composed of the vestibular folds, the true vocal cords,
and the space between these folds ([link]). A vestibular fold, or false vocal
cord, is one of a pair of folded sections of mucous membrane. A true vocal
cord is one of the white, membranous folds attached by muscle to the
thyroid and arytenoid cartilages of the larynx on their outer edges. The
inner edges of the true vocal cords are free, allowing oscillation to produce
sound. The size of the membranous folds of the true vocal cords differs
between individuals, producing voices with different pitch ranges. Folds in
males tend to be larger than those in females, which create a deeper voice.
The act of swallowing causes the pharynx and larynx to lift upward,
allowing the pharynx to expand and the epiglottis of the larynx to swing
downward, closing the opening to the trachea. These movements produce a
larger area for food to pass through, while preventing food and beverages
from entering the trachea.
Vocal Cords
Esophagus Pyriform fossa
Trachea
True vocal cord
Vestibular fold
Glottis
Epiglottis
Tongue
The true vocal cords and vestibular folds of the
larynx are viewed inferiorly from the
laryngopharynx.
Continuous with the laryngopharynx, the superior portion of the larynx is
lined with stratified squamous epithelium, transitioning into
pseudostratified ciliated columnar epithelium that contains goblet cells.
Similar to the nasal cavity and nasopharynx, this specialized epithelium
produces mucus to trap debris and pathogens as they enter the trachea. The
cilia beat the mucus upward towards the laryngopharynx, where it can be
swallowed down the esophagus.
Trachea
The trachea (windpipe) extends from the larynx toward the lungs ((link]a).
The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline
cartilage that are connected by dense connective tissue. The trachealis
muscle and elastic connective tissue together form the fibroelastic
membrane, a flexible membrane that closes the posterior surface of the
trachea, connecting the C-shaped cartilages. The fibroelastic membrane
allows the trachea to stretch and expand slightly during inhalation and
exhalation, whereas the rings of cartilage provide structural support and
prevent the trachea from collapsing. In addition, the trachealis muscle can
be contracted to force air through the trachea during exhalation. The trachea
is lined with pseudostratified ciliated columnar epithelium, which is
continuous with the larynx. The esophagus borders the trachea posteriorly.
Trachea
(a) The tracheal tube is formed by stacked, C-shaped pieces of hyaline
cartilage. (b) The layer visible in this cross-section of tracheal wall
tissue between the hyaline cartilage and the lumen of the trachea is the
mucosa, which is composed of pseudostratified ciliated columnar
epithelium that contains goblet cells. LM x 1220. (Micrograph
provided by the Regents of University of Michigan Medical School ©
2012)
Larynx
Trachea
=) Tracheal
cartilages
Submucosal Pseudostratified
seromucous glands columnar epithelia Cilia =©Lumen
Primary
bronchi
Right lung Left lung Hyaline cartilage
Secondary
bronchi
(a) (b)
Bronchial Tree
The trachea branches into the right and left primary bronchi at the carina.
These bronchi are also lined by pseudostratified ciliated columnar
epithelium containing mucus-producing goblet cells ((link]b). The carina is
a raised structure that contains specialized nervous tissue that induces
violent coughing if a foreign body, such as food, is present. Rings of
cartilage, similar to those of the trachea, support the structure of the bronchi
and prevent their collapse. The primary bronchi enter the lungs at the hilum,
a concave region where blood vessels, lymphatic vessels, and nerves also
enter the lungs. The bronchi continue to branch into bronchial a tree. A
bronchial tree (or respiratory tree) is the collective term used for these
multiple-branched bronchi. The main function of the bronchi, like other
conducting zone structures, is to provide a passageway for air to move into
and out of each lung. In addition, the mucous membrane traps debris and
pathogens.
A bronchiole branches from the tertiary bronchi. Bronchioles, which are
about 1 mm in diameter, further branch until they become the tiny terminal
bronchioles, which lead to the structures of gas exchange. There are more
than 1000 terminal bronchioles in each lung. The muscular walls of the
bronchioles do not contain cartilage like those of the bronchi. This muscular
wall can change the size of the tubing to increase or decrease airflow
through the tube.
Respiratory Zone
In contrast to the conducting zone, the respiratory zone includes structures
that are directly involved in gas exchange. The respiratory zone begins
where the terminal bronchioles join a respiratory bronchiole, the smallest
type of bronchiole ({link]), which then leads to an alveolar duct, opening
into a cluster of alveoli.
Respiratory Zone
Terminal bronchiole
Smooth muscle
Deoxygenated blood from
pulmonary artery
Oxygenated blood to
pulmonary vein
Respiratory bronchiole
Alveolar Alveolus
sac
Capillaries
Alveolar
pores
Bronchioles lead to alveolar sacs in the respiratory zone,
where gas exchange occurs.
Alveoli
An alveolar duct is a tube composed of smooth muscle and connective
tissue, which opens into a cluster of alveoli. An alveolus is one of the many
small, grape-like sacs that are attached to the alveolar ducts.
An alveolar sac is a cluster of many individual alveoli that are responsible
for gas exchange. An alveolus is approximately 200 pm in diameter with
elastic walls that allow the alveolus to stretch during air intake, which
greatly increases the surface area available for gas exchange. Alveoli are
connected to their neighbors by alveolar pores, which help maintain equal
air pressure throughout the alveoli and lung ([link]).
Structures of the Respiratory Zone
(a) The alveolus is responsible for gas exchange. (b) A micrograph
shows the alveolar structures within lung tissue. LM x 178.
(Micrograph provided by the Regents of University of Michigan
Medical School © 2012)
Alveoli Alveolar duct Blood vessels Lumen of bronchiole
U> Alveolar pores
Capillary
Respiratory membrane *
Type | alveolar cell
Macrophage
Alveolus
(gas-filled space)
Type II alveolar cell Alveolar sac
(a) (b)
The alveolar wall consists of three major cell types: type I alveolar cells,
type II alveolar cells, and alveolar macrophages. A type I alveolar cell is a
squamous epithelial cell of the alveoli, which constitute up to 97 percent of
the alveolar surface area. These cells are about 25 nm thick and are highly
permeable to gases. A type II alveolar cell is interspersed among the type I
cells and secretes pulmonary surfactant, a substance composed of
phospholipids and proteins that reduces the surface tension of the alveoli.
Roaming around the alveolar wall is the alveolar macrophage, a
phagocytic cell of the immune system that removes debris and pathogens
that have reached the alveoli.
The simple squamous epithelium formed by type I alveolar cells is attached
to a thin, elastic basement membrane. This epithelium is extremely thin and
borders the endothelial membrane of capillaries. Taken together, the alveoli
and capillary membranes form a respiratory membrane that is
approximately 0.5 mm thick. The respiratory membrane allows gases to
cross by simple diffusion, allowing oxygen to be picked up by the blood for
transport and CO; to be released into the air of the alveoli.
Note:
Diseases of the...
Respiratory System: Asthma
Asthma is common condition that affects the lungs in both adults and
children. Approximately 8.2 percent of adults (18.7 million) and 9.4
percent of children (7 million) in the United States suffer from asthma. In
addition, asthma is the most frequent cause of hospitalization in children.
Asthma is a chronic disease characterized by inflammation and edema of
the airway, and bronchospasms (that is, constriction of the bronchioles),
which can inhibit air from entering the lungs. In addition, excessive mucus
secretion can occur, which further contributes to airway occlusion ((link]).
Cells of the immune system, such as eosinophils and mononuclear cells,
may also be involved in infiltrating the walls of the bronchi and
bronchioles.
Bronchospasms occur periodically and lead to an “asthma attack.” An
attack may be triggered by environmental factors such as dust, pollen, pet
hair, or dander, changes in the weather, mold, tobacco smoke, and
respiratory infections, or by exercise and stress.
Normal and Bronchial Asthma Tissues
Mucus
Goblet cell
Epithelium
Basement membrane
Lamina propria
O Smooth muscle
(e) 2 Es -
y © 6 Gland
Cartilage
Mucus
Goblet cell
Basement membrane
Mast cell
Lamina propria
Macrophage
Eosinophil
Smooth muscle
Neutrophil
(a) Normal lung tissue does not have the
characteristics of lung tissue during (b) an
asthma attack, which include thickened mucosa,
increased mucus-producing goblet cells, and
eosinophil infiltrates.
Symptoms of an asthma attack involve coughing, shortness of breath,
wheezing, and tightness of the chest. Symptoms of a severe asthma attack
that requires immediate medical attention would include difficulty
breathing that results in blue (cyanotic) lips or face, confusion, drowsiness,
a rapid pulse, sweating, and severe anxiety. The severity of the condition,
frequency of attacks, and identified triggers influence the type of
medication that an individual may require. Longer-term treatments are
used for those with more severe asthma. Short-term, fast-acting drugs that
are used to treat an asthma attack are typically administered via an inhaler.
For young children or individuals who have difficulty using an inhaler,
asthma medications can be administered via a nebulizer.
In many cases, the underlying cause of the condition is unknown.
However, recent research has demonstrated that certain viruses, such as
human rhinovirus C (HRVC), and the bacteria Mycoplasma pneumoniae
and Chlamydia pneumoniae that are contracted in infancy or early
childhood, may contribute to the development of many cases of asthma.
Note:
| [a
= epehstan coulece
‘ My
Oe
Visit this site to learn more about what happens during an asthma attack.
What are the three changes that occur inside the airways during an asthma
attack?
Chapter Review
The respiratory system is responsible for obtaining oxygen and getting rid
of carbon dioxide, and aiding in speech production and in sensing odors.
From a functional perspective, the respiratory system can be divided into
two major areas: the conducting zone and the respiratory zone. The
conducting zone consists of all of the structures that provide passageways
for air to travel into and out of the lungs: the nasal cavity, pharynx, trachea,
bronchi, and most bronchioles. The nasal passages contain the conchae and
meatuses that expand the surface area of the cavity, which helps to warm
and humidify incoming air, while removing debris and pathogens. The
pharynx is composed of three major sections: the nasopharynx, which is
continuous with the nasal cavity; the oropharynx, which borders the
nasopharynx and the oral cavity; and the laryngopharynx, which borders the
oropharynx, trachea, and esophagus. The respiratory zone includes the
structures of the lung that are directly involved in gas exchange: the
terminal bronchioles and alveoli.
The lining of the conducting zone is composed mostly of pseudostratified
ciliated columnar epithelium with goblet cells. The mucus traps pathogens
and debris, whereas beating cilia move the mucus superiorly toward the
throat, where it is swallowed. As the bronchioles become smaller and
smaller, and nearer the alveoli, the epithelium thins and is simple squamous
epithelium in the alveoli. The endothelium of the surrounding capillaries,
together with the alveolar epithelium, forms the respiratory membrane. This
is a blood-air barrier through which gas exchange occurs by simple
diffusion.
Interactive Link Questions
Exercise:
Problem:
Visit this site to learn more about what happens during an asthma
attack. What are the three changes that occur inside the airways during
an asthma attack?
Solution:
Inflammation and the production of a thick mucus; constriction of the
airway muscles, or bronchospasm; and an increased sensitivity to
allergens.
Review Questions
Exercise:
Problem:
Which of the following anatomical structures is not part of the
conducting zone?
a. pharynx
b. nasal cavity
c. alveoli
d. bronchi
Solution:
C
Exercise:
Problem: What is the function of the conchae in the nasal cavity?
a. increase surface area
b. exchange gases
c. maintain surface tension
d. maintain air pressure
Solution:
A
Exercise:
Problem:
The fauces connects which of the following structures to the
oropharynx?
a. nasopharynx
b. laryngopharynx
c. nasal cavity
d. oral cavity
Solution:
D
Exercise:
Problem: Which of the following are structural features of the trachea?
a. C-shaped cartilage
b. smooth muscle fibers
c. cilia
d. all of the above
Solution:
A
Exercise:
Problem:
Which of the following structures is not part of the bronchial tree?
a. alveoli
b. bronchi
c. terminal bronchioles
d. respiratory bronchioles
Solution:
C
Exercise:
Problem: What is the role of alveolar macrophages?
a. to secrete pulmonary surfactant
b. to secrete antimicrobial proteins
c. to remove pathogens and debris
d. to facilitate gas exchange
Solution:
C
Critical Thinking Questions
Exercise:
Problem: Describe the three regions of the pharynx and their functions.
Solution:
The pharynx has three major regions. The first region is the
nasopharynx, which is connected to the posterior nasal cavity and
functions as an airway. The second region is the oropharynx, which is
continuous with the nasopharynx and is connected to the oral cavity at
the fauces. The laryngopharynx is connected to the oropharynx and the
esophagus and trachea. Both the oropharynx and laryngopharynx are
passageways for air and food and drink.
Exercise:
Problem:
If a person sustains an injury to the epiglottis, what would be the
physiological result?
Solution:
The epiglottis is a region of the larynx that is important during the
swallowing of food or drink. As a person swallows, the pharynx moves
upward and the epiglottis closes over the trachea, preventing food or
drink from entering the trachea. If a person’s epiglottis were injured,
this mechanism would be impaired. As a result, the person may have
problems with food or drink entering the trachea, and possibly, the
lungs. Over time, this may cause infections such as pneumonia to set
in.
Exercise:
Problem:Compare and contrast the conducting and respiratory zones.
Solution:
The conducting zone of the respiratory system includes the organs and
structures that are not directly involved in gas exchange, but perform
other duties such as providing a passageway for air, trapping and
removing debris and pathogens, and warming and humidifying
incoming air. Such structures include the nasal cavity, pharynx, larynx,
trachea, and most of the bronchial tree. The respiratory zone includes
all the organs and structures that are directly involved in gas exchange,
including the respiratory bronchioles, alveolar ducts, and alveoli.
References
Bizzintino J, Lee WM, Laing IA, Vang F, Pappas T, Zhang G, Martin AC,
Khoo SK, Cox DW, Geelhoed GC, et al. Association between human
rhinovirus C and severity of acute asthma in children. Eur Respir J
[Internet]. 2010 [cited 2013 Mar 22]; 37(5):1037—1042. Available from:
submit=Go&gca=erj%3B37%2F5%2F 1037 &allch=
Kumar V, Ramzi S, Robbins SL. Robbins Basic Pathology. 7th ed.
Philadelphia (PA): Elsevier Ltd; 2005.
Martin RJ, Kraft M, Chu HW, Berns, EA, Cassell GH. A link between
chronic asthma and chronic infection. J Allergy Clin Immunol [Internet].
2001 [cited 2013 Mar 22]; 107(4):595-601. Available from:
submit=Go&gca=erj%3B37%2F5%2F 1037 &allch=
Glossary
ala
(plural = alae) small, flaring structure of a nostril that forms the lateral
side of the nares
alar cartilage
cartilage that supports the apex of the nose and helps shape the nares;
it is connected to the septal cartilage and connective tissue of the alae
alveolar duct
small tube that leads from the terminal bronchiole to the respiratory
bronchiole and is the point of attachment for alveoli
alveolar macrophage
immune system cell of the alveolus that removes debris and pathogens
alveolar pore
opening that allows airflow between neighboring alveoli
alveolar sac
cluster of alveoli
alveolus
small, grape-like sac that performs gas exchange in the lungs
apex
tip of the external nose
bronchial tree
collective name for the multiple branches of the bronchi and
bronchioles of the respiratory system
bridge
portion of the external nose that lies in the area of the nasal bones
bronchiole
branch of bronchi that are 1 mm or less in diameter and terminate at
alveolar sacs
bronchus
tube connected to the trachea that branches into many subsidiaries and
provides a passageway for air to enter and leave the lungs
conducting zone
region of the respiratory system that includes the organs and structures
that provide passageways for air and are not directly involved in gas
exchange
cricoid cartilage
portion of the larynx composed of a ring of cartilage with a wide
posterior region and a thinner anterior region; attached to the
esophagus
dorsum nasi
intermediate portion of the external nose that connects the bridge to the
apex and is supported by the nasal bone
epiglottis
leaf-shaped piece of elastic cartilage that is a portion of the larynx that
swings to close the trachea during swallowing
external nose
region of the nose that is easily visible to others
fauces
portion of the posterior oral cavity that connects the oral cavity to the
oropharynx
fibroelastic membrane
specialized membrane that connects the ends of the C-shape cartilage
in the trachea; contains smooth muscle fibers
glottis
opening between the vocal folds through which air passes when
producing speech
laryngeal prominence
region where the two lamina of the thyroid cartilage join, forming a
protrusion known as “Adam’s apple”
laryngopharynx
portion of the pharynx bordered by the oropharynx superiorly and
esophagus and trachea inferiorly; serves as a route for both air and
food
larynx
cartilaginous structure that produces the voice, prevents food and
beverages from entering the trachea, and regulates the volume of air
that enters and leaves the lungs
lingual tonsil
lymphoid tissue located at the base of the tongue
meatus
one of three recesses (superior, middle, and inferior) in the nasal cavity
attached to the conchae that increase the surface area of the nasal
cavity
naris
(plural = nares) opening of the nostrils
nasal bone
bone of the skull that lies under the root and bridge of the nose and is
connected to the frontal and maxillary bones
nasal septum
wall composed of bone and cartilage that separates the left and right
nasal cavities
nasopharynx
portion of the pharynx flanked by the conchae and oropharynx that
serves as an airway
oropharynx
portion of the pharynx flanked by the nasopharynx, oral cavity, and
laryngopharynx that is a passageway for both air and food
palatine tonsil
one of the paired structures composed of lymphoid tissue located
anterior to the uvula at the roof of isthmus of the fauces
paranasal sinus
one of the cavities within the skull that is connected to the conchae that
serve to warm and humidify incoming air, produce mucus, and lighten
the weight of the skull; consists of frontal, maxillary, sphenoidal, and
ethmoidal sinuses
pharyngeal tonsil
structure composed of lymphoid tissue located in the nasopharynx
pharynx
region of the conducting zone that forms a tube of skeletal muscle
lined with respiratory epithelium; located between the nasal conchae
and the esophagus and trachea
philtrum
concave surface of the face that connects the apex of the nose to the
top lip
pulmonary surfactant
substance composed of phospholipids and proteins that reduces the
surface tension of the alveoli; made by type II alveolar cells
respiratory bronchiole
specific type of bronchiole that leads to alveolar sacs
respiratory epithelium
ciliated lining of much of the conducting zone that is specialized to
remove debris and pathogens, and produce mucus
respiratory membrane
alveolar and capillary wall together, which form an air-blood barrier
that facilitates the simple diffusion of gases
respiratory zone
includes structures of the respiratory system that are directly involved
in gas exchange
root
region of the external nose between the eyebrows
thyroid cartilage
largest piece of cartilage that makes up the larynx and consists of two
lamina
trachea
tube composed of cartilaginous rings and supporting tissue that
connects the lung bronchi and the larynx; provides a route for air to
enter and exit the lung
trachealis muscle
smooth muscle located in the fibroelastic membrane of the trachea
true vocal cord
one of the pair of folded, white membranes that have a free inner edge
that oscillates as air passes through to produce sound
type I alveolar cell
squamous epithelial cells that are the major cell type in the alveolar
wall; highly permeable to gases
type II alveolar cell
cuboidal epithelial cells that are the minor cell type in the alveolar
wall; secrete pulmonary surfactant
vestibular fold
part of the folded region of the glottis composed of mucous membrane;
supports the epiglottis during swallowing
The Lungs
By the end of this section, you will be able to:
¢ Describe the overall function of the lung
e Summarize the blood flow pattern associated with the lungs
¢ Outline the anatomy of the blood supply to the lungs
e Describe the pleura of the lungs and their function
A major organ of the respiratory system, each lung houses structures of
both the conducting and respiratory zones. The main function of the lungs is
to perform the exchange of oxygen and carbon dioxide with air from the
atmosphere. To this end, the lungs exchange respiratory gases across a very
large epithelial surface area—about 70 square meters—that is highly
permeable to gases.
Gross Anatomy of the Lungs
The lungs are pyramid-shaped, paired organs that are connected to the
trachea by the right and left bronchi; on the inferior surface, the lungs are
bordered by the diaphragm. The diaphragm is the flat, dome-shaped muscle
located at the base of the lungs and thoracic cavity. The lungs are enclosed
by the pleurae, which are attached to the mediastinum. The right lung is
shorter and wider than the left lung, and the left lung occupies a smaller
volume than the right. The cardiac notch is an indentation on the surface of
the left lung, and it allows space for the heart ([link]). The apex of the lung
is the superior region, whereas the base is the opposite region near the
diaphragm. The costal surface of the lung borders the ribs. The mediastinal
surface faces the midline.
Gross Anatomy of the Lungs
Trachea
Superior lobe
Main (primary)
bronchus
Superior lobe Lobar
(secondary)
ronchus
Segmental
(tertiary)
bronchus
Middle lobe Cardiac notch
Inferior lobe Inferior lobe
Right lung Left lung
Each lung is composed of smaller units called lobes. Fissures separate these
lobes from each other. The right lung consists of three lobes: the superior,
middle, and inferior lobes. The left lung consists of two lobes: the superior
and inferior lobes. A bronchopulmonary segment is a division of a lobe, and
each lobe houses multiple bronchopulmonary segments. Each segment
receives air from its own tertiary bronchus and is supplied with blood by its
own artery. Some diseases of the lungs typically affect one or more
bronchopulmonary segments, and in some cases, the diseased segments can
be surgically removed with little influence on neighboring segments. A
pulmonary lobule is a subdivision formed as the bronchi branch into
bronchioles. Each lobule receives its own large bronchiole that has multiple
branches. An interlobular septum is a wall, composed of connective tissue,
which separates lobules from one another.
Blood Supply and Nervous Innervation of the Lungs
The blood supply of the lungs plays an important role in gas exchange and
serves as a transport system for gases throughout the body. In addition,
innervation by the both the parasympathetic and sympathetic nervous
systems provides an important level of control through dilation and
constriction of the airway.
Blood Supply
The major function of the lungs is to perform gas exchange, which requires
blood from the pulmonary circulation. This blood supply contains
deoxygenated blood and travels to the lungs where erythrocytes, also
known as red blood cells, pick up oxygen to be transported to tissues
throughout the body. The pulmonary artery is an artery that arises from
the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli.
The pulmonary artery branches multiple times as it follows the bronchi, and
each branch becomes progressively smaller in diameter. One arteriole and
an accompanying venule supply and drain one pulmonary lobule. As they
near the alveoli, the pulmonary arteries become the pulmonary capillary
network. The pulmonary capillary network consists of tiny vessels with
very thin walls that lack smooth muscle fibers. The capillaries branch and
follow the bronchioles and structure of the alveoli. It is at this point that the
capillary wall meets the alveolar wall, creating the respiratory membrane.
Once the blood is oxygenated, it drains from the alveoli by way of multiple
pulmonary veins, which exit the lungs through the hilum.
Nervous Innervation
Dilation and constriction of the airway are achieved through nervous
control by the parasympathetic and sympathetic nervous systems. The
parasympathetic system causes bronchoconstriction, whereas the
sympathetic nervous system stimulates bronchodilation. Reflexes such as
coughing, and the ability of the lungs to regulate oxygen and carbon dioxide
levels, also result from this autonomic nervous system control. Sensory
nerve fibers arise from the vagus nerve, and from the second to fifth
thoracic ganglia. The pulmonary plexus is a region on the lung root
formed by the entrance of the nerves at the hilum. The nerves then follow
the bronchi in the lungs and branch to innervate muscle fibers, glands, and
blood vessels.
Pleura of the Lungs
Each lung is enclosed within a cavity that is surrounded by the pleura. The
pleura (plural = pleurae) is a serous membrane that surrounds the lung. The
right and left pleurae, which enclose the right and left lungs, respectively,
are separated by the mediastinum. The pleurae consist of two layers. The
visceral pleura is the layer that is superficial to the lungs, and extends into
and lines the lung fissures ({link]). In contrast, the parietal pleura is the
outer layer that connects to the thoracic wall, the mediastinum, and the
diaphragm. The visceral and parietal pleurae connect to each other at the
hilum. The pleural cavity is the space between the visceral and parietal
layers.
Parietal and Visceral Pleurae of the Lungs
Intercostal
muscle
Pleural sac |
Intercostal f , Ke ae
muscles <4 I ent pleura
Visceral
pleura
Pleural cavity
Diaphragm
Chest wall
(rib cage, sternum, thoracic vertebrae,
connective tissue, intercostal muscles)
The pleurae perform two major functions: They produce pleural fluid and
create cavities that separate the major organs. Pleural fluid is secreted by
mesothelial cells from both pleural layers and acts to lubricate their
surfaces. This lubrication reduces friction between the two layers to prevent
trauma during breathing, and creates surface tension that helps maintain the
position of the lungs against the thoracic wall. This adhesive characteristic
of the pleural fluid causes the lungs to enlarge when the thoracic wall
expands during ventilation, allowing the lungs to fill with air. The pleurae
also create a division between major organs that prevents interference due
to the movement of the organs, while preventing the spread of infection.
Note:
Everyday Connection
The Effects of Second-Hand Tobacco Smoke
The burning of a tobacco cigarette creates multiple chemical compounds
that are released through mainstream smoke, which is inhaled by the
smoker, and through sidestream smoke, which is the smoke that is given
off by the burning cigarette. Second-hand smoke, which is a combination
of sidestream smoke and the mainstream smoke that is exhaled by the
smoker, has been demonstrated by numerous scientific studies to cause
disease. At least 40 chemicals in sidestream smoke have been identified
that negatively impact human health, leading to the development of cancer
or other conditions, such as immune system dysfunction, liver toxicity,
cardiac arrhythmias, pulmonary edema, and neurological dysfunction.
Furthermore, second-hand smoke has been found to harbor at least 250
compounds that are known to be toxic, carcinogenic, or both. Some major
classes of carcinogens in second-hand smoke are polyaromatic
hydrocarbons (PAHs), N-nitrosamines, aromatic amines, formaldehyde,
and acetaldehyde.
Tobacco and second-hand smoke are considered to be carcinogenic.
Exposure to second-hand smoke can cause lung cancer in individuals who
are not tobacco users themselves. It is estimated that the risk of developing
lung cancer is increased by up to 30 percent in nonsmokers who live with
an individual who smokes in the house, as compared to nonsmokers who
are not regularly exposed to second-hand smoke. Children are especially
affected by second-hand smoke. Children who live with an individual who
smokes inside the home have a larger number of lower respiratory
infections, which are associated with hospitalizations, and higher risk of
sudden infant death syndrome (SIDS). Second-hand smoke in the home
has also been linked to a greater number of ear infections in children, as
well as worsening symptoms of asthma.
Chapter Review
The lungs are the major organs of the respiratory system and are responsible
for performing gas exchange. The lungs are paired and separated into lobes;
The left lung consists of two lobes, whereas the right lung consists of three
lobes. Blood circulation is very important, as blood is required to transport
oxygen from the lungs to other tissues throughout the body. The function of
the pulmonary circulation is to aid in gas exchange. The pulmonary artery
provides deoxygenated blood to the capillaries that form respiratory
membranes with the alveoli, and the pulmonary veins return newly
oxygenated blood to the heart for further transport throughout the body. The
lungs are innervated by the parasympathetic and sympathetic nervous
systems, which coordinate the bronchodilation and bronchoconstriction of
the airways. The lungs are enclosed by the pleura, a membrane that is
composed of visceral and parietal pleural layers. The space between these
two layers is called the pleural cavity. The mesothelial cells of the pleural
membrane create pleural fluid, which serves as both a lubricant (to reduce
friction during breathing) and as an adhesive to adhere the lungs to the
thoracic wall (to facilitate movement of the lungs during ventilation).
Review Questions
Exercise:
Problem:
Which of the following structures separates the lung into lobes?
a. Mediastinum
b. fissure
c. root
d. pleura
Solution:
B
Exercise:
Problem:
A section of the lung that receives its own tertiary bronchus is called
the
a. bronchopulmonary segment
b. pulmonary lobule
c. interpulmonary segment
d. respiratory segment
Solution:
A
Exercise:
Problem:
The circulation picks up oxygen for cellular use and drops
off carbon dioxide for removal from the body.
a. pulmonary
b. interlobular
c. respiratory
d. bronchial
Solution:
C
Exercise:
Problem:
The pleura that surrounds the lungs consists of two layers, the
a. visceral and parietal pleurae.
b. mediastinum and parietal pleurae.
c. visceral and mediastinum pleurae.
d. none of the above
Solution:
A
Critical Thinking Questions
Exercise:
Problem:Compare and contrast the right and left lungs.
Solution:
The right and left lungs differ in size and shape to accommodate other
organs that encroach on the thoracic region. The right lung consists of
three lobes and is shorter than the left lung, due to the position of the
liver underneath it. The left lung consist of two lobes and is longer and
narrower than the right lung. The left lung has a concave region on the
mediastinal surface called the cardiac notch that allows space for the
heart.
Exercise:
Problem: Why are the pleurae not damaged during normal breathing?
Solution:
There is a cavity, called the pleural cavity, between the parietal and
visceral layers of the pleura. Mesothelial cells produce and secrete
pleural fluid into the pleural cavity that acts as a lubricant. Therefore,
as you breathe, the pleural fluid prevents the two layers of the pleura
from rubbing against each other and causing damage due to friction.
Glossary
bronchoconstriction
decrease in the size of the bronchiole due to contraction of the
muscular wall
bronchodilation
increase in the size of the bronchiole due to contraction of the
muscular wall
cardiac notch
indentation on the surface of the left lung that allows space for the
heart
hilum
concave structure on the mediastinal surface of the lungs where blood
vessels, lymphatic vessels, nerves, and a bronchus enter the lung
lung
organ of the respiratory system that performs gas exchange
parietal pleura
outermost layer of the pleura that connects to the thoracic wall,
mediastinum, and diaphragm
pleural cavity
space between the visceral and parietal pleurae
pleural fluid
substance that acts as a lubricant for the visceral and parietal layers of
the pleura during the movement of breathing
pulmonary artery
artery that arises from the pulmonary trunk and carries deoxygenated,
arterial blood to the alveoli
pulmonary plexus
network of autonomic nervous system fibers found near the hilum of
the lung
visceral pleura
innermost layer of the pleura that is superficial to the lungs and
extends into the lung fissures
Overview of the Digestive System
By the end of this section, you will be able to:
Identify the organs of the alimentary canal from proximal to distal, and
briefly state their function
Identify the accessory digestive organs and briefly state their function
Describe the four fundamental tissue layers of the alimentary canal
Contrast the contributions of the enteric and autonomic nervous
systems to digestive system functioning
Explain how the peritoneum anchors the digestive organs
The function of the digestive system is to break down the foods you eat,
release their nutrients, and absorb those nutrients into the body. Although
the small intestine is the workhorse of the system, where the majority of
digestion occurs, and where most of the released nutrients are absorbed into
the blood or lymph, each of the digestive system organs makes a vital
contribution to this process ([link]).
Components of the Digestive System
Salivary glands:
Mouth Ca Parotid gland
Tongue
r Sublingual gland
Pe Submandibular gland
Pharynx
Esophagus
Liver
Gallbladder Stomach
y 7 Spleen
Small intestine:
Duodenum
Jejunum
lleum
Pancreas
Large intestine:
Transverse colon
Ascending colon
Descending colon
Cecum
Sigmoid colon
Appendix
Rectum
Anus Anal canal
All digestive organs play integral roles in
the life-sustaining process of digestion.
As is the case with all body systems, the digestive system does not work in
isolation; it functions cooperatively with the other systems of the body.
Consider for example, the interrelationship between the digestive and
cardiovascular systems. Arteries supply the digestive organs with oxygen
and processed nutrients, and veins drain the digestive tract. These intestinal
veins, constituting the hepatic portal system, are unique; they do not return
blood directly to the heart. Rather, this blood is diverted to the liver where
its nutrients are off-loaded for processing before blood completes its circuit
back to the heart. At the same time, the digestive system provides nutrients
to the heart muscle and vascular tissue to support their functioning. The
interrelationship of the digestive and endocrine systems is also critical.
Hormones secreted by several endocrine glands, as well as endocrine cells
of the pancreas, the stomach, and the small intestine, contribute to the
control of digestion and nutrient metabolism. In turn, the digestive system
provides the nutrients to fuel endocrine function. [link] gives a quick
glimpse at how these other systems contribute to the functioning of the
digestive system.
Contribution of Other Body Systems to the Digestive System
Body system
Cardiovascular
Endocrine
Integumentary
Lymphatic
Muscular
Benefits received by the digestive system
Blood supplies digestive organs with oxygen and
processed nutrients
Endocrine hormones help regulate secretion in
digestive glands and accessory organs
Skin helps protect digestive organs and
synthesizes vitamin D for calcium absorption
Mucosa-associated lymphoid tissue and other
lymphatic tissue defend against entry of
pathogens; lacteals absorb lipids; and lymphatic
vessels transport lipids to bloodstream
Skeletal muscles support and protect abdominal
organs
Contribution of Other Body Systems to the Digestive System
Body system Benefits received by the digestive system
Sensory and motor neurons help regulate
Nervous secretions and muscle contractions in the
digestive tract
Respiratory organs provide oxygen and remove
Respirator aa
P e carbon dioxide
Skeletal Bones help protect and support digestive organs
; Kidneys convert vitamin D into its active form,
Urinary
allowing calcium absorption in the small intestine
Digestive System Organs
The easiest way to understand the digestive system is to divide its organs
into two main categories. The first group is the organs that make up the
alimentary canal. Accessory digestive organs comprise the second group
and are critical for orchestrating the breakdown of food and the assimilation
of its nutrients into the body. Accessory digestive organs, despite their
name, are critical to the function of the digestive system.
Alimentary Canal Organs
Also called the gastrointestinal (GI) tract or gut, the alimentary canal
(aliment- = “to nourish’) is a one-way tube about 7.62 meters (25 feet) in
length during life and closer to 10.67 meters (35 feet) in length when
measured after death, once smooth muscle tone is lost. The main function
of the organs of the alimentary canal is to nourish the body. This tube
begins at the mouth and terminates at the anus. Between those two points,
the canal is modified as the pharynx, esophagus, stomach, and small and
large intestines to fit the functional needs of the body. Both the mouth and
anus are open to the external environment; thus, food and wastes within the
alimentary canal are technically considered to be outside the body. Only
through the process of absorption do the nutrients in food enter into and
3 cc
nourish the body’s “inner space.”
Accessory Structures
Each accessory digestive organ aids in the breakdown of food ((Link]).
Within the mouth, the teeth and tongue begin mechanical digestion,
whereas the salivary glands begin chemical digestion. Once food products
enter the small intestine, the gallbladder, liver, and pancreas release
secretions—such as bile and enzymes—essential for digestion to continue.
Together, these are called accessory organs because they sprout from the
lining cells of the developing gut (mucosa) and augment its function;
indeed, you could not live without their vital contributions, and many
significant diseases result from their malfunction. Even after development
is complete, they maintain a connection to the gut by way of ducts.
Histology of the Alimentary Canal
Throughout its length, the alimentary tract is composed of the same four
tissue layers; the details of their structural arrangements vary to fit their
specific functions. Starting from the lumen and moving outwards, these
layers are the mucosa, submucosa, muscularis, and serosa, which is
continuous with the mesentery (see [link]).
Layers of the Alimentary Canal
Vein
}
Submucosal plexus a’, (| |
(plexus of Meissner) | i Mesentery
Glands in pelaly
submucosa <=
— y Nerve
Submucosa | y a
Gland in mucosa
Duct of gland
outside tract
Myenteric plexus
Lymphatic tissue
Serosa:
Areolar connective tissue
Epithelium
Lumen
Mucosa:
Epithelium
Lamina propria
Muscularis mucosae
Muscularis:
Circular muscle
Longitudinal muscle
The wall of the alimentary canal has four basic tissue
layers: the mucosa, submucosa, muscularis, and
serosa.
The mucosa is referred to as a mucous membrane, because mucus
production is a characteristic feature of gut epithelium. The membrane
consists of epithelium, which is in direct contact with ingested food, and the
lamina propria, a layer of connective tissue analogous to the dermis. In
addition, the mucosa has a thin, smooth muscle layer, called the muscularis
mucosa (not to be confused with the muscularis layer, described below).
Epithelium—lIn the mouth, pharynx, esophagus, and anal canal, the
epithelium is primarily a non-keratinized, stratified squamous epithelium.
In the stomach and intestines, it is a simple columnar epithelium. Notice
that the epithelium is in direct contact with the lumen, the space inside the
alimentary canal. Interspersed among its epithelial cells are goblet cells,
which secrete mucus and fluid into the lumen, and enteroendocrine cells,
which secrete hormones into the interstitial spaces between cells. Epithelial
cells have a very brief lifespan, averaging from only a couple of days (in the
mouth) to about a week (in the gut). This process of rapid renewal helps
preserve the health of the alimentary canal, despite the wear and tear
resulting from continued contact with foodstuffs.
Lamina propria—In addition to loose connective tissue, the lamina propria
contains numerous blood and lymphatic vessels that transport nutrients
absorbed through the alimentary canal to other parts of the body. The
lamina propria also serves an immune function by housing clusters of
lymphocytes, making up the mucosa-associated lymphoid tissue (MALT).
These lymphocyte clusters are particularly substantial in the distal ileum
where they are known as Peyer’s patches. When you consider that the
alimentary canal is exposed to foodborne bacteria and other foreign matter,
it is not hard to appreciate why the immune system has evolved a means of
defending against the pathogens encountered within it.
Muscularis mucosa—This thin layer of smooth muscle is in a constant state
of tension, pulling the mucosa of the stomach and small intestine into
undulating folds. These folds dramatically increase the surface area
available for digestion and absorption.
As its name implies, the submucosa lies immediately beneath the mucosa.
A broad layer of dense connective tissue, it connects the overlying mucosa
to the underlying muscularis. It includes blood and lymphatic vessels
(which transport absorbed nutrients), and a scattering of submucosal glands
that release digestive secretions. Additionally, it serves as a conduit for a
dense branching network of nerves, the submucosal plexus, which functions
as described below.
The third layer of the alimentary canal is the muscularis (also called the
muscularis externa). The muscularis in the small intestine is made up of a
double layer of smooth muscle: an inner circular layer and an outer
longitudinal layer. The contractions of these layers promote mechanical
digestion, expose more of the food to digestive chemicals, and move the
food along the canal. In the most proximal and distal regions of the
alimentary canal, including the mouth, pharynx, anterior part of the
esophagus, and external anal sphincter, the muscularis is made up of
skeletal muscle, which gives you voluntary control over swallowing and
defecation. The basic two-layer structure found in the small intestine is
modified in the organs proximal and distal to it. The stomach is equipped
for its churning function by the addition of a third layer, the oblique muscle.
While the colon has two layers like the small intestine, its longitudinal layer
is segregated into three narrow parallel bands, the tenia coli, which make it
look like a series of pouches rather than a simple tube.
The serosa is the portion of the alimentary canal superficial to the
muscularis. Present only in the region of the alimentary canal within the
abdominal cavity, it consists of a layer of visceral peritoneum overlying a
layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and
esophagus have a dense sheath of collagen fibers called the adventitia.
These tissues serve to hold the alimentary canal in place near the ventral
surface of the vertebral column.
Nerve Supply
As soon as food enters the mouth, it is detected by receptors that send
impulses along the sensory neurons of cranial nerves. Without these nerves,
not only would your food be without taste, but you would also be unable to
feel either the food or the structures of your mouth, and you would be
unable to avoid biting yourself as you chew, an action enabled by the motor
branches of cranial nerves.
Intrinsic innervation of much of the alimentary canal is provided by the
enteric nervous system, which runs from the esophagus to the anus, and
contains approximately 100 million motor, sensory, and interneurons
(unique to this system compared to all other parts of the peripheral nervous
system). These enteric neurons are grouped into two plexuses. The
myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the
alimentary canal and is responsible for motility, especially the rhythm and
force of the contractions of the muscularis. The submucosal plexus (plexus
of Meissner) lies in the submucosal layer and is responsible for regulating
digestive secretions and reacting to the presence of food (see [link]).
Extrinsic innervations of the alimentary canal are provided by the
autonomic nervous system, which includes both sympathetic and
parasympathetic nerves. In general, sympathetic activation (the fight-or-
flight response) restricts the activity of enteric neurons, thereby decreasing
GI secretion and motility. In contrast, parasympathetic activation (the rest-
and-digest response) increases GI secretion and motility by stimulating
neurons of the enteric nervous system.
Blood Supply
The blood vessels serving the digestive system have two functions. They
transport the protein and carbohydrate nutrients absorbed by mucosal cells
after food is digested in the lumen. Lipids are absorbed via lacteals, tiny
structures of the lymphatic system. The blood vessels’ second function is to
supply the organs of the alimentary canal with the nutrients and oxygen
needed to drive their cellular processes.
Specifically, the more anterior parts of the alimentary canal are supplied
with blood by arteries branching off the aortic arch and thoracic aorta.
Below this point, the alimentary canal is supplied with blood by arteries
branching from the abdominal aorta. The celiac trunk services the liver,
stomach, and duodenum, whereas the superior and inferior mesenteric
arteries supply blood to the remaining small and large intestines.
The veins that collect nutrient-rich blood from the small intestine (where
most absorption occurs) empty into the hepatic portal system. This venous
network takes the blood into the liver where the nutrients are either
processed or stored for later use. Only then does the blood drained from the
alimentary canal viscera circulate back to the heart. To appreciate just how
demanding the digestive process is on the cardiovascular system, consider
that while you are “resting and digesting,” about one-fourth of the blood
pumped with each heartbeat enters arteries serving the intestines.
The Peritoneum
The digestive organs within the abdominal cavity are held in place by the
peritoneum, a broad serous membranous sac made up of squamous
epithelial tissue surrounded by connective tissue. It is composed of two
different regions: the parietal peritoneum, which lines the abdominal wall,
and the visceral peritoneum, which envelopes the abdominal organs ([link]).
The peritoneal cavity is the space bounded by the visceral and parietal
peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to
minimize friction between the serosal surfaces of the peritoneum.
The Peritoneum
Spinal cord
Vertebra
Kidney Kidney
Pancreas Spleen
Liver Small intestine
Gallbladder Large intestine
Large intestine Stomach
Small intestine
Visceral peritoneum Peritoneal cavity Parietal peritoneum
A cross-section of the abdomen shows the
relationship between abdominal organs and the
peritoneum (darker lines).
Note:
Disorders of the...
Digestive System: Peritonitis
Inflammation of the peritoneum is called peritonitis. Chemical peritonitis
can develop any time the wall of the alimentary canal is breached, allowing
the contents of the lumen entry into the peritoneal cavity. For example,
when an ulcer perforates the stomach wall, gastric juices spill into the
peritoneal cavity. Hemorrhagic peritonitis occurs after a ruptured tubal
pregnancy or traumatic injury to the liver or spleen fills the peritoneal
cavity with blood. Even more severe peritonitis is associated with bacterial
infections seen with appendicitis, colonic diverticulitis, and pelvic
inflammatory disease (infection of uterine tubes, usually by sexually
transmitted bacteria). Peritonitis is life threatening and often results in
emergency surgery to correct the underlying problem and intensive
antibiotic therapy. When your great grandparents and even your parents
were young, the mortality from peritonitis was high. Aggressive surgery,
improvements in anesthesia safety, the advance of critical care expertise,
and antibiotics have greatly improved the mortality rate from this
condition. Even so, the mortality rate still ranges from 30 to 40 percent.
The visceral peritoneum includes multiple large folds that envelope various
abdominal organs, holding them to the dorsal surface of the body wall.
Within these folds are blood vessels, lymphatic vessels, and nerves that
innervate the organs with which they are in contact, supplying their adjacent
organs. The five major peritoneal folds are described in [link]. Note that
during fetal development, certain digestive structures, including the first
portion of the small intestine (called the duodenum), the pancreas, and
portions of the large intestine (the ascending and descending colon, and the
rectum) remain completely or partially posterior to the peritoneum. Thus,
the location of these organs is described as retroperitoneal.
The Five Major Peritoneal Folds
Fold Description
ore Apron-like structure that lies superficial to the small
intestine and transverse colon; a site of fat deposition
omentum ; ;
in people who are overweight
Falciform Anchors the liver to the anterior abdominal wall and
ligament inferior border of the diaphragm
The Five Major Peritoneal Folds
Fold Description
heccar Suspends the stomach from the inferior border of the
liver; provides a pathway for structures connecting to
omentum
the liver
Vertical band of tissue anterior to the lumbar
Mesentery vertebrae and anchoring all of the small intestine
except the initial portion (the duodenum)
Attaches two portions of the large intestine (the
Mesocolon transverse and sigmoid colon) to the posterior
abdominal wall
Note:
wees Openstax COLLEGE
oe ee
-
By clicking on this link you can watch a short video of what happens to the
food you eat, as it passes from your mouth to your intestine. Along the
way, note how the food changes consistency and form. How does this
change in consistency facilitate your gaining nutrients from food?
Chapter Review
The digestive system includes the organs of the alimentary canal and
accessory structures. The alimentary canal forms a continuous tube that is
open to the outside environment at both ends. The organs of the alimentary
canal are the mouth, pharynx, esophagus, stomach, small intestine, and
large intestine. The accessory digestive structures include the teeth, tongue,
salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary
canal is composed of four basic tissue layers: mucosa, submucosa,
muscularis, and serosa. The enteric nervous system provides intrinsic
innervation, and the autonomic nervous system provides extrinsic
innervation.
Interactive Link Questions
Exercise:
Problem:
By clicking on this link, you can watch a short video of what happens
to the food you eat as it passes from your mouth to your intestine.
Along the way, note how the food changes consistency and form. How
does this change in consistency facilitate your gaining nutrients from
food?
Solution:
Answers may vary.
Review Questions
Exercise:
Problem:
Which of these organs is not considered an accessory digestive
structure?
a. mouth
b. salivary glands
c. pancreas
d. liver
Solution:
A
Exercise:
Problem:
Which of the following organs is supported by a layer of adventitia
rather than serosa?
a. esophagus
b. stomach
c. small intestine
d. large intestine
Solution:
A
Exercise:
Problem: Which of the following membranes covers the stomach?
a. falciform ligament
b. mesocolon
c. parietal peritoneum
d. visceral peritoneum
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
Explain how the enteric nervous system supports the digestive system.
What might occur that could result in the autonomic nervous system
having a negative impact on digestion?
Solution:
The enteric nervous system helps regulate alimentary canal motility
and the secretion of digestive juices, thus facilitating digestion. If a
person becomes overly anxious, sympathetic innervation of the
alimentary canal is stimulated, which can result in a slowing of
digestive activity.
Exercise:
Problem:
What layer of the alimentary canal tissue is capable of helping to
protect the body against disease, and through what mechanism?
Solution:
The lamina propria of the mucosa contains lymphoid tissue that makes
up the MALT and responds to pathogens encountered in the alimentary
canal.
Glossary
accessory digestive organ
includes teeth, tongue, salivary glands, gallbladder, liver, and pancreas
alimentary canal
continuous muscular digestive tube that extends from the mouth to the
anus
motility
movement of food through the GI tract
mucosa
innermost lining of the alimentary canal
muscularis
muscle (skeletal or smooth) layer of the alimentary canal wall
myenteric plexus
(plexus of Auerbach) major nerve supply to alimentary canal wall;
controls motility
retroperitoneal
located posterior to the peritoneum
serosa
outermost layer of the alimentary canal wall present in regions within
the abdominal cavity
submucosa
layer of dense connective tissue in the alimentary canal wall that binds
the overlying mucosa to the underlying muscularis
submucosal plexus
(plexus of Meissner) nerve supply that regulates activity of glands and
smooth muscle
The Mouth, Pharynx, and Esophagus
By the end of this section, you will be able to:
e Describe the structures of the mouth, including its three accessory
digestive organs
e Group the 32 adult teeth according to name, location, and function
e Describe the process of swallowing, including the roles of the tongue,
upper esophageal sphincter, and epiglottis
e Trace the pathway food follows from ingestion into the mouth through
release into the stomach
In this section, you will examine the anatomy and functions of the three
main organs of the upper alimentary canal—the mouth, pharynx, and
esophagus—as well as three associated accessory organs—the tongue,
salivary glands, and teeth.
The Mouth
The cheeks, tongue, and palate frame the mouth, which is also called the
oral cavity (or buccal cavity). The structures of the mouth are illustrated in
[link].
At the entrance to the mouth are the lips, or labia (singular = labium). Their
outer covering is skin, which transitions to a mucous membrane in the
mouth proper. Lips are very vascular with a thin layer of keratin; hence, the
reason they are "red." They have a huge representation on the cerebral
cortex, which probably explains the human fascination with kissing! The
lips cover the orbicularis oris muscle, which regulates what comes in and
goes out of the mouth. The labial frenulum is a midline fold of mucous
membrane that attaches the inner surface of each lip to the gum. The cheeks
make up the oral cavity’s sidewalls. While their outer covering is skin, their
inner covering is mucous membrane. This membrane is made up of non-
keratinized, stratified squamous epithelium. Between the skin and mucous
membranes are connective tissue and buccinator muscles. The next time
you eat some food, notice how the buccinator muscles in your cheeks and
the orbicularis oris muscle in your lips contract, helping you keep the food
from falling out of your mouth. Additionally, notice how these muscles
work when you are speaking.
The pocket-like part of the mouth that is framed on the inside by the gums
and teeth, and on the outside by the cheeks and lips is called the oral
vestibule. Moving farther into the mouth, the opening between the oral
cavity and throat (oropharynx) is called the fauces (like the kitchen
faucet"). The main open area of the mouth, or oral cavity proper, runs from
the gums and teeth to the fauces.
When you are chewing, you do not find it difficult to breathe
simultaneously. The next time you have food in your mouth, notice how the
arched shape of the roof of your mouth allows you to handle both digestion
and respiration at the same time. This arch is called the palate. The anterior
region of the palate serves as a wall (or septum) between the oral and nasal
cavities as well as a rigid shelf against which the tongue can push food. It is
created by the maxillary and palatine bones of the skull and, given its bony
structure, is known as the hard palate. If you run your tongue along the roof
of your mouth, you’! notice that the hard palate ends in the posterior oral
cavity, and the tissue becomes fleshier. This part of the palate, known as the
soft palate, is composed mainly of skeletal muscle. You can therefore
manipulate, subconsciously, the soft palate—for instance, to yawn, swallow,
or sing (see [link]).
Mouth
Superior lip
Superior labial frenulum
wr
ages AK, Gingivae (gums)
fg Sy Palatoglossal arch
Fauces
Hard palate a
oe Palatopharyngeal arch
Soft palate ih 42
via & [ -\ <0 4
Palatine tonsil
Cheek ———————— Vv \
Sa ea |
Tongue (underside)
IN
Molars Sa We Lingual frenulum
~ yee (ix Opening duct of
Premolars ey ~~) FE py submandibular gland
y AT)
: Gingivae (gums)
Cuspid (canine) we
Incisors =a ea /
NS es as, Inferior labial frenulum
Oral vestibule
Ne, ee Inferior lip
Anterior view
The mouth includes the lips, tongue, palate, gums, and
teeth.
A fleshy bead of tissue called the uvula drops down from the center of the
posterior edge of the soft palate. Although some have suggested that the
uvula is a vestigial organ, it serves an important purpose. When you
swallow, the soft palate and uvula move upward, helping to keep foods and
liquid from entering the nasal cavity. Unfortunately, it can also contribute to
the sound produced by snoring. Two muscular folds extend downward from
the soft palate, on either side of the uvula. Toward the front, the
palatoglossal arch lies next to the base of the tongue; behind it, the
palatopharyngeal arch forms the superior and lateral margins of the
fauces. Between these two arches are the palatine tonsils, clusters of
lymphoid tissue that protect the pharynx. The lingual tonsils are located at
the base of the tongue.
The Tongue
Perhaps you have heard it said that the tongue is the strongest muscle in the
body. Those who stake this claim cite its strength proportionate to its size.
Although it is difficult to quantify the relative strength of different muscles,
it remains indisputable that the tongue is a workhorse, facilitating ingestion,
mechanical digestion, chemical digestion (lingual lipase), sensation (of
taste, texture, and temperature of food), swallowing, and vocalization.
The tongue is attached to the mandible, the styloid processes of the
temporal bones, and the hyoid bone. The hyoid is unique in that it only
distantly/indirectly articulates with other bones. The tongue is positioned
over the floor of the oral cavity. A medial septum extends the entire length
of the tongue, dividing it into symmetrical halves.
Beneath its mucous membrane covering, each half of the tongue is
composed of the same number and type of intrinsic and extrinsic skeletal
muscles. The intrinsic muscles (those within the tongue) are the
longitudinalis inferior, longitudinalis superior, transversus linguae, and
verticalis linguae muscles. These allow you to change the size and shape of
your tongue, as well as to stick it out, if you wish. Having such a flexible
tongue facilitates both swallowing and speech.
As you learned in your study of the muscular system, the extrinsic muscles
of the tongue are the mylohyoid, hyoglossus, styloglossus, and genioglossus
muscles. These muscles originate outside the tongue and insert into
connective tissues within the tongue. The mylohyoid is responsible for
raising the tongue, the hyoglossus pulls it down and back, the styloglossus
pulls it up and back, and the genioglossus pulls it forward. Working in
concert, these muscles perform three important digestive functions in the
mouth: (1) position food for optimal chewing, (2) gather food into a bolus
(rounded mass), and (3) position food so it can be swallowed.
The top and sides of the tongue are studded with papillae, extensions of
lamina propria of the mucosa, which are covered in stratified squamous
epithelium ({link]). Fungiform papillae, which are mushroom shaped, cover
a large area of the tongue; they tend to be larger toward the rear of the
tongue and smaller on the tip and sides. In contrast, filiform papillae are
long and thin. Fungiform papillae contain taste buds, and filiform papillae
have touch receptors that help the tongue move food around in the mouth.
The filiform papillae create an abrasive surface that performs mechanically,
much like a cat’s rough tongue that is used for grooming. Lingual glands in
the lamina propria of the tongue secrete mucus and a watery serous fluid
that contains the enzyme lingual lipase, which plays a minor role in
breaking down triglycerides but does not begin working until it is activated
in the stomach. A fold of mucous membrane on the underside of the tongue,
the lingual frenulum, tethers the tongue to the floor of the mouth. People
with the congenital anomaly ankyloglossia, also known by the non-medical
term “tongue tie,” have a lingual frenulum that is too short or otherwise
malformed. Severe ankyloglossia can impair speech and must be corrected
with surgery.
Tongue
Epiglottis
Palatopharyngeal
arch
Palatine tonsil
Lingual tonsil
Palatoglossal
arch
Cg
<9
©
Terminal sulcus
Foliate papillae
Circumvallate
papilla
Dorsum of tongue
Fungiform papilla
Filiform papilla
This superior view of the tongue
shows the locations and types of
lingual papillae.
The Salivary Glands
Many small salivary glands are housed within the mucous membranes of
the mouth and tongue. These minor exocrine glands are constantly secreting
saliva, either directly into the oral cavity or indirectly through ducts, even
while you sleep. In fact, an average of 1 to 1.5 liters of saliva is secreted
each day. Usually just enough saliva is present to moisten the mouth and
teeth. Secretion increases when you eat, because saliva is essential to
moisten food and initiate the chemical breakdown of carbohydrates. Small
amounts of saliva are also secreted by the labial glands in the lips. In
addition, the buccal glands in the cheeks, palatal glands in the palate, and
lingual glands in the tongue help ensure that all areas of the mouth are
supplied with adequate saliva.
The Major Salivary Glands
Outside the oral mucosa are three pairs of major salivary glands, which
secrete the majority of saliva into ducts that open into the mouth:
e The submandibular glands, which are in the floor of the mouth,
secrete saliva into the mouth through the submandibular ducts.
e The sublingual glands, which lie below the tongue, use the lesser
sublingual ducts to secrete saliva into the oral cavity.
e The parotid glands lie between the skin and the masseter muscle, near
the ears. They secrete saliva into the mouth through the parotid duct,
which is located near the second upper molar tooth ((Link]).
Saliva
Saliva is essentially (98 to 99.5 percent) water. The remaining 4.5 percent is
a complex mixture of ions, glycoproteins, enzymes, growth factors, and
waste products. Perhaps the most important ingredient in saliva from the
perspective of digestion is the enzyme salivary amylase, which initiates the
breakdown of carbohydrates. Food does not spend enough time in the
mouth to allow all the carbohydrates to break down, but salivary amylase
continues acting until it is inactivated by stomach acids. Bicarbonate and
phosphate ions function as chemical buffers, maintaining saliva at a pH
between 6.35 and 6.85. Salivary mucus helps lubricate food, facilitating
movement in the mouth, bolus formation, and swallowing. Saliva contains
immunoglobulin A, which prevents microbes from penetrating the
epithelium, and lysozyme, which makes saliva antimicrobial. Saliva also
contains epidermal growth factor, which might have given rise to the adage
“a mother’s kiss can heal a wound.”
Each of the major salivary glands secretes a unique formulation of saliva
according to its cellular makeup. For example, the parotid glands secrete a
watery solution that contains salivary amylase. The submandibular glands
have cells similar to those of the parotid glands, as well as mucus-secreting
cells. Therefore, saliva secreted by the submandibular glands also contains
amylase but in a liquid thickened with mucus. The sublingual glands
contain mostly mucous cells, and they secrete the thickest saliva with the
least amount of salivary amylase.
Salivary glands
Parotid salivary gland
Parotid duct
Sublingual
ducts
Sublingual salivary gland
Submandibular salivary gland
Submandibular duct
The major salivary glands are
located outside the oral mucosa
and deliver saliva into the
mouth through ducts.
Note:
Homeostatic Imbalances
The Parotid Glands: Mumps
Infections of the nasal passages and pharynx can attack any salivary gland.
The parotid glands are the usual site of infection with the virus that causes
mumps (paramyxovirus). Mumps manifests by enlargement and
inflammation of the parotid glands, causing a characteristic swelling
between the ears and the jaw. Symptoms include fever and throat pain,
which can be severe when swallowing acidic substances such as orange
juice.
In about one-third of men who are past puberty, mumps also causes
testicular inflammation, typically affecting only one testis and rarely
resulting in sterility. With the increasing use and effectiveness of mumps
vaccines, the incidence of mumps has decreased dramatically. According to
the U.S. Centers for Disease Control and Prevention (CDC), the number of
mumps cases dropped from more than 150,000 in 1968 to fewer than 1700
in 1993 to only 11 reported cases in 2011.
Regulation of Salivation
The autonomic nervous system regulates salivation (the secretion of
Saliva). In the absence of food, parasympathetic stimulation keeps saliva
flowing at just the right level for comfort as you speak, swallow, sleep, and
generally go about life. Over-salivation can occur, for example, if you are
stimulated by the smell of food, but that food is not available for you to eat.
Drooling is an extreme instance of the overproduction of saliva. During
times of stress, such as before speaking in public, sympathetic stimulation
takes over, reducing salivation and producing the symptom of dry mouth
often associated with anxiety. When you are dehydrated, salivation is
reduced, causing the mouth to feel dry and prompting you to take action to
quench your thirst.
Salivation can be stimulated by the sight, smell, and taste of food. It can
even be stimulated by thinking about food. You might notice whether
reading about food and salivation right now has had any effect on your
production of saliva.
How does the salivation process work while you are eating? Food contains
chemicals that stimulate taste receptors on the tongue, which send impulses
to the superior and inferior salivatory nuclei in the brain stem. These two
nuclei then send back parasympathetic impulses through fibers in the
glossopharyngeal and facial nerves, which stimulate salivation. Even after
you swallow food, salivation is increased to cleanse the mouth and to water
down and neutralize any irritating chemical remnants, such as that hot sauce
in your burrito. Most saliva is swallowed along with food and is reabsorbed,
so that fluid is not lost.
The Teeth
The teeth, or dentes (singular = dens), are organs similar to bones that you
use to tear, grind, and otherwise mechanically break down food.
Types of Teeth
During the course of your lifetime, you have two sets of teeth (one set of
teeth is a dentition). Your 20 deciduous teeth, or baby teeth, first begin to
appear at about 6 months of age. Between approximately age 6 and 12,
these teeth are replaced by 32 permanent teeth. Moving from the center of
the mouth toward the side, these are as follows ([link]):
e The eight incisors, four top and four bottom, are the sharp front teeth
you use for biting into food.
e The four cuspids (or canines) flank the incisors and have a pointed
edge (cusp) to tear up food. These fang-like teeth are superb for
piercing tough or fleshy foods.
e Posterior to the cuspids are the eight premolars (or bicuspids), which
have an overall flatter shape with two rounded cusps useful for
mashing foods.
e The most posterior and largest are the 12 molars, which have several
pointed cusps used to crush food so it is ready for swallowing. The
third members of each set of three molars, top and bottom, are
commonly referred to as the wisdom teeth, because their eruption is
commonly delayed until early adulthood. It is not uncommon for
wisdom teeth to fail to erupt; that is, they remain impacted. In these
cases, the teeth are typically removed by orthodontic surgery.
Permanent and Deciduous Teeth
Central incisor (7-8 yr)
Lateral incisor (8-9 yr)
Cuspid or canine (11-12 yr)
First premolar or
bicuspid (9-10 yr)
Second premolar or
bicuspid (10—12 yr)
First molar (6-7 yr)
Second molar
(12-13 yr)
Third molar or
Central incisor wisdom tooth
(8-12 mo)
Lateral incisor (12-24 mo)
Cuspid or canine
(16-24 mo)
First molar (12-16 mo)
Second molar (24-32 mo)
Second molar (24-32 mo)
First molar (12-16 mo)
Cuspid or canine
(16-24 mo)
Lateral incisor (12-15 mo)
Central incisor
(6-8 mo) Third molar or
wisdom tooth
Second molar
(11-13 yr)
First molar (6-7 yr)
Second premolar or
bicuspid (11-12 yr)
First premolar or
bicuspid (9-10 yr)
Cuspid or canine (9-10 yr)
Lateral incisor (7-8 yr)
Central incisor (7-8 yr)
This figure of two human dentitions
shows the arrangement of teeth in the
maxilla and mandible, and the
relationship between the deciduous
and permanent teeth.
Anatomy of a Tooth
The teeth are secured in the alveolar processes (sockets) of the maxilla and
the mandible. Gingivae (commonly called the gums) are soft tissues that
line the alveolar processes and surround the necks of the teeth. Teeth are
also held in their sockets by a connective tissue called the periodontal
ligament.
The two main parts of a tooth are the crown, which is the portion projecting
above the gum line, and the root, which is embedded within the maxilla and
mandible. Both parts contain an inner pulp cavity, containing loose
connective tissue through which run nerves and blood vessels. The region
of the pulp cavity that runs through the root of the tooth is called the root
canal. Surrounding the pulp cavity is dentin, a bone-like tissue. In the root
of each tooth, the dentin is covered by an even harder bone-like layer called
cementum. In the crown of each tooth, the dentin is covered by an outer
layer of enamel, the hardest substance in the body ([link]).
Although enamel protects the underlying dentin and pulp cavity, it is still
nonetheless susceptible to mechanical and chemical erosion, or what is
known as tooth decay. The most common form, dental caries (cavities)
develops when colonies of bacteria feeding on sugars in the mouth release
acids that cause soft tissue inflammation and degradation of the calcium
crystals of the enamel. The digestive functions of the mouth are
summarized in [link].
The Structure of the Tooth
Enamel
crown Dentin
—— Gingiva
Neck _ \ (gum)
\
\ ‘
=m Pulp cavity
~; & \ (contains
| |g blood vessels
TD tt V&,
Lf ') 7S and nerves)
16 i a | (0
Root (| ad l ime. Periodontal
NL) i » ee ligament
\\ “ Lt] f :
HO {| 771 Root canal
|G, / > Ae
| V4 I . \
WM! SS
Lp ey 26 7 — Bone
This longitudinal section through a
molar in its alveolar socket shows the
relationships between enamel, dentin,
Digestive Functions of the Mouth
Structure
Lips and
cheeks
Salivary
glands
Tongue’s
extrinsic
muscles
and pulp.
Action
Confine
food
between
teeth
Secrete
saliva
Move
tongue
sideways,
and in
and out
Outcome
Food is chewed evenly during
mastication
Moisten and lubricate the lining
of the mouth and pharynx
Moisten, soften, and dissolve food
Clean the mouth and teeth
Salivary amylase breaks down
starch
Manipulate food for chewing
Shape food into a bolus
Manipulate food for swallowing
Digestive Functions of the Mouth
Structure Action Outcome
Tongue’s Change
intrinsic tongue Manipulate food for swallowing
muscles shape
Sense Nerve impulses from taste buds
T food in are conducted to salivary nuclei in
aste
mouth the brain stem and then to salivary
buds é
and sense glands, stimulating saliva
taste secretion
; Secrete Activated in the stomach
Lingual ; ; eager
fande lingual Break down triglycerides into
s lipase fatty acids and diglycerides
Sales Break down solid food into
Teeth and crush -
smaller particles for deglutition
food
The Pharynx
The pharynx (throat) is involved in both digestion and respiration. It
receives food and air from the mouth, and air from the nasal cavities. When
food enters the pharynx, involuntary muscle contractions close off the air
passageways.
A short tube of skeletal muscle lined with a mucous membrane, the pharynx
runs from the posterior oral and nasal cavities to the opening of the
esophagus and larynx. It has three subdivisions. The most superior, the
nasopharynx, is involved only in breathing and speech. The other two
subdivisions, the oropharynx and the laryngopharynx, are used for both
breathing and digestion. The oropharynx begins inferior to the nasopharynx
and is continuous below with the laryngopharynx ([link]). The inferior
border of the laryngopharynx connects to the esophagus, whereas the
anterior portion connects to the larynx, allowing air to flow into the
bronchial tree.
Pharynx
Soft palate
Nasopharynx
Hard palate
Uvula
Oropharynx
Epiglottis
Glottis \ Laryngopharynx
Larynx
Trachea
—~
Esophagus
Es Nasal cavity
| Oral cavity S) —/
= Pharynx
aa Larynx a ae —
The pharynx runs from the nostrils to the
esophagus and the larynx.
Histologically, the wall of the oropharynx is similar to that of the oral
cavity. The mucosa includes a stratified squamous epithelium that is
endowed with mucus-producing glands. During swallowing, the elevator
skeletal muscles of the pharynx contract, raising and expanding the pharynx
to receive the bolus of food. Once received, these muscles relax and the
constrictor muscles of the pharynx contract, forcing the bolus into the
esophagus and initiating peristalsis.
Usually during swallowing, the soft palate and uvula rise reflexively to
close off the entrance to the nasopharynx. At the same time, the larynx is
pulled superiorly and the cartilaginous epiglottis, its most superior structure,
folds inferiorly, covering the glottis (the opening to the larynx); this process
effectively blocks access to the trachea and bronchi. When the food “goes
down the wrong way,” it goes into the trachea. When food enters the
trachea, the reaction is to cough, which usually forces the food up and out
of the trachea, and back into the pharynx.
The Esophagus
The esophagus is a muscular tube that connects the pharynx to the stomach.
It is approximately 25.4 cm (10 in) in length, located posterior to the
trachea, and remains in a collapsed form when not engaged in swallowing.
As you can see in [link], the esophagus runs a mainly straight route through
the mediastinum of the thorax. To enter the abdomen, the esophagus
penetrates the diaphragm through an opening called the esophageal hiatus.
Passage of Food through the Esophagus
The upper esophageal sphincter, which is continuous with the inferior
pharyngeal constrictor, controls the movement of food from the pharynx
into the esophagus. The upper two-thirds of the esophagus consists of both
smooth and skeletal muscle fibers, with the latter fading out in the bottom
third of the esophagus. Rhythmic waves of peristalsis, which begin in the
upper esophagus, propel the bolus of food toward the stomach. Meanwhile,
secretions from the esophageal mucosa lubricate the esophagus and food.
Food passes from the esophagus into the stomach at the lower esophageal
sphincter (also called the gastroesophageal or cardiac sphincter). Recall
that sphincters are muscles that surround tubes and serve as valves, closing
the tube when the sphincters contract and opening it when they relax. The
lower esophageal sphincter relaxes to let food pass into the stomach, and
then contracts to prevent stomach acids from backing up into the
esophagus. Surrounding this sphincter is the muscular diaphragm, which
helps close off the sphincter when no food is being swallowed. When the
lower esophageal sphincter does not completely close, the stomach’s
contents can reflux (that is, back up into the esophagus), causing heartburn
or gastroesophageal reflux disease (GERD).
Esophagus
Upper esophageal
sphincter
Esophagus Lower esophageal
sphincter
{8)
Stomach SS
TY
The upper esophageal sphincter
controls the movement of food
from the pharynx to the
esophagus. The lower esophageal
sphincter controls the movement
of food from the esophagus to the
stomach.
Histology of the Esophagus
The mucosa of the esophagus is made up of an epithelial lining that
contains non-keratinized, stratified squamous epithelium, with a layer of
basal and parabasal cells. This epithelium protects against erosion from
food particles. The mucosa’s lamina propria contains mucus-secreting
glands. The muscularis layer changes according to location: In the upper
third of the esophagus, the muscularis is skeletal muscle. In the middle
third, it is both skeletal and smooth muscle. In the lower third, it is smooth
muscle. As mentioned previously, the most superficial layer of the
esophagus is called the adventitia, not the serosa. In contrast to the stomach
and intestines, the loose connective tissue of the adventitia is not covered by
a fold of visceral peritoneum. The digestive functions of the esophagus are
identified in [link].
Digestive Functions of the Esophagus
Action Outcome
Upper
esophageal Allows the bolus to move from the
sphincter laryngopharynx to the esophagus
relaxation
Peristalsis Propels the bolus through the esophagus
powel Allows the bolus to move from the esophagus
esophageal
into the stomach and prevents chime from
hincter '
ee entering the esophagus
relaxation
Digestive Functions of the Esophagus
Action Outcome
Mucus Lubricates the esophagus, allowing easy passage
secretion of the bolus
Deglutition
Deglutition is another word for swallowing—the movement of food from
the mouth to the stomach. The entire process takes about 4 to 8 seconds for
solid or semisolid food, and about 1 second for very soft food and liquids.
Although this sounds quick and effortless, deglutition is, in fact, a complex
process that involves both the skeletal muscle of the tongue and the muscles
of the pharynx and esophagus. It is aided by the presence of mucus and
saliva. There are three stages in deglutition: the voluntary phase, the
pharyngeal phase, and the esophageal phase ([link]). The autonomic
nervous system controls the latter two phases.
Deglutition
je = “aS
Y Kg
HSS
bn. m
_— < ¢
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)
Superior
pharyngeal
constrictor
muscle
Medial
pharyngeal
constrictor
muscle
Medial and
inferior
\ i pharyngeal
\@\\ constrictor
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Inferior
pharyngeal
| and esophageal
\\\—— constrictor
muscles
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Deglutition includes the voluntary phase and two
involuntary phases: the pharyngeal phase and the
esophageal phase.
The Voluntary Phase
The voluntary phase of deglutition (also known as the oral or buccal
phase) is so called because you can control when you swallow food. In this
phase, chewing has been completed and swallowing is set in motion. The
tongue moves upward and backward against the palate, pushing the bolus to
the back of the oral cavity and into the oropharynx. Other muscles keep the
mouth closed and prevent food from falling out. At this point, the two
involuntary phases of swallowing begin.
The Pharyngeal Phase
In the pharyngeal phase, stimulation of receptors in the oropharynx sends
impulses to the deglutition center (a collection of neurons that controls
swallowing) in the medulla oblongata. Impulses are then sent back to the
uvula and soft palate, causing them to move upward and close off the
nasopharynx. The laryngeal muscles also constrict to prevent aspiration of
food into the trachea. At this point, deglutition apnea takes place, which
means that breathing ceases for a very brief time. Contractions of the
pharyngeal constrictor muscles move the bolus through the oropharynx and
laryngopharynx. Relaxation of the upper esophageal sphincter then allows
food to enter the esophagus.
The Esophageal Phase
The entry of food into the esophagus marks the beginning of the esophageal
phase of deglutition and the initiation of peristalsis. As in the previous
phase, the complex neuromuscular actions are controlled by the medulla
oblongata. Peristalsis propels the bolus through the esophagus and toward
the stomach. The circular muscle layer of the muscularis contracts, pinching
the esophageal wall and forcing the bolus forward. At the same time, the
longitudinal muscle layer of the muscularis also contracts, shortening this
area and pushing out its walls to receive the bolus. In this way, a series of
contractions keeps moving food toward the stomach. When the bolus nears
the stomach, distention of the esophagus initiates a short reflex relaxation of
the lower esophageal sphincter that allows the bolus to pass into the
stomach. During the esophageal phase, esophageal glands secrete mucus
that lubricates the bolus and minimizes friction.
Note:
I
OR ero
HS Erk ep
wees OPenstax COLLEGE”
one
Watch this_animation to see how swallowing is a complex process that
involves the nervous system to coordinate the actions of upper respiratory
and digestive activities. During which stage of swallowing is there a risk of
food entering respiratory pathways and how is this risk blocked?
Chapter Review
In the mouth, the tongue and the teeth begin mechanical digestion, and
saliva begins chemical digestion. The pharynx, which plays roles in
breathing and vocalization as well as digestion, runs from the nasal and oral
cavities superiorly to the esophagus inferiorly (for digestion) and to the
larynx anteriorly (for respiration). During deglutition (swallowing), the soft
palate rises to close off the nasopharynx, the larynx elevates, and the
epiglottis folds over the glottis. The esophagus includes an upper
esophageal sphincter made of skeletal muscle, which regulates the
movement of food from the pharynx to the esophagus. It also has a lower
esophageal sphincter, made of smooth muscle, which controls the passage
of food from the esophagus to the stomach. Cells in the esophageal wall
secrete mucus that eases the passage of the food bolus.
Interactive Link Questions
Exercise:
Problem:
Watch this animation to see how swallowing is a complex process that
involves the nervous system to coordinate the actions of upper
respiratory and digestive activities. During which stage of swallowing
is there a risk of food entering respiratory pathways and how is this
risk blocked?
Solution:
Answers may vary.
Review Questions
Exercise:
Problem:
Which of these ingredients in saliva is responsible for activating
salivary amylase?
a. mucus
b. phosphate ions
c. chloride ions
d. urea
Solution:
C
Exercise:
Problem: Which of these statements about the pharynx is true?
a. It extends from the nasal and oral cavities superiorly to the
esophagus anteriorly.
b. The oropharynx is continuous superiorly with the nasopharynx.
c. The nasopharynx is involved in digestion.
d. The laryngopharynx is composed partially of cartilage.
Solution:
B
Exercise:
Problem:
Which structure is located where the esophagus penetrates the
diaphragm?
a. esophageal hiatus
b. cardiac orifice
c. upper esophageal sphincter
d. lower esophageal sphincter
Solution:
A
Exercise:
Problem:
Which phase of deglutition involves contraction of the longitudinal
muscle layer of the muscularis?
a. voluntary phase
b. buccal phase
c. pharyngeal phase
d. esophageal phase
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
The composition of saliva varies from gland to gland. Discuss how
saliva produced by the parotid gland differs in action from saliva
produced by the sublingual gland.
Solution:
Parotid gland saliva is watery with little mucus but a lot of amylase,
which allows it to mix freely with food during mastication and begin
the digestion of carbohydrates. In contrast, sublingual gland saliva has
a lot of mucus with the least amount of amylase of all the salivary
glands. The high mucus content serves to lubricate the food for
swallowing.
Exercise:
Problem:
During a hockey game, the puck hits a player in the mouth, knocking
out all eight of his most anterior teeth. Which teeth did the player lose
and how does this loss affect food ingestion?
Solution:
The incisors. Since these teeth are used for tearing off pieces of food
during ingestion, the player will need to ingest foods that have already
been cut into bite-sized pieces until the broken teeth are replaced.
Exercise:
Problem: What prevents swallowed food from entering the airways?
Solution:
Usually when food is swallowed, involuntary muscle contractions
cause the soft palate to rise and close off the nasopharynx. The larynx
also is pulled up, and the epiglottis folds over the glottis. These actions
block off the air passages.
Exercise:
Problem:
Explain the mechanism responsible for gastroesophageal reflux.
Solution:
If the lower esophageal sphincter does not close completely, the
stomach’s acidic contents can back up into the esophagus, a
phenomenon known as GERD.
Exercise:
Problem:
Describe the three processes involved in the esophageal phase of
deglutition.
Solution:
Peristalsis moves the bolus down the esophagus and toward the
stomach. Esophageal glands secrete mucus that lubricates the bolus
and reduces friction. When the bolus nears the stomach, the lower
esophageal sphincter relaxes, allowing the bolus to pass into the
stomach.
References
van Loon FPL, Holmes SJ, Sirotkin B, Williams W, Cochi S, Hadler S,
Lindegren ML. Morbidity and Mortality Weekly Report: Mumps
surveillance -- United States, 1988—1993 [Internet]. Atlanta, GA: Center for
Disease Control; [cited 2013 Apr 3]. Available from:
http://www.cdc.gov/mmwr/preview/mmwrhtml/00038546.htm.
Glossary
bolus
mass of chewed food
cementum
bone-like tissue covering the root of a tooth
crown
portion of tooth visible superior to the gum line
cuspid
(also, canine) pointed tooth used for tearing and shredding food
deciduous tooth
one of 20 “baby teeth”
deglutition
three-stage process of swallowing
dens
tooth
dentin
bone-like tissue immediately deep to the enamel of the crown or
cementum of the root of a tooth
dentition
set of teeth
enamel
covering of the dentin of the crown of a tooth
esophagus
muscular tube that runs from the pharynx to the stomach
fauces
opening between the oral cavity and the oropharynx
gingiva
gum
incisor
midline, chisel-shaped tooth used for cutting into food
labium
lip
labial frenulum
midline mucous membrane fold that attaches the inner surface of the
lips to the gums
laryngopharynx
part of the pharynx that functions in respiration and digestion
lingual frenulum
mucous membrane fold that attaches the bottom of the tongue to the
floor of the mouth
lingual lipase
digestive enzyme from glands in the tongue that acts on triglycerides
lower esophageal sphincter
smooth muscle sphincter that regulates food movement from the
esophagus to the stomach
molar
tooth used for crushing and grinding food
oral cavity
(also, buccal cavity) mouth
oral vestibule
part of the mouth bounded externally by the cheeks and lips, and
internally by the gums and teeth
oropharynx
part of the pharynx continuous with the oral cavity that functions in
respiration and digestion
palatoglossal arch
muscular fold that extends from the lateral side of the soft palate to the
base of the tongue
palatopharyngeal arch
muscular fold that extends from the lateral side of the soft palate to the
side of the pharynx
parotid gland
one of a pair of major salivary glands located inferior and anterior to
the ears
permanent tooth
one of 32 adult teeth
pharynx
throat
premolar
(also, bicuspid) transitional tooth used for mastication, crushing, and
grinding food
pulp cavity
deepest portion of a tooth, containing nerve endings and blood vessels
root
portion of a tooth embedded in the alveolar processes beneath the gum
line
saliva
aqueous solution of proteins and ions secreted into the mouth by the
salivary glands
salivary amylase
digestive enzyme in saliva that acts on starch
salivary gland
an exocrine gland that secretes a digestive fluid called saliva
salivation
secretion of saliva
soft palate
posterior region of the bottom portion of the nasal cavity that consists
of skeletal muscle
sublingual gland
one of a pair of major salivary glands located beneath the tongue
submandibular gland
one of a pair of major salivary glands located in the floor of the mouth
tongue
accessory digestive organ of the mouth, the bulk of which is composed
of skeletal muscle
upper esophageal sphincter
skeletal muscle sphincter that regulates food movement from the
pharynx to the esophagus
voluntary phase
initial phase of deglutition, in which the bolus moves from the mouth
to the oropharynx
The Stomach
By the end of this section, you will be able to:
e Label on a diagram the four main regions of the stomach, its curvatures, and its
sphincter
e Identify the four main types of secreting cells in gastric glands, and their important
products
e Explain why the stomach does not digest itself
¢ Describe the mechanical and chemical digestion of food entering the stomach
Although a minimal amount of carbohydrate digestion occurs in the mouth, chemical
digestion really gets underway in the stomach. An expansion of the alimentary canal that
lies immediately inferior to the esophagus, the stomach links the esophagus to the first
part of the small intestine (the duodenum) and is relatively fixed in place at its esophageal
and duodenal ends. In between, however, it can be a highly active structure, contracting
and continually changing position and size. These contractions provide mechanical
assistance to digestion. The empty stomach is only about the size of your fist, but can
stretch to hold as much as 4 liters of food and fluid, or more than 75 times its empty
volume, and then return to its resting size when empty. Although you might think that the
size of a person’s stomach is related to how much food that individual consumes, body
weight does not correlate with stomach size. Rather, when you eat greater quantities of
food—such as at holiday dinner—you stretch the stomach more than when you eat less.
Popular culture tends to refer to the stomach as the location where all digestion takes
place. Of course, this is not true. An important function of the stomach is to serve as a
temporary holding chamber. You can ingest a meal far more quickly than it can be
digested and absorbed by the small intestine. Thus, the stomach holds food and parses
only small amounts into the small intestine at a time. Foods are not processed in the order
they are eaten; rather, they are mixed together with digestive juices in the stomach until
they are converted into chyme, which is released into the small intestine.
As you will see in the sections that follow, the stomach plays several important roles in
chemical digestion, including the continued digestion of carbohydrates and the initial
digestion of proteins and triglycerides. Little if any nutrient absorption occurs in the
stomach, with the exception of the negligible amount of nutrients in alcohol.
Structure
There are four main regions in the stomach: the cardia, fundus, body, and pylorus
([link]). The cardia (or cardiac region) is the point where the esophagus connects to the
stomach and through which food passes into the stomach. Located inferior to the
diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the
fundus is the body, the main part of the stomach. The funnel-shaped pylorus connects
the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects
to the body of the stomach. The narrower end is called the pyloric canal, which connects
to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of
connection and controls stomach emptying. In the absence of food, the stomach deflates
inward, and its mucosa and submucosa fall into a large fold called a ruga.
Stomach
Cardia
Esophagus
Muscularis externa:
Longitudinal layer
Circular layer
Oblique layer
Fundus
Serosa
Lesser curvature Body
Pyloric sphincter
(valve) at pylorus Lumen
Rugae of mucosa
Duodenum F ~~
Pyloric canal
Pyloric antrum Greater curvature
The stomach has four major regions: the cardia, fundus,
body, and pylorus. The addition of an inner oblique smooth
muscle layer gives the muscularis the ability to vigorously
churn and mix food.
The convex lateral surface of the stomach is called the greater curvature; the concave
medial border is the lesser curvature. The stomach is held in place by the lesser omentum,
which extends from the liver to the lesser curvature, and the greater omentum, which runs
from the greater curvature to the posterior abdominal wall.
Histology
The wall of the stomach is made of the same four layers as most of the rest of the
alimentary canal, but with adaptations to the mucosa and muscularis for the unique
functions of this organ. In addition to the typical circular and longitudinal smooth muscle
layers, the muscularis has an inner oblique smooth muscle layer ({link]). As a result, in
addition to moving food through the canal, the stomach can vigorously churn food,
mechanically breaking it down into smaller particles.
Histology of the Stomach
Parietal cell
Surface
epithelium
Gastric pit
Gastric gland
Lamina
propria
Chief cell
Muscularis
mucosae
Submucosa ———_—— aS >
Oblique layer .
Muscularis Circular layer —9 : » | Enteroendocrine
externa Longitudinal SS / cell
layer ~ y
Serosa
The stomach wall is adapted for the functions of the
stomach. In the epithelium, gastric pits lead to gastric
glands that secrete gastric juice. The gastric glands (one
gland is shown enlarged on the right) contain different
types of cells that secrete a variety of enzymes, including
hydrochloride acid, which activates the protein-digesting
enzyme pepsin.
The stomach mucosa’s epithelial lining consists only of surface mucus cells, which
secrete a protective coat of alkaline mucus. A vast number of gastric pits dot the surface
of the epithelium, giving it the appearance of a well-used pincushion, and mark the entry
to each gastric gland, which secretes a complex digestive fluid referred to as gastric
juice.
Although the walls of the gastric pits are made up primarily of mucus cells, the gastric
glands are made up of different types of cells. The glands of the cardia and pylorus are
composed primarily of mucus-secreting cells. Cells that make up the pyloric antrum
secrete mucus and a number of hormones, including the majority of the stimulatory
hormone, gastrin. The much larger glands of the fundus and body of the stomach, the site
of most chemical digestion, produce most of the gastric secretions. These glands are
made up of a variety of secretory cells. These include parietal cells, chief cells, mucous
neck cells, and enteroendocrine cells.
Parietal cells—Located primarily in the middle region of the gastric glands are parietal
cells, which are among the most highly differentiated of the body’s epithelial cells. These
relatively large cells produce both hydrochloric acid (HCI) and intrinsic factor. HCl is
responsible for the high acidity (pH 1.5 to 3.5) of the stomach contents and is needed to
activate the protein-digesting enzyme, pepsin. The acidity also kills much of the bacteria
you ingest with food and helps to denature proteins, making them more available for
enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of
vitamin Bj, in the small intestine.
Chief cells—Located primarily in the basal regions of gastric glands are chief cells,
which secrete pepsinogen, the inactive proenzyme form of pepsin. HCl is necessary for
the conversion of pepsinogen to pepsin.
Mucous neck cells—Gastric glands in the upper part of the stomach contain mucous
neck cells that secrete thin, acidic mucus that is much different from the mucus secreted
by the goblet cells of the surface epithelium. The role of this mucus is not currently
known.
Enteroendocrine cells—Finally, enteroendocrine cells found in the gastric glands secrete
various hormones into the interstitial fluid of the lamina propria. These include gastrin,
which is released mainly by enteroendocrine G cells.
[link] describes the digestive functions of important hormones secreted by the stomach.
Note:
Op eC
=> openstax COLLEGE
arene:
Watch this animation that depicts the structure of the stomach and how this structure
functions in the initiation of protein digestion. This view of the stomach shows the
characteristic rugae. What is the function of these rugae?
Hormones Secreted by the Stomach
Production Production
Hormone site stimulus Target organ Action
Hormones Secreted by the Stomach
Hormone
Gastrin
Gastrin
Gastrin
Gastrin
Ghrelin
Production
site
Stomach
mucosa,
mainly G
cells of the
pyloric
antrum
Stomach
mucosa,
mainly G
cells of the
pyloric
antrum
Stomach
mucosa,
mainly G
cells of the
pyloric
antrum
Stomach
mucosa,
mainly G
cells of the
pyloric
antrum
Stomach
mucosa,
mainly
fundus
Production
stimulus
Presence of
peptides
and amino
acids in
stomach
Presence of
peptides
and amino
acids in
stomach
Presence of
peptides
and amino
acids in
stomach
Presence of
peptides
and amino
acids in
stomach
Fasting
state (levels
increase
just prior to
meals)
Target organ
Stomach
Small
intestine
Ileocecal
valve
Large
intestine
Hypothalamus
Action
Increases
secretion
by gastric
glands;
promotes
gastric
emptying
Promotes
intestinal
muscle
contraction
Relaxes
valve
Triggers
mass
movements
Regulates
food
intake,
primarily
by
stimulating
hunger and
satiety
Hormones Secreted by the Stomach
Hormone
Histamine
Serotonin
Somatostatin
Somatostatin
Somatostatin
Gastric Secretion
Production
site
Stomach
mucosa
Stomach
mucosa
Mucosa of
stomach,
especially
pyloric
antrum;
also
duodenum
Mucosa of
stomach,
especially
pyloric
antrum;
also
duodenum
Mucosa of
stomach,
especially
pyloric
antrum;
also
duodenum
Production
stimulus
Presence of
food in the
stomach
Presence of
food in the
stomach
Presence of
food in the
stomach;
sympathetic
axon
stimulation
Presence of
food in the
stomach;
sympathetic
axon
stimulation
Presence of
food in the
stomach;
sympathetic
axon
stimulation
Target organ
Stomach
Stomach
Stomach
Pancreas
Small
intestine
Action
Stimulates
parietal
cells to
release
HCl
Contracts
stomach
muscle
Restricts
all gastric
secretions,
gastric
motility,
and
emptying
Restricts
pancreatic
secretions
Reduces
intestinal
absorption
by
reducing
blood flow
The secretion of gastric juice is controlled by both nerves and hormones. Stimuli in the
brain, stomach, and small intestine activate or inhibit gastric juice production. This is
why the three phases of gastric secretion are called the cephalic, gastric, and intestinal
phases ([link]). However, once gastric secretion begins, all three phases can occur
simultaneously.
The Three Phases of Gastric Secretion
Stimulates stomach
secretory activity
CEPHALIC PHASE
Inhibits stomach
secretory activity
Stimulates stomach
secretory activity
GASTRIC PHASE
Inhibits stomach
secretory activity
Stimulates stomach
secretory activity
INTESTINAL PHASE
Inhibits stomach
secretory activity
Gastric secretion occurs in three phases: cephalic,
gastric, and intestinal. During each phase, the
secretion of gastric juice can be stimulated or
inhibited.
The cephalic phase (reflex phase) of gastric secretion, which is relatively brief, takes
place before food enters the stomach. The smell, taste, sight, or thought of food triggers
this phase. For example, when you bring a piece of sushi to your lips, impulses from
receptors in your taste buds or the nose are relayed to your brain, which returns signals
that increase gastric secretion to prepare your stomach for digestion. This enhanced
secretion is a conditioned reflex, meaning it occurs only if you like or want a particular
food. Depression and loss of appetite can suppress the cephalic reflex.
The gastric phase of secretion lasts 3 to 4 hours, and is set in motion by local neural and
hormonal mechanisms triggered by the entry of food into the stomach. For example,
when your sushi reaches the stomach, it creates distention that activates the stretch
receptors. This stimulates parasympathetic neurons to release acetylcholine, which then
provokes increased secretion of gastric juice. Partially digested proteins, caffeine, and
rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn
induces parietal cells to increase their production of HCl, which is needed to create an
acidic environment for the conversion of pepsinogen to pepsin, and protein digestion.
Additionally, the release of gastrin activates vigorous smooth muscle contractions.
However, it should be noted that the stomach does have a natural means of avoiding
excessive acid secretion and potential heartburn. Whenever pH levels drop too low, cells
in the stomach react by suspending HCI secretion and increasing mucous secretions.
The intestinal phase of gastric secretion has both excitatory and inhibitory elements. The
duodenum has a major role in regulating the stomach and its emptying. When partially
digested food fills the duodenum, intestinal mucosal cells release a hormone called
intestinal (enteric) gastrin, which further excites gastric juice secretion. This stimulatory
activity is brief, however, because when the intestine distends with chyme, the
enterogastric reflex inhibits secretion. One of the effects of this reflex is to close the
pyloric sphincter, which blocks additional chyme from entering the duodenum.
The Mucosal Barrier
The mucosa of the stomach is exposed to the highly corrosive acidity of gastric juice.
Gastric enzymes that can digest protein can also digest the stomach itself. The stomach is
protected from self-digestion by the mucosal barrier. This barrier has several
components. First, the stomach wall is covered by a thick coating of bicarbonate-rich
mucus. This mucus forms a physical barrier, and its bicarbonate ions neutralize acid.
Second, the epithelial cells of the stomach's mucosa meet at tight junctions, which block
gastric juice from penetrating the underlying tissue layers. Finally, stem cells located
where gastric glands join the gastric pits quickly replace damaged epithelial mucosal
cells, when the epithelial cells are shed. In fact, the surface epithelium of the stomach is
completely replaced every 3 to 6 days.
Note:
Homeostatic Imbalances
Ulcers: When the Mucosal Barrier Breaks Down
As effective as the mucosal barrier is, it is not a “fail-safe” mechanism. Sometimes,
gastric juice eats away at the superficial lining of the stomach mucosa, creating erosions,
which mostly heal on their own. Deeper and larger erosions are called ulcers.
Why does the mucosal barrier break down? A number of factors can interfere with its
ability to protect the stomach lining. The majority of all ulcers are caused by either
excessive intake of non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin,
or Helicobacter pylori infection.
Antacids help relieve symptoms of ulcers such as “burning” pain and indigestion. When
ulcers are caused by NSAID use, switching to other classes of pain relievers allows
healing. When caused by H. pylori infection, antibiotics are effective.
A potential complication of ulcers is perforation: Perforated ulcers create a hole in the
stomach wall, resulting in peritonitis (inflammation of the peritoneum). These ulcers
must be repaired surgically.
Digestive Functions of the Stomach
The stomach participates in virtually all the digestive activities with the exception of
ingestion and defecation. Although almost all absorption takes place in the small
intestine, the stomach does absorb some nonpolar substances, such as alcohol and aspirin.
Mechanical Digestion
Within a few moments after food after enters your stomach, mixing waves begin to occur
at intervals of approximately 20 seconds. A mixing wave is a unique type of peristalsis
that mixes and softens the food with gastric juices to create chyme. The initial mixing
waves are relatively gentle, but these are followed by more intense waves, starting at the
body of the stomach and increasing in force as they reach the pylorus. It is fair to say that
long before your sushi exits through the pyloric sphincter, it bears little resemblance to
the sushi you ate.
The pylorus, which holds around 30 mL (1 fluid ounce) of chyme, acts as a filter,
permitting only liquids and small food particles to pass through the mostly, but not fully,
closed pyloric sphincter. In a process called gastric emptying, rhythmic mixing waves
force about 3 mL of chyme at a time through the pyloric sphincter and into the
duodenum. Release of a greater amount of chyme at one time would overwhelm the
capacity of the small intestine to handle it. The rest of the chyme is pushed back into the
body of the stomach, where it continues mixing. This process is repeated when the next
mixing waves force more chyme into the duodenum.
Gastric emptying is regulated by both the stomach and the duodenum. The presence of
chyme in the duodenum activates receptors that inhibit gastric secretion. This prevents
additional chyme from being released by the stomach before the duodenum is ready to
process it.
Chemical Digestion
The fundus plays an important role, because it stores both undigested food and gases that
are released during the process of chemical digestion. Food may sit in the fundus of the
stomach for a while before being mixed with the chyme. While the food is in the fundus,
the digestive activities of salivary amylase continue until the food begins mixing with the
acidic chyme. Ultimately, mixing waves incorporate this food with the chyme, the acidity
of which inactivates salivary amylase and activates lingual lipase. Lingual lipase then
begins breaking down triglycerides into free fatty acids, and mono- and diglycerides.
The breakdown of protein begins in the stomach through the actions of HCl and the
enzyme pepsin. During infancy, gastric glands also produce rennin, an enzyme that helps
digest milk protein.
Its numerous digestive functions notwithstanding, there is only one stomach function
necessary to life: the production of intrinsic factor. The intestinal absorption of vitamin
By», which is necessary for both the production of mature red blood cells and normal
neurological functioning, cannot occur without intrinsic factor. People who undergo total
gastrectomy (stomach removal)—for life-threatening stomach cancer, for example—can
survive with minimal digestive dysfunction if they receive vitamin B,> injections.
The contents of the stomach are completely emptied into the duodenum within 2 to 4
hours after you eat a meal. Different types of food take different amounts of time to
process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods.
Meals with a high triglyceride content remain in the stomach the longest. Since enzymes
in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer
when the duodenum is processing fatty chyme. However, note that this is still a fraction
of the 24 to 72 hours that full digestion typically takes from start to finish.
Chapter Review
The stomach participates in all digestive activities except ingestion and defecation. It
vigorously churns food. It secretes gastric juices that break down food and absorbs
certain drugs, including aspirin and some alcohol. The stomach begins the digestion of
protein and continues the digestion of carbohydrates and fats. It stores food as an acidic
liquid called chyme, and releases it gradually into the small intestine through the pyloric
sphincter.
Interactive Link Questions
Exercise:
Problem:
Watch this animation that depicts the structure of the stomach and how this structure
functions in the initiation of protein digestion. This view of the stomach shows the
characteristic rugae. What is the function of these rugae?
Solution:
Answers may vary.
Review Questions
Exercise:
Problem: Which of these cells secrete hormones?
a. parietal cells
b. mucous neck cells
c. enteroendocrine cells
d. chief cells
Solution:
C
Exercise:
Problem:Where does the majority of chemical digestion in the stomach occur?
a. fundus and body
b. cardia and fundus
c. body and pylorus
d. body
Solution:
A
Exercise:
Problem:
During gastric emptying, chyme is released into the duodenum through the
a. esophageal hiatus
b. pyloric antrum
c. pyloric canal
d. pyloric sphincter
Solution:
D
Exercise:
Problem: Parietal cells secrete
a. gastrin
b. hydrochloric acid
c. pepsin
d. pepsinogen
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Explain how the stomach is protected from self-digestion and why this is necessary.
Solution:
The mucosal barrier protects the stomach from self-digestion. It includes a thick
coating of bicarbonate-rich mucus; the mucus is physically protective, and
bicarbonate neutralizes gastric acid. Epithelial cells meet at tight junctions, which
block gastric juice from penetrating the underlying tissue layers, and stem cells
quickly replace sloughed off epithelial mucosal cells.
Exercise:
Problem:
Describe unique anatomical features that enable the stomach to perform digestive
functions.
Solution:
The stomach has an additional inner oblique smooth muscle layer that helps the
muscularis churn and mix food. The epithelium includes gastric glands that secrete
gastric fluid. The gastric fluid consists mainly of mucous, HCl, and the enzyme
pepsin released as pepsinogen.
Glossary
body
mid-portion of the stomach
cardia
(also, cardiac region) part of the stomach surrounding the cardiac orifice (esophageal
hiatus)
cephalic phase
(also, reflex phase) initial phase of gastric secretion that occurs before food enters
the stomach
chief cell
gastric gland cell that secretes pepsinogen
enteroendocrine cell
gastric gland cell that releases hormones
fundus
dome-shaped region of the stomach above and to the left of the cardia
G cell
gastrin-secreting enteroendocrine cell
gastric emptying
process by which mixing waves gradually cause the release of chyme into the
duodenum
gastric gland
gland in the stomach mucosal epithelium that produces gastric juice
gastric phase
phase of gastric secretion that begins when food enters the stomach
gastric pit
narrow channel formed by the epithelial lining of the stomach mucosa
gastrin
peptide hormone that stimulates secretion of hydrochloric acid and gut motility
hydrochloric acid (HCl)
digestive acid secreted by parietal cells in the stomach
intrinsic factor
glycoprotein required for vitamin B,> absorption in the small intestine
intestinal phase
phase of gastric secretion that begins when chyme enters the intestine
mixing wave
unique type of peristalsis that occurs in the stomach
mucosal barrier
protective barrier that prevents gastric juice from destroying the stomach itself
mucous neck cell
gastric gland cell that secretes a uniquely acidic mucus
parietal cell
gastric gland cell that secretes hydrochloric acid and intrinsic factor
pepsinogen
inactive form of pepsin
pyloric antrum
wider, more superior part of the pylorus
pyloric canal
narrow, more inferior part of the pylorus
pyloric sphincter
sphincter that controls stomach emptying
pylorus
lower, funnel-shaped part of the stomach that is continuous with the duodenum
ruga
fold of alimentary canal mucosa and submucosa in the empty stomach and other
organs
stomach
alimentary canal organ that contributes to chemical and mechanical digestion of
food from the esophagus before releasing it, as chyme, to the small intestine
The Small and Large Intestines
By the end of this section, you will be able to:
¢ Compare and contrast the location and gross anatomy of the small and
large intestines
e Identify three main adaptations of the small intestine wall that increase
its absorptive capacity
e Describe the mechanical and chemical digestion of chyme upon its
release into the small intestine
e List three features unique to the wall of the large intestine and identify
their contributions to its function
e Identify the beneficial roles of the bacterial flora in digestive system
functioning
e Trace the pathway of food waste from its point of entry into the large
intestine through its exit from the body as feces
The word intestine is derived from a Latin root meaning “internal,” and
indeed, the two organs together nearly fill the interior of the abdominal
cavity. In addition, called the small and large bowel, or colloquially the
“suts,” they constitute the greatest mass and length of the alimentary canal
and, with the exception of ingestion, perform all digestive system functions.
The Small Intestine
Chyme released from the stomach enters the small intestine, which is the
primary digestive organ in the body. Not only is this where most digestion
occurs, it is also where practically all absorption occurs. The longest part of
the alimentary canal, the small intestine is about 3.05 meters (10 feet) long
in a living person (but about twice as long in a cadaver due to the loss of
muscle tone). Since this makes it about five times longer than the large
intestine, you might wonder why it is called “small.” In fact, its name
derives from its relatively smaller diameter of only about 2.54 cm (1 in),
compared with 7.62 cm (3 in) for the large intestine. As we’ll see shortly, in
addition to its length, the folds and projections of the lining of the small
intestine work to give it an enormous surface area, which is approximately
200 m?, more than 100 times the surface area of your skin. This large
surface area is necessary for complex processes of digestion and absorption
that occur within it.
Structure
The coiled tube of the small intestine is subdivided into three regions. From
proximal (at the stomach) to distal, these are the duodenum, jejunum, and
ileum ([link]).
The shortest region is the 25.4-cm (10-in) duodenum, which begins at the
pyloric sphincter. Just past the pyloric sphincter, it bends posteriorly behind
the peritoneum, becoming retroperitoneal, and then makes a C-shaped curve
around the head of the pancreas before ascending anteriorly again to return
to the peritoneal cavity and join the jejunum. The duodenum can therefore
be subdivided into four segments: the superior, descending, horizontal, and
ascending duodenum.
Of particular interest is the hepatopancreatic ampulla (ampulla of Vater).
Located in the duodenal wall, the ampulla marks the transition from the
anterior portion of the alimentary canal to the mid-region, and is where the
bile duct (through which bile passes from the liver) and the main
pancreatic duct (through which pancreatic juice passes from the pancreas)
join. This ampulla opens into the duodenum at a tiny volcano-shaped
structure called the major duodenal papilla. The hepatopancreatic
sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic
juice from the ampulla into the duodenum.
Small Intestine
Duodenum
Jejunum
lleum
Large intestine
Rectum ———___—>=_
The three regions of the small intestine are the
duodenum, jejunum, and ileum.
The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the
duodenum to the ileum. Jejunum means “empty” in Latin and supposedly
was so named by the ancient Greeks who noticed it was always empty at
death. No clear demarcation exists between the jejunum and the final
segment of the small intestine, the ileum.
The ileum is the longest part of the small intestine, measuring about 1.8
meters (6 feet) in length. It is thicker, more vascular, and has more
developed mucosal folds than the jejunum. The ileum joins the cecum, the
first portion of the large intestine, at the ileocecal sphincter (or valve). The
jejunum and ileum are tethered to the posterior abdominal wall by the
mesentery. The large intestine frames these three parts of the small
intestine.
Parasympathetic nerve fibers from the vagus nerve and sympathetic nerve
fibers from the thoracic splanchnic nerve provide extrinsic innervation to
the small intestine. The superior mesenteric artery is its main arterial
supply. Veins run parallel to the arteries and drain into the superior
mesenteric vein. Nutrient-rich blood from the small intestine is then carried
to the liver via the hepatic portal vein.
Histology
The wall of the small intestine is composed of the same four layers typically
present in the alimentary system. However, three features of the mucosa and
submucosa are unique. These features, which increase the absorptive
surface area of the small intestine more than 600-fold, include circular
folds, villi, and microvilli ([link]). These adaptations are most abundant in
the proximal two-thirds of the small intestine, where the majority of
absorption occurs.
~~ of the Small ie
Absorptive cells
Capillary oe
Artery (brush border)
Goblet cell
trl@t JURee Jet Be!
° ony OS ED Vac
|i
Lymphatic vesicle
Muscularis mucosae Duodenal gland
(a)
(a) The absorptive surface of the small intestine is vastly
enlarged by the presence of circular folds, villi, and microvilli.
(b) Micrograph of the circular folds. (c) Micrograph of the villi.
(d) Electron micrograph of the microvilli. From left to right,
LM x 56, LM x 508, EM x 196,000. (credit b-d: Micrograph
provided by the Regents of University of Michigan Medical
School © 2012)
Circular folds
Also called a plica circulare, a circular fold is a deep ridge in the mucosa
and submucosa. Beginning near the proximal part of the duodenum and
ending near the middle of the ileum, these folds facilitate absorption. Their
shape causes the chyme to spiral, rather than move in a straight line,
through the small intestine. Spiraling slows the movement of chyme and
provides the time needed for nutrients to be fully absorbed.
Villi
Within the circular folds are small (0.5—1 mm long) hairlike vascularized
projections called villi (singular = villus) that give the mucosa a furry
texture. There are about 20 to 40 villi per square millimeter, increasing the
surface area of the epithelium tremendously. The mucosal epithelium,
primarily composed of absorptive cells, covers the villi. In addition to
muscle and connective tissue to support its structure, each villus contains a
capillary bed composed of one arteriole and one venule, as well as a
lymphatic capillary called a lacteal. The breakdown products of
carbohydrates and proteins (sugars and amino acids) can enter the
bloodstream directly, but lipid breakdown products are absorbed by the
lacteals and transported to the bloodstream via the lymphatic system.
Microvilli
As their name suggests, microvilli (singular = microvillus) are much
smaller (1 pm) than villi. They are cylindrical apical surface extensions of
the plasma membrane of the mucosa’s epithelial cells, and are supported by
microfilaments within those cells. Although their small size makes it
difficult to see each microvillus, their combined microscopic appearance
suggests a mass of bristles, which is termed the brush border. Fixed to the
surface of the microvilli membranes are enzymes that finish digesting
carbohydrates and proteins. There are an estimated 200 million microvilli
per square millimeter of small intestine, greatly expanding the surface area
of the plasma membrane and thus greatly enhancing absorption.
Intestinal Glands
In addition to the three specialized absorptive features just discussed, the
mucosa between the villi is dotted with deep crevices that each lead into a
tubular intestinal gland (crypt of Lieberkiihn), which is formed by cells
that line the crevices (see [link]). These produce intestinal juice, a slightly
alkaline (pH 7.4 to 7.8) mixture of water and mucus. Each day, about 0.95
to 1.9 liters (1 to 2 quarts) are secreted in response to the distention of the
small intestine or the irritating effects of chyme on the intestinal mucosa.
The submucosa of the duodenum is the only site of the complex mucus-
secreting duodenal glands (Brunner’s glands), which produce a
bicarbonate-rich alkaline mucus that buffers the acidic chyme as it enters
from the stomach.
The roles of the cells in the small intestinal mucosa are detailed in [link].
Cells of the Small Intestinal Mucosa
Cells of the SmdlbhatéstialtNéucosa
Cell type
Cell type
Absorptive
Goblet
Paneth
G cells
I cells
K cells
mucosa
Location in the
mucosa
Epithelium/intestinal
glands
Epithelium/intestinal
glands
Intestinal glands
Intestinal glands of
duodenum
Intestinal glands of
duodenum
Intestinal glands
Function
Function
Digestion and absorption
of nutrients in chyme
Secretion of mucus
Secretion of the
bactericidal enzyme
lysozyme; phagocytosis
Secretion of the hormone
intestinal gastrin
Secretion of the hormone
cholecystokinin, which
stimulates release of
pancreatic juices and bile
Secretion of the hormone
glucose-dependent
insulinotropic peptide,
which stimulates the
release of insulin
Cells of the Small Intestinal Mucosa
Location in the
Cell type mucosa Function
Secretion of the hormone
motilin, which accelerates
Intestinal glands of ;
gastric emptying,
M cells duodenum and : ; ‘
oe stimulates intestinal
jejunum ; : :
peristalsis, and stimulates
the production of pepsin
? retion of the h
S cells Intestinal glands Sec eno Die OENOn’
secretin
Intestinal MALT
The lamina propria of the small intestine mucosa is studded with quite a bit
of MALT. In addition to solitary lymphatic nodules, aggregations of
intestinal MALT, which are typically referred to as Peyer’s patches, are
concentrated in the distal ileum, and serve to keep bacteria from entering
the bloodstream. Peyer’s patches are most prominent in young people and
become less distinct as you age, which coincides with the general activity of
our immune system.
Note:
—
meee OPENStAX COLLEGE
Watch this animation that depicts the structure of the small intestine, and,
in particular, the villi. Epithelial cells continue the digestion and absorption
of nutrients and transport these nutrients to the lymphatic and circulatory
systems. In the small intestine, the products of food digestion are absorbed
by different structures in the villi. Which structure absorbs and transports
fats?
Mechanical Digestion in the Small Intestine
The movement of intestinal smooth muscles includes both segmentation
and a form of peristalsis called migrating motility complexes. The kind of
peristaltic mixing waves seen in the stomach are not observed here.
If you could see into the small intestine when it was going through
segmentation, it would look as if the contents were being shoved
incrementally back and forth, as the rings of smooth muscle repeatedly
contract and then relax. Segmentation in the small intestine does not force
chyme through the tract. Instead, it combines the chyme with digestive
juices and pushes food particles against the mucosa to be absorbed. The
duodenum is where the most rapid segmentation occurs, at a rate of about
12 times per minute. In the ileum, segmentations are only about eight times
per minute ([link]).
Segmentation
Segmentation separates
chyme and then pushes it
back together, mixing it
and providing time for
digestion and absorption.
When most of the chyme has been absorbed, the small intestinal wall
becomes less distended. At this point, the localized segmentation process is
replaced by transport movements. The duodenal mucosa secretes the
hormone motilin, which initiates peristalsis in the form of a migrating
motility complex. These complexes, which begin in the duodenum, force
chyme through a short section of the small intestine and then stop. The next
contraction begins a little bit farther down than the first, forces chyme a bit
farther through the small intestine, then stops. These complexes move
slowly down the small intestine, forcing chyme on the way, taking around
90 to 120 minutes to finally reach the end of the ileum. At this point, the
process is repeated, starting in the duodenum.
The ileocecal valve, a sphincter, is usually in a constricted state, but when
motility in the ileum increases, this sphincter relaxes, allowing food residue
to enter the first portion of the large intestine, the cecum. Relaxation of the
ileocecal sphincter is controlled by both nerves and hormones. First,
digestive activity in the stomach provokes the gastroileal reflex, which
increases the force of ileal segmentation. Second, the stomach releases the
hormone gastrin, which enhances ileal motility, thus relaxing the ileocecal
sphincter. After chyme passes through, backward pressure helps close the
sphincter, preventing backflow into the ileum. Because of this reflex, your
lunch is completely emptied from your stomach and small intestine by the
time you eat your dinner. It takes about 3 to 5 hours for all chyme to leave
the small intestine.
Chemical Digestion in the Small Intestine
The digestion of proteins and carbohydrates, which partially occurs in the
stomach, is completed in the small intestine with the aid of intestinal and
pancreatic juices. Lipids arrive in the intestine largely undigested, so much
of the focus here is on lipid digestion, which is facilitated by bile and the
enzyme pancreatic lipase.
Moreover, intestinal juice combines with pancreatic juice to provide a liquid
medium that facilitates absorption. The intestine is also where most water is
absorbed, via osmosis. The small intestine’s absorptive cells also synthesize
digestive enzymes and then place them in the plasma membranes of the
microvilli. This distinguishes the small intestine from the stomach; that is,
enzymatic digestion occurs not only in the lumen, but also on the luminal
surfaces of the mucosal cells.
For optimal chemical digestion, chyme must be delivered from the stomach
slowly and in small amounts. This is because chyme from the stomach is
typically hypertonic, and if large quantities were forced all at once into the
small intestine, the resulting osmotic water loss from the blood into the
intestinal lumen would result in potentially life-threatening low blood
volume. In addition, continued digestion requires an upward adjustment of
the low pH of stomach chyme, along with rigorous mixing of the chyme
with bile and pancreatic juices. Both processes take time, so the pumping
action of the pylorus must be carefully controlled to prevent the duodenum
from being overwhelmed with chyme.
Note:
Disorders of the...
Small Intestine: Lactose Intolerance
Lactose intolerance is a condition characterized by indigestion caused by
dairy products. It occurs when the absorptive cells of the small intestine do
not produce enough lactase, the enzyme that digests the milk sugar lactose.
In most mammals, lactose intolerance increases with age. In contrast, some
human populations, most notably Caucasians, are able to maintain the
ability to produce lactase as adults.
In people with lactose intolerance, the lactose in chyme is not digested.
Bacteria in the large intestine ferment the undigested lactose, a process that
produces gas. In addition to gas, symptoms include abdominal cramps,
bloating, and diarrhea. Symptom severity ranges from mild discomfort to
severe pain; however, symptoms resolve once the lactose is eliminated in
feces.
The hydrogen breath test is used to help diagnose lactose intolerance.
Lactose-tolerant people have very little hydrogen in their breath. Those
with lactose intolerance exhale hydrogen, which is one of the gases
produced by the bacterial fermentation of lactose in the colon. After the
hydrogen is absorbed from the intestine, it is transported through blood
vessels into the lungs. There are a number of lactose-free dairy products
available in grocery stores. In addition, dietary supplements are available.
Taken with food, they provide lactase to help digest lactose.
The Large Intestine
The large intestine is the terminal part of the alimentary canal. The primary
function of this organ is to finish absorption of nutrients and water,
synthesize certain vitamins, form feces, and eliminate feces from the body.
Structure
The large intestine runs from the appendix to the anus. It frames the small
intestine on three sides. Despite its being about one-half as long as the small
intestine, it is called large because it is more than twice the diameter of the
small intestine, about 3 inches.
Subdivisions
The large intestine is subdivided into four main regions: the cecum, the
colon, the rectum, and the anus. The ileocecal valve, located at the opening
between the ileum and the large intestine, controls the flow of chyme from
the small intestine to the large intestine.
Cecum
The first part of the large intestine is the cecum, a sac-like structure that is
suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long,
receives the contents of the ileum, and continues the absorption of water
and salts. The appendix (or vermiform appendix) is a winding tube that
attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains
lymphoid tissue, suggesting an immunologic function, this organ is
generally considered vestigial. However, at least one recent report
postulates a survival advantage conferred by the appendix: In diarrheal
illness, the appendix may serve as a bacterial reservoir to repopulate the
enteric bacteria for those surviving the initial phases of the illness.
Moreover, its twisted anatomy provides a haven for the accumulation and
multiplication of enteric bacteria. The mesoappendix, the mesentery of the
appendix, tethers it to the mesentery of the ileum.
Colon
The cecum blends seamlessly with the colon. Upon entering the colon, the
food residue first travels up the ascending colon on the right side of the
abdomen. At the inferior surface of the liver, the colon bends to form the
right colic flexure (hepatic flexure) and becomes the transverse colon.
The region defined as hindgut begins with the last third of the transverse
colon and continues on. Food residue passing through the transverse colon
travels across to the left side of the abdomen, where the colon angles
sharply immediately inferior to the spleen, at the left colic flexure (splenic
flexure). From there, food residue passes through the descending colon,
which runs down the left side of the posterior abdominal wall. After
entering the pelvis inferiorly, it becomes the s-shaped sigmoid colon, which
extends medially to the midline ([{link]). The ascending and descending
colon, and the rectum (discussed next) are located in the retroperitoneum.
The transverse and sigmoid colon are tethered to the posterior abdominal
wall by the mesocolon.
Large Intestine
Right colic
(hepatic) flexure Left colic
(splenic)
flexure
Transverse
colon
Ascending Descending
colon colon
lleum
Cecum
Vermiform Sigmoid
appendix colon
Anal canal Rectum
The large intestine includes the
cecum, colon, and rectum.
Note:
Homeostatic Imbalances
Colorectal Cancer
Each year, approximately 140,000 Americans are diagnosed with
colorectal cancer, and another 49,000 die from it, making it one of the most
deadly malignancies. People with a family history of colorectal cancer are
at increased risk. Smoking, excessive alcohol consumption, and a diet high
in animal fat and protein also increase the risk. Despite popular opinion to
the contrary, studies support the conclusion that dietary fiber and calcium
do not reduce the risk of colorectal cancer.
Colorectal cancer may be signaled by constipation or diarrhea, cramping,
abdominal pain, and rectal bleeding. Bleeding from the rectum may be
either obvious or occult (hidden in feces). Since most colon cancers arise
from benign mucosal growths called polyps, cancer prevention is focused
on identifying these polyps. The colonoscopy is both diagnostic and
therapeutic. Colonoscopy not only allows identification of precancerous
polyps, the procedure also enables them to be removed before they become
malignant. Screening for fecal occult blood tests and colonoscopy is
recommended for those over 50 years of age.
Rectum
Food residue leaving the sigmoid colon enters the rectum in the pelvis,
near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary
canal, the rectum extends anterior to the sacrum and coccyx. Even though
rectum is Latin for “straight,” this structure follows the curved contour of
the sacrum and has three lateral bends that create a trio of internal
transverse folds called the rectal valves. These valves help separate the
feces from gas to prevent the simultaneous passage of feces and gas.
Anal Canal
Finally, food residue reaches the last part of the large intestine, the anal
canal, which is located in the perineum, completely outside of the
abdominopelvic cavity. This 3.8—5 cm (1.5—2 in) long structure opens to the
exterior of the body at the anus. The anal canal includes two sphincters. The
internal anal sphincter is made of smooth muscle, and its contractions are
involuntary. The external anal sphincter is made of skeletal muscle, which
is under voluntary control. Except when defecating, both usually remain
closed.
Histology
There are several notable differences between the walls of the large and
small intestines ([link]). For example, few enzyme-secreting cells are found
in the wall of the large intestine, and there are no circular folds or villi.
Other than in the anal canal, the mucosa of the colon is simple columnar
epithelium made mostly of enterocytes (absorptive cells) and goblet cells.
In addition, the wall of the large intestine has far more intestinal glands,
which contain a vast population of enterocytes and goblet cells. These
goblet cells secrete mucus that eases the movement of feces and protects the
intestine from the effects of the acids and gases produced by enteric
bacteria. The enterocytes absorb water and salts as well as vitamins
produced by your intestinal bacteria.
Histology of the large Intestine
Openings of Microvilli
intestinal glands ‘ond aN — =
~~ NI \\(] Absorptive cell
4 absorbs water
Large intestine
Goblet cell
secretes mucus
@We
eas ]
Smooth muscle fiber —————— ae
Lymphatic nodule
Muscularis mucosae
Submucosa
(a) The histologies of the large intestine and small
intestine (not shown) are adapted for the digestive
functions of each organ. (b) This micrograph shows
the colon’s simple columnar epithelium and goblet
cells. LM x 464. (credit b: Micrograph provided by the
Regents of University of Michigan Medical School ©
2012)
Anatomy
Three features are unique to the large intestine: teniae coli, haustra, and
epiploic appendages ([link]). The teniae coli are three bands of smooth
muscle that make up the longitudinal muscle layer of the muscularis of the
large intestine, except at its terminal end. Tonic contractions of the teniae
coli bunch up the colon into a succession of pouches called haustra
(singular = haustrum), which are responsible for the wrinkled appearance of
the colon. Attached to the teniae coli are small, fat-filled sacs of visceral
peritoneum called epiploic appendages. The purpose of these is unknown.
Although the rectum and anal canal have neither teniae coli nor haustra,
they do have well-developed layers of muscularis that create the strong
contractions needed for defecation.
Teniae Coli, Haustra, and Epiploic Appendages
Epiploic appendages
The stratified squamous epithelial mucosa of the anal canal connects to the
skin on the outside of the anus. This mucosa varies considerably from that
of the rest of the colon to accommodate the high level of abrasion as feces
pass through. The anal canal’s mucous membrane is organized into
longitudinal folds, each called an anal column, which house a grid of
arteries and veins. Two superficial venous plexuses are found in the anal
canal: one within the anal columns and one at the anus.
Depressions between the anal columns, each called an anal sinus, secrete
mucus that facilitates defecation. The pectinate line (or dentate line) is a
horizontal, jagged band that runs circumferentially just below the level of
the anal sinuses, and represents the junction between the hindgut and
external skin. The mucosa above this line is fairly insensitive, whereas the
area below is very sensitive. The resulting difference in pain threshold is
due to the fact that the upper region is innervated by visceral sensory fibers,
and the lower region is innervated by somatic sensory fibers.
Bacterial Flora
Most bacteria that enter the alimentary canal are killed by lysozyme,
defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria
live within the large intestine and are referred to as the bacterial flora.
Most of the more than 700 species of these bacteria are nonpathogenic
commensal organisms that cause no harm as long as they stay in the gut
lumen. In fact, many facilitate chemical digestion and absorption, and some
synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K.
Some are linked to increased immune response. A refined system prevents
these bacteria from crossing the mucosal barrier. First, peptidoglycan, a
component of bacterial cell walls, activates the release of chemicals by the
mucosa’s epithelial cells, which draft immune cells, especially dendritic
cells, into the mucosa. Dendritic cells open the tight junctions between
epithelial cells and extend probes into the lumen to evaluate the microbial
antigens. The dendritic cells with antigens then travel to neighboring
lymphoid follicles in the mucosa where T cells inspect for antigens. This
process triggers an IgA-mediated response, if warranted, in the lumen that
blocks the commensal organisms from infiltrating the mucosa and setting
off a far greater, widespread systematic reaction.
Digestive Functions of the Large Intestine
The residue of chyme that enters the large intestine contains few nutrients
except water, which is reabsorbed as the residue lingers in the large
intestine, typically for 12 to 24 hours. Thus, it may not surprise you that the
large intestine can be completely removed without significantly affecting
digestive functioning. For example, in severe cases of inflammatory bowel
disease, the large intestine can be removed by a procedure known as a
colectomy. Often, a new fecal pouch can be crafted from the small intestine
and sutured to the anus, but if not, an ileostomy can be created by bringing
the distal ileum through the abdominal wall, allowing the watery chyme to
be collected in a bag-like adhesive appliance.
Mechanical Digestion
In the large intestine, mechanical digestion begins when chyme moves from
the ileum into the cecum, an activity regulated by the ileocecal sphincter.
Right after you eat, peristalsis in the ileum forces chyme into the cecum.
When the cecum is distended with chyme, contractions of the ileocecal
sphincter strengthen. Once chyme enters the cecum, colon movements
begin.
Mechanical digestion in the large intestine includes a combination of three
types of movements. The presence of food residues in the colon stimulates a
slow-moving haustral contraction. This type of movement involves
sluggish segmentation, primarily in the transverse and descending colons.
When a haustrum is distended with chyme, its muscle contracts, pushing the
residue into the next haustrum. These contractions occur about every 30
minutes, and each last about 1 minute. These movements also mix the food
residue, which helps the large intestine absorb water. The second type of
movement is peristalsis, which, in the large intestine, is slower than in the
more proximal portions of the alimentary canal. The third type is a mass
movement. These strong waves start midway through the transverse colon
and quickly force the contents toward the rectum. Mass movements usually
occur three or four times per day, either while you eat or immediately
afterward. Distension in the stomach and the breakdown products of
digestion in the small intestine provoke the gastrocolic reflex, which
increases motility, including mass movements, in the colon. Fiber in the diet
both softens the stool and increases the power of colonic contractions,
optimizing the activities of the colon.
Chemical Digestion
Although the glands of the large intestine secrete mucus, they do not secrete
digestive enzymes. Therefore, chemical digestion in the large intestine
occurs exclusively because of bacteria in the lumen of the colon. Through
the process of saccharolytic fermentation, bacteria break down some of
the remaining carbohydrates. This results in the discharge of hydrogen,
carbon dioxide, and methane gases that create flatus (gas) in the colon;
flatulence is excessive flatus. Each day, up to 1500 mL of flatus is produced
in the colon. More is produced when you eat foods such as beans, which are
rich in otherwise indigestible sugars and complex carbohydrates like
soluble dietary fiber.
Absorption, Feces Formation, and Defecation
The small intestine absorbs about 90 percent of the water you ingest (either
as liquid or within solid food). The large intestine absorbs most of the
remaining water, a process that converts the liquid chyme residue into
semisolid feces (“stool”). Feces is composed of undigested food residues,
unabsorbed digested substances, millions of bacteria, old epithelial cells
from the GI mucosa, inorganic salts, and enough water to let it pass
smoothly out of the body. Of every 500 mL (17 ounces) of food residue that
enters the cecum each day, about 150 mL (5 ounces) become feces.
Feces are eliminated through contractions of the rectal muscles. You help
this process by a voluntary procedure called Valsalva’s maneuver, in
which you increase intra-abdominal pressure by contracting your
diaphragm and abdominal wall muscles, and closing your glottis.
The process of defecation begins when mass movements force feces from
the colon into the rectum, stretching the rectal wall and provoking the
defecation reflex, which eliminates feces from the rectum. This
parasympathetic reflex is mediated by the spinal cord. It contracts the
sigmoid colon and rectum, relaxes the internal anal sphincter, and initially
contracts the external anal sphincter. The presence of feces in the anal canal
sends a signal to the brain, which gives you the choice of voluntarily
opening the external anal sphincter (defecating) or keeping it temporarily
closed. If you decide to delay defecation, it takes a few seconds for the
reflex contractions to stop and the rectal walls to relax. The next mass
movement will trigger additional defecation reflexes until you defecate.
If defecation is delayed for an extended time, additional water is absorbed,
making the feces firmer and potentially leading to constipation. On the
other hand, if the waste matter moves too quickly through the intestines, not
enough water is absorbed, and diarrhea can result. This can be caused by
the ingestion of foodborne pathogens. In general, diet, health, and stress
determine the frequency of bowel movements. The number of bowel
movements varies greatly between individuals, ranging from two or three
per day to three or four per week.
Note:
eee
——
mss" OPENStax COLLEGE
By watching this animation you will see that for the various food groups—
proteins, fats, and carbohydrates—digestion begins in different parts of the
digestion system, though all end in the same place. Of the three major food
classes (carbohydrates, fats, and proteins), which is digested in the mouth,
the stomach, and the small intestine?
Chapter Review
The three main regions of the small intestine are the duodenum, the
jejunum, and the ileum. The small intestine is where digestion is completed
and virtually all absorption occurs. These two activities are facilitated by
structural adaptations that increase the mucosal surface area by 600-fold,
including circular folds, villi, and microvilli. There are around 200 million
microvilli per square millimeter of small intestine, which contain brush
border enzymes that complete the digestion of carbohydrates and proteins.
Combined with pancreatic juice, intestinal juice provides the liquid medium
needed to further digest and absorb substances from chyme. The small
intestine is also the site of unique mechanical digestive movements.
Segmentation moves the chyme back and forth, increasing mixing and
opportunities for absorption. Migrating motility complexes propel the
residual chyme toward the large intestine.
The main regions of the large intestine are the cecum, the colon, and the
rectum. The large intestine absorbs water and forms feces, and is
responsible for defecation. Bacterial flora break down additional
carbohydrate residue, and synthesize certain vitamins. The mucosa of the
large intestinal wall is generously endowed with goblet cells, which secrete
mucus that eases the passage of feces. The entry of feces into the rectum
activates the defecation reflex.
Interactive Link Questions
Exercise:
Problem:
Watch this animation that depicts the structure of the small intestine,
and, in particular, the villi. Epithelial cells continue the digestion and
absorption of nutrients and transport these nutrients to the lymphatic
and circulatory systems. In the small intestine, the products of food
digestion are absorbed by different structures in the villi. Which
structure absorbs and transports fats?
Solution:
Answers may vary.
Exercise:
Problem:
By watching this animation, you will see that for the various food
groups—proteins, fats, and carbohydrates—digestion begins in
different parts of the digestion system, though all end in the same
place. Of the three major food classes (carbohydrates, fats, and
proteins), which is digested in the mouth, the stomach, and the small
intestine?
Solution:
Answers may vary.
Review Questions
Exercise:
Problem:
In which part of the alimentary canal does most digestion occur?
a. stomach
b. proximal small intestine
c. distal small intestine
d. ascending colon
Solution:
B
Exercise:
Problem: Which of these is most associated with villi?
a. haustra
b. lacteals
c. bacterial flora
d. intestinal glands
Solution:
B
Exercise:
Problem: What is the role of the small intestine’s MALT?
a. secreting mucus
b. buffering acidic chyme
c. activating pepsin
d. preventing bacteria from entering the bloodstream
Solution:
D
Exercise:
Problem: Which part of the large intestine attaches to the appendix?
a. cecum
b. ascending colon
c. transverse colon
d. descending colon
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Explain how nutrients absorbed in the small intestine pass into the
general circulation.
Solution:
Nutrients from the breakdown of carbohydrates and proteins are
absorbed through a capillary bed in the villi of the small intestine.
Lipid breakdown products are absorbed into a lacteal in the villi, and
transported via the lymphatic system to the bloodstream.
Exercise:
Problem:
Why is it important that chyme from the stomach is delivered to the
small intestine slowly and in small amounts?
Solution:
If large quantities of chyme were forced into the small intestine, it
would result in osmotic water loss from the blood into the intestinal
lumen that could cause potentially life-threatening low blood volume
and erosion of the duodenum.
Exercise:
Problem:
Describe three of the differences between the walls of the large and
small intestines.
Solution:
The mucosa of the small intestine includes circular folds, villi, and
microvilli. The wall of the large intestine has a thick mucosal layer,
and deeper and more abundant mucus-secreting glands that facilitate
the smooth passage of feces. There are three features that are unique to
the large intestine: teniae coli, haustra, and epiploic appendages.
References
American Cancer Society (US). Cancer facts and figures: colorectal cancer:
2011-2013 [Internet]. c2013 [cited 2013 Apr 3]. Available from:
http://www.cancer.org/Research/CancerFactsFigures/ColorectalCancerFacts
Figures/colorectal-cancer-facts-figures-2011-2013-page.
The Nutrition Source. Fiber and colon cancer: following the scientific trail
[Internet]. Boston (MA): Harvard School of Public Health; c2012 [cited
2013 Apr 3]. Available from:
http://www.hsph.harvard.edu/nutritionsource/nutrition-news/fiber-and-
colon-cancer/index. html.
Centers for Disease Control and Prevention (US). Morbidity and mortality
weekly report: notifiable diseases and mortality tables [Internet]. Atlanta
(GA); [cited 2013 Apr 3]. Available from:
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6101md.htm?
s_cid=mm6101imd_w.
Glossary
anal canal
final segment of the large intestine
anal column
long fold of mucosa in the anal canal
anal sinus
recess between anal columns
appendix
(vermiform appendix) coiled tube attached to the cecum
ascending colon
first region of the colon
bacterial flora
bacteria in the large intestine
brush border
fuzzy appearance of the small intestinal mucosa created by microvilli
cecum
pouch forming the beginning of the large intestine
circular fold
(also, plica circulare) deep fold in the mucosa and submucosa of the
small intestine
colon
part of the large intestine between the cecum and the rectum
descending colon
part of the colon between the transverse colon and the sigmoid colon
duodenal gland
(also, Brunner’s gland) mucous-secreting gland in the duodenal
submucosa
duodenum
first part of the small intestine, which starts at the pyloric sphincter and
ends at the jejunum
epiploic appendage
small sac of fat-filled visceral peritoneum attached to teniae coli
external anal sphincter
voluntary skeletal muscle sphincter in the anal canal
feces
semisolid waste product of digestion
flatus
gas in the intestine
gastrocolic reflex
propulsive movement in the colon activated by the presence of food in
the stomach
gastroileal reflex
long reflex that increases the strength of segmentation in the ileum
haustrum
small pouch in the colon created by tonic contractions of teniae coli
haustral contraction
slow segmentation in the large intestine
hepatopancreatic ampulla
(also, ampulla of Vater) bulb-like point in the wall of the duodenum
where the bile duct and main pancreatic duct unite
hepatopancreatic sphincter
(also, sphincter of Oddi) sphincter regulating the flow of bile and
pancreatic juice into the duodenum
ileocecal sphincter
sphincter located where the small intestine joins with the large
intestine
ileum
end of the small intestine between the jejunum and the large intestine
internal anal sphincter
involuntary smooth muscle sphincter in the anal canal
intestinal gland
(also, crypt of Lieberktihn) gland in the small intestinal mucosa that
secretes intestinal juice
intestinal juice
mixture of water and mucus that helps absorb nutrients from chyme
jejunum
middle part of the small intestine between the duodenum and the ileum
lacteal
lymphatic capillary in the villi
large intestine
terminal portion of the alimentary canal
left colic flexure
(also, splenic flexure) point where the transverse colon curves below
the inferior end of the spleen
main pancreatic duct
(also, duct of Wirsung) duct through which pancreatic juice drains
from the pancreas
major duodenal papilla
point at which the hepatopancreatic ampulla opens into the duodenum
mass movement
long, slow, peristaltic wave in the large intestine
mesoappendix
mesentery of the appendix
microvillus
small projection of the plasma membrane of the absorptive cells of the
small intestinal mucosa
migrating motility complex
form of peristalsis in the small intestine
motilin
hormone that initiates migrating motility complexes
pectinate line
horizontal line that runs like a ring, perpendicular to the inferior
margins of the anal sinuses
rectal valve
one of three transverse folds in the rectum where feces is separated
from flatus
rectum
part of the large intestine between the sigmoid colon and anal canal
right colic flexure
(also, hepatic flexure) point, at the inferior surface of the liver, where
the ascending colon turns abruptly to the left
saccharolytic fermentation
anaerobic decomposition of carbohydrates
sigmoid colon
end portion of the colon, which terminates at the rectum
small intestine
section of the alimentary canal where most digestion and absorption
occurs
tenia coli
one of three smooth muscle bands that make up the longitudinal
muscle layer of the muscularis in all of the large intestine except the
terminal end
transverse colon
part of the colon between the ascending colon and the descending
colon
Valsalva’s maneuver
voluntary contraction of the diaphragm and abdominal wall muscles
and closing of the glottis, which increases intra-abdominal pressure
and facilitates defecation
villus
projection of the mucosa of the small intestine
Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder
By the end of this section, you will be able to:
e State the main digestive roles of the liver, pancreas, and gallbladder
e Identify three main features of liver histology that are critical to its
function
Discuss the composition and function of bile
Identify the major types of enzymes and buffers present in pancreatic
juice
Chemical digestion in the small intestine relies on the activities of three
accessory digestive organs: the liver, pancreas, and gallbladder ({link]). The
digestive role of the liver is to produce bile and export it to the duodenum.
The gallbladder primarily stores, concentrates, and releases bile. The
pancreas produces pancreatic juice, which contains digestive enzymes and
bicarbonate ions, and delivers it to the duodenum.
Accessory Organs
Liver:
Right lobe
Quadrate lobe
Left lobe
Caudate lobe
Gallbladder Spleen
Right hepatic duct
Pancreas
Cystic duct
Common hepatic duct Pancreatic duct
Common bile duct
Left hepatic duct
The liver, pancreas, and gallbladder are
considered accessory digestive organs, but
their roles in the digestive system are vital.
The Liver
The liver is the largest gland in the body, weighing about three pounds in an
adult. It is also one of the most important organs. In addition to being an
accessory digestive organ, it plays a number of roles in metabolism and
regulation. The liver lies inferior to the diaphragm in the right upper
quadrant of the abdominal cavity and receives protection from the
surrounding ribs.
The liver is divided into two primary lobes: a large right lobe and a much
smaller left lobe. In the right lobe, some anatomists also identify an inferior
quadrate lobe and a posterior caudate lobe, which are defined by internal
features. The liver is connected to the abdominal wall and diaphragm by
five peritoneal folds referred to as ligaments. These are the falciform
ligament, the coronary ligament, two lateral ligaments, and the ligamentum
teres hepatis. The falciform ligament and ligamentum teres hepatis are
actually remnants of the umbilical vein, and separate the right and left lobes
anteriorly. The lesser omentum tethers the liver to the lesser curvature of the
stomach.
The porta hepatis (“gate to the liver”) is where the hepatic artery and
hepatic portal vein enter the liver. These two vessels, along with the
common hepatic duct, run behind the lateral border of the lesser omentum
on the way to their destinations. As shown in [link], the hepatic artery
delivers oxygenated blood from the heart to the liver. The hepatic portal
vein delivers partially deoxygenated blood containing nutrients absorbed
from the small intestine and actually supplies more oxygen to the liver than
do the much smaller hepatic arteries. In addition to nutrients, drugs and
toxins are also absorbed. After processing the bloodborne nutrients and
toxins, the liver releases nutrients needed by other cells back into the blood,
which drains into the central vein and then through the hepatic vein to the
inferior vena cava. With this hepatic portal circulation, all blood from the
alimentary canal passes through the liver. This largely explains why the
liver is the most common site for the metastasis of cancers that originate in
the alimentary canal.
Microscopic Anatomy of the Liver
Central vein
Connective
tissue
Lobules
Interlobular vein
(to hepatic vein)
Central vein Sinusoids
Plates of Portal venule
hepatocytes
From portal vein
The liver receives oxygenated blood from the
hepatic artery and nutrient-rich deoxygenated
blood from the hepatic portal vein.
Histology
The liver has three main components: hepatocytes, bile canaliculi, and
hepatic sinusoids. A hepatocyte is the liver’s main cell type, accounting for
around 80 percent of the liver's volume. These cells play a role in a wide
variety of secretory, metabolic, and endocrine functions. Plates of
hepatocytes called hepatic laminae radiate outward from the portal vein in
each hepatic lobule.
Between adjacent hepatocytes, grooves in the cell membranes provide room
for each bile canaliculus (plural = canaliculi). These small ducts
accumulate the bile produced by hepatocytes. From here, bile flows first
into bile ductules and then into bile ducts. The bile ducts unite to form the
larger right and left hepatic ducts, which themselves merge and exit the
liver as the common hepatic duct. This duct then joins with the cystic duct
from the gallbladder, forming the common bile duct through which bile
flows into the small intestine.
A hepatic sinusoid is an open, porous blood space formed by fenestrated
capillaries from nutrient-rich hepatic portal veins and oxygen-rich hepatic
arteries. Hepatocytes are tightly packed around the fenestrated endothelium
of these spaces, giving them easy access to the blood. From their central
position, hepatocytes process the nutrients, toxins, and waste materials
carried by the blood. Materials such as bilirubin are processed and excreted
into the bile canaliculi. Other materials including proteins, lipids, and
carbohydrates are processed and secreted into the sinusoids or just stored in
the cells until called upon. The hepatic sinusoids combine and send blood to
a central vein. Blood then flows through a hepatic vein into the inferior
vena cava. This means that blood and bile flow in opposite directions. The
hepatic sinusoids also contain star-shaped reticuloendothelial cells
(Kupffer cells), phagocytes that remove dead red and white blood cells,
bacteria, and other foreign material that enter the sinusoids. The portal
triad is a distinctive arrangement around the perimeter of hepatic lobules,
consisting of three basic structures: a bile duct, a hepatic artery branch, and
a hepatic portal vein branch.
Bile
Recall that lipids are hydrophobic, that is, they do not dissolve in water.
Thus, before they can be digested in the watery environment of the small
intestine, large lipid globules must be broken down into smaller lipid
globules, a process called emulsification. Bile is a mixture secreted by the
liver to accomplish the emulsification of lipids in the small intestine.
Hepatocytes secrete about one liter of bile each day. A yellow-brown or
yellow-green alkaline solution (pH 7.6 to 8.6), bile is a mixture of water,
bile salts, bile pigments, phospholipids (such as lecithin), electrolytes,
cholesterol, and triglycerides. The components most critical to
emulsification are bile salts and phospholipids, which have a nonpolar
(hydrophobic) region as well as a polar (hydrophilic) region. The
hydrophobic region interacts with the large lipid molecules, whereas the
hydrophilic region interacts with the watery chyme in the intestine. This
results in the large lipid globules being pulled apart into many tiny lipid
fragments of about 1 ym in diameter. This change dramatically increases
the surface area available for lipid-digesting enzyme activity. This is the
same way dish soap works on fats mixed with water.
Bile salts act as emulsifying agents, so they are also important for the
absorption of digested lipids. While most constituents of bile are eliminated
in feces, bile salts are reclaimed by the enterohepatic circulation. Once
bile salts reach the ileum, they are absorbed and returned to the liver in the
hepatic portal blood. The hepatocytes then excrete the bile salts into newly
formed bile. Thus, this precious resource is recycled.
Bilirubin, the main bile pigment, is a waste product produced when the
spleen removes old or damaged red blood cells from the circulation. These
breakdown products, including proteins, iron, and toxic bilirubin, are
transported to the liver via the splenic vein of the hepatic portal system. In
the liver, proteins and iron are recycled, whereas bilirubin is excreted in the
bile. It accounts for the green color of bile. Bilirubin is eventually
transformed by intestinal bacteria into stercobilin, a brown pigment that
gives your stool its characteristic color! In some disease states, bile does not
enter the intestine, resulting in white (‘acholic’) stool with a high fat
content, since virtually no fats are broken down or absorbed.
Hepatocytes work non-stop, but bile production increases when fatty chyme
enters the duodenum and stimulates the secretion of the gut hormone
secretin. Between meals, bile is produced but conserved. The valve-like
hepatopancreatic ampulla closes, allowing bile to divert to the gallbladder,
where it is concentrated and stored until the next meal.
Note:
. .
mss’ OPENStax COLLEGE
Watch this video to see the structure of the liver and how this structure
supports the functions of the liver, including the processing of nutrients,
toxins, and wastes. At rest, about 1500 mL of blood per minute flow
through the liver. What percentage of this blood flow comes from the
hepatic portal system?
The Pancreas
The soft, oblong, glandular pancreas lies transversely in the
retroperitoneum behind the stomach. Its head is nestled into the “c-shaped”
curvature of the duodenum with the body extending to the left about 15.2
cm (6 in) and ending as a tapering tail in the hilum of the spleen. It is a
curious mix of exocrine (secreting digestive enzymes) and endocrine
(releasing hormones into the blood) functions ([link]).
Exocrine and Endocrine Pancreas
Common bile duct Pancreatic duct
Tail of
pancreas
Lobules
Acinar cells secrete
Head of pancreas digestive enzymes.
Pancreatic islet) _ ® Sie as
cells secrete We =
hormones. ‘@
Pancreatic duct
Exocrine cells secrete pancreatic juice.
The pancreas has a head, a body, and a
tail. It delivers pancreatic juice to the
duodenum through the pancreatic
duct.
The exocrine part of the pancreas arises as little grape-like cell clusters,
each called an acinus (plural = acini), located at the terminal ends of
pancreatic ducts. These acinar cells secrete enzyme-rich pancreatic juice
into tiny merging ducts that form two dominant ducts. The larger duct fuses
with the common bile duct (carrying bile from the liver and gallbladder)
just before entering the duodenum via a common opening (the
hepatopancreatic ampulla). The smooth muscle sphincter of the
hepatopancreatic ampulla controls the release of pancreatic juice and bile
into the small intestine. The second and smaller pancreatic duct, the
accessory duct (duct of Santorini), runs from the pancreas directly into the
duodenum, approximately 1 inch above the hepatopancreatic ampulla.
When present, it is a persistent remnant of pancreatic development.
Scattered through the sea of exocrine acini are small islands of endocrine
cells, the islets of Langerhans. These vital cells produce the hormones
pancreatic polypeptide, insulin, glucagon, and somatostatin.
Pancreatic Juice
The pancreas produces over a liter of pancreatic juice each day. Unlike bile,
it is clear and composed mostly of water along with some salts, sodium
bicarbonate, and several digestive enzymes. Sodium bicarbonate is
responsible for the slight alkalinity of pancreatic juice (pH 7.1 to 8.2),
which serves to buffer the acidic gastric juice in chyme, inactivate pepsin
from the stomach, and create an optimal environment for the activity of pH-
sensitive digestive enzymes in the small intestine. Pancreatic enzymes are
active in the digestion of sugars, proteins, and fats.
The pancreas produces protein-digesting enzymes in their inactive forms.
These enzymes are activated in the duodenum. If produced in an active
form, they would digest the pancreas (which is exactly what occurs in the
disease, pancreatitis). The intestinal brush border enzyme enteropeptidase
stimulates the activation of trypsin from trypsinogen of the pancreas, which
in turn changes the pancreatic enzymes procarboxypeptidase and
chymotrypsinogen into their active forms, carboxypeptidase and
chymotrypsin.
The enzymes that digest starch (amylase), fat (lipase), and nucleic acids
(nuclease) are secreted in their active forms, since they do not attack the
pancreas as do the protein-digesting enzymes.
Pancreatic Secretion
Regulation of pancreatic secretion is the job of hormones and the
parasympathetic nervous system. The entry of acidic chyme into the
duodenum stimulates the release of secretin, which in turn causes the duct
cells to release bicarbonate-rich pancreatic juice. The presence of proteins
and fats in the duodenum stimulates the secretion of CCK, which then
stimulates the acini to secrete enzyme-rich pancreatic juice and enhances
the activity of secretin. Parasympathetic regulation occurs mainly during
the cephalic and gastric phases of gastric secretion, when vagal stimulation
prompts the secretion of pancreatic juice.
Usually, the pancreas secretes just enough bicarbonate to counterbalance the
amount of HCl produced in the stomach. Hydrogen ions enter the blood
when bicarbonate is secreted by the pancreas. Thus, the acidic blood
draining from the pancreas neutralizes the alkaline blood draining from the
stomach, maintaining the pH of the venous blood that flows to the liver.
The Gallbladder
The gallbladder is 8—10 cm (~3-4 in) long and is nested in a shallow area
on the posterior aspect of the right lobe of the liver. This muscular sac
stores, concentrates, and, when stimulated, propels the bile into the
duodenum via the common bile duct. It is divided into three regions. The
fundus is the widest portion and tapers medially into the body, which in turn
narrows to become the neck. The neck angles slightly superiorly as it
approaches the hepatic duct. The cystic duct is 1—2 cm (less than 1 in) long
and turns inferiorly as it bridges the neck and hepatic duct.
The simple columnar epithelium of the gallbladder mucosa is organized in
rugae, similar to those of the stomach. There is no submucosa in the
gallbladder wall. The wall’s middle, muscular coat is made of smooth
muscle fibers. When these fibers contract, the gallbladder’s contents are
ejected through the cystic duct and into the bile duct ([link]). Visceral
peritoneum reflected from the liver capsule holds the gallbladder against the
liver and forms the outer coat of the gallbladder. The gallbladder's mucosa
absorbs water and ions from bile, concentrating it by up to 10-fold.
Gallbladder
Left hepatic duct
Right hepatic duct
Cystic duct
Gallbladder:
Body
Fundus
Neck
Common
hepatic duct
F Common
Liver bile duct
The gallbladder stores and concentrates
bile, and releases it into the two-way
cystic duct when it is needed by the small
intestine.
Chapter Review
Chemical digestion in the small intestine cannot occur without the help of
the liver and pancreas. The liver produces bile and delivers it to the
common hepatic duct. Bile contains bile salts and phospholipids, which
emulsify large lipid globules into tiny lipid droplets, a necessary step in
lipid digestion and absorption. The gallbladder stores and concentrates bile,
releasing it when it is needed by the small intestine.
The pancreas produces the enzyme- and bicarbonate-rich pancreatic juice
and delivers it to the small intestine through ducts. Pancreatic juice buffers
the acidic gastric juice in chyme, inactivates pepsin from the stomach, and
enables the optimal functioning of digestive enzymes in the small intestine.
Interactive Link Questions
Exercise:
Problem:
Watch this video to see the structure of the liver and how this structure
supports the functions of the liver, including the processing of
nutrients, toxins, and wastes. At rest, about 1500 mL of blood per
minute flow through the liver. What percentage of this blood flow
comes from the hepatic portal system?
Solution:
Answers may vary.
Review Questions
Exercise:
Problem: Which of these statements about bile is true?
a. About 500 mL is secreted daily.
b. Its main function is the denaturation of proteins.
c. It is synthesized in the gallbladder.
d. Bile salts are recycled.
Solution:
D
Exercise:
Problem: Pancreatic juice
a. deactivates bile.
b. is secreted by pancreatic islet cells.
c. buffers chyme.
d. is released into the cystic duct.
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Why does the pancreas secrete some enzymes in their inactive forms,
and where are these enzymes activated?
Solution:
The pancreas secretes protein-digesting enzymes in their inactive
forms. If secreted in their active forms, they would self-digest the
pancreas. These enzymes are activated in the duodenum.
Exercise:
Problem:
Describe the location of hepatocytes in the liver and how this
arrangement enhances their function.
Solution:
The hepatocytes are the main cell type of the liver. They process, store,
and release nutrients into the blood. Radiating out from the central
vein, they are tightly packed around the hepatic sinusoids, allowing the
hepatocytes easy access to the blood flowing through the sinusoids.
Glossary
accessory duct
(also, duct of Santorini) duct that runs from the pancreas into the
duodenum
acinus
cluster of glandular epithelial cells in the pancreas that secretes
pancreatic juice in the pancreas
bile
alkaline solution produced by the liver and important for the
emulsification of lipids
bile canaliculus
small duct between hepatocytes that collects bile
bilirubin
main bile pigment, which is responsible for the brown color of feces
central vein
vein that receives blood from hepatic sinusoids
common bile duct
structure formed by the union of the common hepatic duct and the
gallbladder’s cystic duct
common hepatic duct
duct formed by the merger of the two hepatic ducts
cystic duct
duct through which bile drains and enters the gallbladder
enterohepatic circulation
recycling mechanism that conserves bile salts
enteropeptidase
intestinal brush-border enzyme that activates trypsinogen to trypsin
gallbladder
accessory digestive organ that stores and concentrates bile
hepatic artery
artery that supplies oxygenated blood to the liver
hepatic lobule
hexagonal-shaped structure composed of hepatocytes that radiate
outward from a central vein
hepatic portal vein
vein that supplies deoxygenated nutrient-rich blood to the liver
hepatic sinusoid
blood capillaries between rows of hepatocytes that receive blood from
the hepatic portal vein and the branches of the hepatic artery
hepatic vein
vein that drains into the inferior vena cava
hepatocytes
major functional cells of the liver
liver
largest gland in the body whose main digestive function is the
production of bile
pancreas
accessory digestive organ that secretes pancreatic juice
pancreatic juice
secretion of the pancreas containing digestive enzymes and
bicarbonate
porta hepatis
“gateway to the liver” where the hepatic artery and hepatic portal vein
enter the liver
portal triad
bile duct, hepatic artery branch, and hepatic portal vein branch
reticuloendothelial cell
(also, Kupffer cell) phagocyte in hepatic sinusoids that filters out
material from venous blood from the alimentary canal
Anatomy of the Lymphatic and Immune Systems
By the end of this section, you will be able to:
e Describe the structure and function of the lymphatic tissue (lymph
fluid, vessels, ducts, and organs)
e Describe the structure and function of the primary and secondary
lymphatic organs
e Discuss the cells of the immune system, how they function, and their
relationship with the lymphatic system
The immune system is the complex collection of cells and organs that
destroys or neutralizes pathogens that would otherwise cause disease or
death. The lymphatic system, for most people, is associated with the
immune system to such a degree that the two systems are virtually
indistinguishable. The lymphatic system is the system of vessels, cells, and
organs that carries excess fluids to the bloodstream and filters pathogens
from the blood. The swelling of lymph nodes during an infection and the
transport of lymphocytes via the lymphatic vessels are but two examples of
the many connections between these critical organ systems.
Functions of the Lymphatic System
A major function of the lymphatic system is to drain body fluids and return
them to the bloodstream. Blood pressure causes leakage of fluid from the
capillaries, resulting in the accumulation of fluid in the interstitial space—
that is, spaces between individual cells in the tissues. In humans, 20 liters of
plasma is released into the interstitial space of the tissues each day due to
capillary filtration. Once this filtrate is out of the bloodstream and in the
tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is
reabsorbed directly by the blood vessels. But what happens to the remaining
three liters? This is where the lymphatic system comes into play. It drains
the excess fluid and empties it back into the bloodstream via a series of
vessels, trunks, and ducts. Lymph is the term used to describe interstitial
fluid once it has entered the lymphatic system. When the lymphatic system
is damaged in some way, such as by being blocked by cancer cells or
destroyed by injury, protein-rich interstitial fluid accumulates (sometimes
“backs up” from the lymph vessels) in the tissue spaces. This inappropriate
accumulation of fluid referred to as lymphedema may lead to serious
medical consequences.
As the vertebrate immune system evolved, the network of lymphatic vessels
became convenient avenues for transporting the cells of the immune
system. Additionally, the transport of dietary lipids and fat-soluble vitamins
absorbed in the gut uses this system.
Cells of the immune system not only use lymphatic vessels to make their
way from interstitial spaces back into the circulation, but they also use
lymph nodes as major staging areas for the development of critical immune
responses. A lymph node is one of the small, bean-shaped organs located
throughout the lymphatic system.
Note:
openstax COLLEGE”
Visit this website for an overview of the lymphatic system. What are the
three main components of the lymphatic system?
Structure of the Lymphatic System
The lymphatic vessels begin as open-ended capillaries, which feed into
larger and larger lymphatic vessels, and eventually empty into the
bloodstream by a series of ducts. Along the way, the lymph travels through
the lymph nodes, which are commonly found near the groin, armpits, neck,
chest, and abdomen. Humans have about 500—600 lymph nodes throughout
the body ([link]).
Anatomy of the Lymphatic System
Adenoid
Tonsil
J
Ah Lymph nodes
_ j 4\ Thymus
| Lymphatic
| vessel
; 7 Thymus
Right lymphatic duct,
entering vein
Tissue cell
Spleen
Interstitial
fluid
capillary ot
Lymphatic
capillary
Masses of lymphocytes
and macrophages
Bone marrow
Lymph node
Lymphatic vessels in the arms and legs convey lymph
to the larger lymphatic vessels in the torso.
A major distinction between the lymphatic and cardiovascular systems in
humans is that lymph is not actively pumped by the heart, but is forced
through the vessels by the movements of the body, the contraction of
skeletal muscles during body movements, and breathing. One-way valves
(semi-lunar valves) in lymphatic vessels keep the lymph moving toward the
heart. Lymph flows from the lymphatic capillaries, through lymphatic
vessels, and then is dumped into the circulatory system via the lymphatic
ducts located at the junction of the jugular and subclavian veins in the neck.
Lymphatic Capillaries
Lymphatic capillaries, also called the terminal lymphatics, are vessels
where interstitial fluid enters the lymphatic system to become lymph fluid.
Located in almost every tissue in the body, these vessels are interlaced
among the arterioles and venules of the circulatory system in the soft
connective tissues of the body ({link]). Exceptions are the central nervous
system, bone marrow, bones, teeth, and the cornea of the eye, which do not
contain lymph vessels.
Lymphatic Capillaries
Lymph capillaries in the tissue spaces
Lymph capillary.
Gy x Collagen fiber
Arteriole
Interstitial fluid
Lymph (interstitial fluid)
x Lymphatic
vessel
Tissue fluid
Endothelial
“flaps”
Lymph vessel
endothelial cells
prevention
valve
Lymphatic capillaries are interlaced with the arterioles
and venules of the cardiovascular system. Collagen
fibers anchor a lymphatic capillary in the tissue (inset).
Interstitial fluid slips through spaces between the
overlapping endothelial cells that compose the lymphatic
capillary.
Lymphatic capillaries are formed by a one cell-thick layer of endothelial
cells and represent the open end of the system, allowing interstitial fluid to
flow into them via overlapping cells (see [link]). When interstitial pressure
is low, the endothelial flaps close to prevent “backflow.” As interstitial
pressure increases, the spaces between the cells open up, allowing the fluid
to enter. Entry of fluid into lymphatic capillaries is also enabled by the
collagen filaments that anchor the capillaries to surrounding structures. As
interstitial pressure increases, the filaments pull on the endothelial cell
flaps, opening up them even further to allow easy entry of fluid.
In the small intestine, lymphatic capillaries called lacteals are critical for the
transport of dietary lipids and lipid-soluble vitamins to the bloodstream. In
the small intestine, dietary triglycerides combine with other lipids and
proteins, and enter the lacteals to form a milky fluid called chyle. The chyle
then travels through the lymphatic system, eventually entering the
bloodstream.
Larger Lymphatic Vessels, Trunks, and Ducts
The lymphatic capillaries empty into larger lymphatic vessels, which are
similar to veins in terms of their three-tunic structure and the presence of
valves. These one-way valves are located fairly close to one another, and
each one causes a bulge in the lymphatic vessel, giving the vessels a beaded
appearance (see [link]).
The superficial and deep lymphatics eventually merge to form larger
lymphatic vessels known as lymphatic trunks. On the right side of the
body, the right sides of the head, thorax, and right upper limb drain lymph
fluid into the right subclavian vein via the right lymphatic duct ([link]). On
the left side of the body, the remaining portions of the body drain into the
larger thoracic duct, which drains into the left subclavian vein. The thoracic
duct itself begins just beneath the diaphragm in the cisterna chyli, a sac-
like chamber that receives lymph from the lower abdomen, pelvis, and
lower limbs by way of the left and right lumbar trunks and the intestinal
trunk.
Major Trunks and Ducts of the Lymphatic System
Right lymphatic Right internal Left internal Thoracic duct
duct jugular vein jugular vein drains into
subclavian vein
Left subclavian vein
Right subclavian vein
Thoracic duct
Cisterna chyli of
Drained by right thoracic duct
lymphatic duct
Drained by
thoracic duct
The thoracic duct drains a much larger portion of the
body than does the right lymphatic duct.
The overall drainage system of the body is asymmetrical (see [link]). The
right lymphatic duct receives lymph from only the upper right side of the
body. The lymph from the rest of the body enters the bloodstream through
the thoracic duct via all the remaining lymphatic trunks. In general,
lymphatic vessels of the subcutaneous tissues of the skin, that is, the
superficial lymphatics, follow the same routes as veins, whereas the deep
lymphatic vessels of the viscera generally follow the paths of arteries.
The Organization of Immune Function
The immune system is a collection of barriers, cells, and soluble proteins
that interact and communicate with each other in extraordinarily complex
ways. The modern model of immune function is organized into three phases
based on the timing of their effects. The three temporal phases consist of the
following:
¢ Barrier defenses such as the skin and mucous membranes, which act
instantaneously to prevent pathogenic invasion into the body tissues
e The rapid but nonspecific innate immune response, which consists of
a variety of specialized cells and soluble factors
e The slower but more specific and effective adaptive immune
response, which involves many cell types and soluble factors, but is
primarily controlled by white blood cells (leukocytes) known as
lymphocytes, which help control immune responses
The cells of the blood, including all those involved in the immune response,
arise in the bone marrow via various differentiation pathways from
hematopoietic stem cells ({link]). In contrast with embryonic stem cells,
hematopoietic stem cells are present throughout adulthood and allow for the
continuous differentiation of blood cells to replace those lost to age or
function. These cells can be divided into three classes based on function:
e Phagocytic cells, which ingest pathogens to destroy them
e Lymphocytes, which specifically coordinate the activities of adaptive
immunity
¢ Cells containing cytoplasmic granules, which help mediate immune
responses against parasites and intracellular pathogens such as viruses
Hematopoietic System of the Bone Marrow
After division some cells
remain stem cells.
Multipotent hematopoietic
stem cell (hemocytoblast)
a
e
, The remaining cell goes down one of two paths
@ depending on the chemical signals received.
e
Myeloid stem cell Lymphoid stem cell
Megakaryoblast Proerythroblast Myeloblast Monoblast Lymphoblast
@
Reticulocyte
=i oo
yy rc ae Le Small lymphocyte
ork A 2 : —_— arge granular
Mm @ ovr YY
| — “
Megakaryocyte Erythrocyte Basophil Neutrophil Eosinophil Monocyte sy) @&
T lymphocyte —_B lymphocyte
b)
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Ad)
BY)
7
7X SS
Platelets 7) => o
° Plasma cell
Macrophage
All the cells of the immune response as well as of the blood arise by
differentiation from hematopoietic stem cells. Platelets are cell
fragments involved in the clotting of blood.
Lymphocytes: B Cells, T Cells, Plasma Cells, and Natural
Killer Cells
As stated above, lymphocytes are the primary cells of adaptive immune
responses ({link]). The two basic types of lymphocytes, B cells and T cells,
are identical morphologically with a large central nucleus surrounded by a
thin layer of cytoplasm. They are distinguished from each other by their
surface protein markers as well as by the molecules they secrete. While B
cells mature in red bone marrow and T cells mature in the thymus, they
both initially develop from bone marrow. T cells migrate from bone marrow
to the thymus gland where they further mature. B cells and T cells are found
in many parts of the body, circulating in the bloodstream and lymph, and
residing in secondary lymphoid organs, including the spleen and lymph
nodes, which will be described later in this section. The human body
contains approximately 10'* lymphocytes.
B Cells
B cells are immune cells that function primarily by producing antibodies.
An antibody is any of the group of proteins that binds specifically to
pathogen-associated molecules known as antigens. An antigen is a
chemical structure on the surface of a pathogen that binds to T or B
lymphocyte antigen receptors. Once activated by binding to antigen, B cells
differentiate into cells that secrete a soluble form of their surface antibodies.
These activated B cells are known as plasma cells.
T Cells
The T cell, on the other hand, does not secrete antibody but performs a
variety of functions in the adaptive immune response. Different T cell types
have the ability to either secrete soluble factors that communicate with
other cells of the adaptive immune response or destroy cells infected with
intracellular pathogens. The roles of T and B lymphocytes in the adaptive
immune response will be discussed further in this chapter.
Plasma Cells
Another type of lymphocyte of importance is the plasma cell. A plasma cell
is a B cell that has differentiated in response to antigen binding, and has
thereby gained the ability to secrete soluble antibodies. These cells differ in
morphology from standard B and T cells in that they contain a large amount
of cytoplasm packed with the protein-synthesizing machinery known as
rough endoplasmic reticulum.
Natural Killer Cells
A fourth important lymphocyte is the natural killer cell, a participant in the
innate immune response. A natural killer cell (NK) is a circulating blood
cell that contains cytotoxic (cell-killing) granules in its extensive
cytoplasm. It shares this mechanism with the cytotoxic T cells of the
adaptive immune response. NK cells are among the body’s first lines of
defense against viruses and certain types of cancer.
Lymphocytes
Type of lymphocyte Primary function
B lymphocyte Generates diverse antibodies
T lymphocyte Secretes chemical messengers
Plasma cell Secretes antibodies
NK cell Destroys virally infected cells
Note:
Visit this website to learn about the many different cell types in the
immune system and their very specialized jobs. What is the role of the
dendritic cell in an HIV infection?
Primary Lymphoid Organs and Lymphocyte Development
Understanding the differentiation and development of B and T cells is
critical to the understanding of the adaptive immune response. It is through
this process that the body (ideally) learns to destroy only pathogens and
leaves the body’s own cells relatively intact. The primary lymphoid
organs are the bone marrow and thymus gland. The lymphoid organs are
where lymphocytes mature, proliferate, and are selected, which enables
them to attack pathogens without harming the cells of the body.
Bone Marrow
In the embryo, blood cells are made in the yolk sac. As development
proceeds, this function is taken over by the spleen, lymph nodes, and liver.
Later, the bone marrow takes over most hematopoietic functions, although
the final stages of the differentiation of some cells may take place in other
organs. The red bone marrow is a loose collection of cells where
hematopoiesis occurs, and the yellow bone marrow is a site of energy
storage, which consists largely of fat cells ({link]). The B cell undergoes
nearly all of its development in the red bone marrow, whereas the immature
T cell, called a thymocyte, leaves the bone marrow and matures largely in
the thymus gland.
Bone Marrow
Red bone marrow fills the head
of the femur, and a spot of
yellow bone marrow is visible
in the center. The white
reference bar is 1 cm.
Thymus
The thymus gland is a bilobed organ found in the space between the
sternum and the aorta of the heart ({link]). Connective tissue holds the lobes
closely together but also separates them and forms a capsule.
Location, Structure, and Histology of the Thymus
Cortex Trabeculae
Fibrous
capsule
Right lymphatic duct,
entering vein
Lymph nodes
Cortical epithelial cell Thymocytes _Trabecula
Heart
FONG oy Hai eee —}— Fibrous
Oma \ Wows ke capsule
3 5, e: . Cortex
t <9
Spleen A a Yo a Medulla
Dendritic cell », ry. t
Macrophage Blood vessel Medullary
epithelial cell
The thymus lies above the heart. The trabeculae and
lobules, including the darkly staining cortex and the
lighter staining medulla of each lobule, are clearly
visible in the light micrograph of the thymus of a
newborn. LM x 100. (Micrograph provided by the
Regents of the University of Michigan Medical School
© 2012)
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
The connective tissue capsule further divides the thymus into lobules via
extensions called trabeculae. The outer region of the organ is known as the
cortex and contains large numbers of thymocytes with some epithelial cells,
macrophages, and dendritic cells (two types of phagocytic cells that are
derived from monocytes). The cortex is densely packed so it stains more
intensely than the rest of the thymus (see [link]). The medulla, where
thymocytes migrate before leaving the thymus, contains a less dense
collection of thymocytes, epithelial cells, and dendritic cells.
Note:
Aging and the...
Immune System
By the year 2050, 25 percent of the population of the United States will be
60 years of age or older. The CDC estimates that 80 percent of those 60
years and older have one or more chronic disease associated with
deficiencies of the immune systems. This loss of immune function with age
is called immunosenescence. To treat this growing population, medical
professionals must better understand the aging process. One major cause of
age-related immune deficiencies is thymic involution, the shrinking of the
thymus gland that begins at birth, at a rate of about three percent tissue loss
per year, and continues until 35-45 years of age, when the rate declines to
about one percent loss per year for the rest of one’s life. At that pace, the
total loss of thymic epithelial tissue and thymocytes would occur at about
120 years of age. Thus, this age is a theoretical limit to a healthy human
lifespan.
Thymic involution has been observed in all vertebrate species that have a
thymus gland. Animal studies have shown that transplanted thymic grafts
between inbred strains of mice involuted according to the age of the donor
and not of the recipient, implying the process is genetically programmed.
There is evidence that the thymic microenvironment, so vital to the
development of naive T cells, loses thymic epithelial cells according to the
decreasing expression of the FOXNI1 gene with age.
It is also known that thymic involution can be altered by hormone levels.
Sex hormones such as estrogen and testosterone enhance involution, and
the hormonal changes in pregnant women cause a temporary thymic
involution that reverses itself, when the size of the thymus and its hormone
levels return to normal, usually after lactation ceases. What does all this
tell us? Can we reverse immunosenescence, or at least slow it down? The
potential is there for using thymic transplants from younger donors to keep
thymic output of naive T cells high. Gene therapies that target gene
expression are also seen as future possibilities. The more we learn through
immunosenescence research, the more opportunities there will be to
develop therapies, even though these therapies will likely take decades to
develop. The ultimate goal is for everyone to live and be healthy longer,
but there may be limits to immortality imposed by our genes and
hormones.
Secondary Lymphoid Organs and their Roles in Active
Immune Responses
Lymphocytes develop and mature in the primary lymphoid organs, but they
mount immune responses from the secondary lymphoid organs. A naive
lymphocyte is one that has left the primary organ and entered a secondary
lymphoid organ. Naive lymphocytes are fully functional immunologically,
but have yet to encounter an antigen to respond to. In addition to circulating
in the blood and lymph, lymphocytes concentrate in secondary lymphoid
organs, which include the lymph nodes, spleen, and lymphoid nodules. All
of these tissues have many features in common, including the following:
e The presence of lymphoid follicles, the sites of the formation of
lymphocytes, with specific B cell-rich and T cell-rich areas
e An internal structure of reticular fibers with associated fixed
macrophages
¢ Germinal centers, which are the sites of rapidly dividing and
differentiating B lymphocytes
e Specialized post-capillary vessels known as high endothelial venules;
the cells lining these venules are thicker and more columnar than
normal endothelial cells, which allow cells from the blood to directly
enter these tissues
Lymph Nodes
Lymph nodes function to remove debris and pathogens from the lymph, and
are thus sometimes referred to as the “filters of the lymph” ({link]). Any
bacteria that infect the interstitial fluid are taken up by the lymphatic
capillaries and transported to a regional lymph node. Dendritic cells and
macrophages within this organ internalize and kill many of the pathogens
that pass through, thereby removing them from the body. The lymph node is
also the site of adaptive immune responses mediated by T cells, B cells, and
accessory Cells of the adaptive immune system. Like the thymus, the bean-
shaped lymph nodes are surrounded by a tough capsule of connective tissue
and are separated into compartments by trabeculae, the extensions of the
capsule. In addition to the structure provided by the capsule and trabeculae,
the structural support of the lymph node is provided by a series of reticular
fibers laid down by fibroblasts.
Structure and Histology of a Lymph Node
Efferent lymphatic
Connective tissue Cortex vessels
capsule
Ofc Z
. J
ie : Rive.
fad | 2 Prats
fe 2,
~~ Roh . ‘
ey 3 :
7 ‘ 3 ~
i
-
Connective
tissue capsule
Subcapsular
Subcapsular
sinus Afferent lymphatic vessels
Lymph nodes are masses of lymphatic tissue located
along the larger lymph vessels. The micrograph of the
lymph nodes shows a germinal center, which consists of
rapidly dividing B cells surrounded by a layer of T cells
and other accessory cells. LM x 128. (Micrograph
provided by the Regents of the University of Michigan
Medical School © 2012)
Note:
[=] [a
ro
= pense COLLEGE
. rg
Ott lt
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
The major routes into the lymph node are via afferent lymphatic vessels
(see [link]). Cells and lymph fluid that leave the lymph node may do so by
another set of vessels known as the efferent lymphatic vessels. Lymph
enters the lymph node via the subcapsular sinus, which is occupied by
dendritic cells, macrophages, and reticular fibers. Within the cortex of the
lymph node are lymphoid follicles, which consist of germinal centers of
rapidly dividing B cells surrounded by a layer of T cells and other accessory
cells. As the lymph continues to flow through the node, it enters the
medulla, which consists of medullary cords of B cells and plasma cells, and
the medullary sinuses where the lymph collects before leaving the node via
the efferent lymphatic vessels.
Spleen
In addition to the lymph nodes, the spleen is a major secondary lymphoid
organ ({link]). It is about 12 cm (5 in) long and is attached to the lateral
border of the stomach via the gastrosplenic ligament. The spleen is a fragile
organ without a strong capsule, and is dark red due to its extensive
vascularization. The spleen is sometimes called the “filter of the blood”
because of its extensive vascularization and the presence of macrophages
and dendritic cells that remove microbes and other materials from the
blood, including dying red blood cells. The spleen also functions as the
location of immune responses to blood-borne pathogens.
Spleen
(a) Cross section of the spleen Hilum
Trabecula
Splenic vein
Diaphragm White pulp Arteriole Venule
Trabecula
Marginal zone
Central artery or
arteriole
= Germinal center
Arterial capillaries =< = 2S
2
at
Bp 4
Venous sinus
(a) The spleen is attached to the stomach. (b) A
micrograph of spleen tissue shows the germinal center.
The marginal zone is the region between the red pulp and
white pulp, which sequesters particulate antigens from
the circulation and presents these antigens to
lymphocytes in the white pulp. EM x 660. (Micrograph
provided by the Regents of the University of Michigan
Medical School © 2012)
The spleen is also divided by trabeculae of connective tissue, and within
each splenic nodule is an area of red pulp, consisting of mostly red blood
cells, and white pulp, which resembles the lymphoid follicles of the lymph
nodes. Upon entering the spleen, the splenic artery splits into several
arterioles (surrounded by white pulp) and eventually into sinusoids. Blood
from the capillaries subsequently collects in the venous sinuses and leaves
via the splenic vein. The red pulp consists of reticular fibers with fixed
macrophages attached, free macrophages, and all of the other cells typical
of the blood, including some lymphocytes. The white pulp surrounds a
central arteriole and consists of germinal centers of dividing B cells
surrounded by T cells and accessory cells, including macrophages and
dendritic cells. Thus, the red pulp primarily functions as a filtration system
of the blood, using cells of the relatively nonspecific immune response, and
white pulp is where adaptive T and B cell responses are mounted.
Lymphoid Nodules
The other lymphoid tissues, the lymphoid nodules, have a simpler
architecture than the spleen and lymph nodes in that they consist of a dense
cluster of lymphocytes without a surrounding fibrous capsule. These
nodules are located in the respiratory and digestive tracts, areas routinely
exposed to environmental pathogens.
Tonsils are lymphoid nodules located along the inner surface of the pharynx
and are important in developing immunity to oral pathogens ([link]). The
tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes
referred to as the adenoid when swollen. Such swelling is an indication of
an active immune response to infection. Histologically, tonsils do not
contain a complete capsule, and the epithelial layer invaginates deeply into
the interior of the tonsil to form tonsillar crypts. These structures, which
accumulate all sorts of materials taken into the body through eating and
breathing, actually “encourage” pathogens to penetrate deep into the
tonsillar tissues where they are acted upon by numerous lymphoid follicles
and eliminated. This seems to be the major function of tonsils—to help
children’s bodies recognize, destroy, and develop immunity to common
environmental pathogens so that they will be protected in their later lives.
Tonsils are often removed in those children who have recurring throat
infections, especially those involving the palatine tonsils on either side of
the throat, whose swelling may interfere with their breathing and/or
swallowing.
Locations and Histology of the Tonsils
(a) Locations of the tonsils
Brain
Palatine Sphenoidal sinus
tonsil
Palatine Sphenoid bone
bone
Tongue ie in
Mandible Nasopharynx
Hyoid
Trachea
Esophagus
Palatine Hard palate
tonsil
Soft palate
Uvula
Lingual
tonsil Palatine tonsils
(swollen due to infection)
Epiglottis
Tongue
(b) Histology of palatine tonsil
Crypt
Stratified
squamous
epithelium
Germinal
centers
(a) The pharyngeal tonsil is located on the roof of the
posterior superior wall of the nasopharynx. The
palatine tonsils lay on each side of the pharynx. (b) A
micrograph shows the palatine tonsil tissue. LM x 40.
(Micrograph provided by the Regents of the
University of Michigan Medical School © 2012)
Note:
—
mess OPenstax COLLEGE
Poo
-
fi
ae
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of
lymphoid follicles directly associated with the mucous membrane epithelia.
MALT makes up dome-shaped structures found underlying the mucosa of
the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a
type of MALT in the small intestine, are especially important for immune
responses against ingested substances ({link]). Peyer’s patches contain
specialized endothelial cells called M (or microfold) cells that sample
material from the intestinal lumen and transport it to nearby follicles so that
adaptive immune responses to potential pathogens can be mounted.
Mucosa-associated Lymphoid Tissue (MALT) Nodule
Peyer's patches
LM ~x 40. (Micrograph provided by the Regents of the
University of Michigan Medical School © 2012)
Bronchus-associated lymphoid tissue (BALT) consists of lymphoid
follicular structures with an overlying epithelial layer found along the
bifurcations of the bronchi, and between bronchi and arteries. They also
have the typically less-organized structure of other lymphoid nodules.
These tissues, in addition to the tonsils, are effective against inhaled
pathogens.
Chapter Review
The lymphatic system is a series of vessels, ducts, and trunks that remove
interstitial fluid from the tissues and return it the blood. The lymphatics are
also used to transport dietary lipids and cells of the immune system. Cells
of the immune system all come from the hematopoietic system of the bone
marrow. Primary lymphoid organs, the bone marrow and thymus gland, are
the locations where lymphocytes of the adaptive immune system proliferate
and mature. Secondary lymphoid organs are site in which mature
lymphocytes congregate to mount immune responses. Many immune
system cells use the lymphatic and circulatory systems for transport
throughout the body to search for and then protect against pathogens.
Interactive Link Questions
Exercise:
Problem:
Visit this website for an overview of the lymphatic system. What are
the three main components of the lymphatic system?
Solution:
The three main components are the lymph vessels, the lymph nodes,
and the lymph.
Exercise:
Problem:
Visit this website to learn about the many different cell types in the
immune system and their very specialized jobs. What is the role of the
dendritic cell in infection by HIV?
Solution:
The dendritic cell transports the virus to a lymph node.
Review Questions
Exercise:
Problem: Which of the following cells is phagocytic?
a. plasma cell
b. macrophage
c. B cell
d. NK cell
Solution:
B
Exercise:
Problem:
Which structure allows lymph from the lower right limb to enter the
bloodstream?
a. thoracic duct
b. right lymphatic duct
c. right lymphatic trunk
d. left lymphatic trunk
Solution:
A
Exercise:
Problem:
Which of the following cells is important in the innate immune
response?
a. B cells
b. T cells
c. macrophages
d. plasma cells
Solution:
c
Exercise:
Problem:
Which of the following cells would be most active in early, antiviral
immune responses the first time one is exposed to pathogen?
a. macrophage
b. T cell
c. neutrophil
d. natural killer cell
Solution:
D
Exercise:
Problem:
Which of the lymphoid nodules is most likely to see food antigens
first?
a. tonsils
b. Peyer’s patches
c. bronchus-associated lymphoid tissue
d. mucosa-associated lymphoid tissue
Solution:
A
Critical Thinking Questions
Exercise:
Problem:
Describe the flow of lymph from its origins in interstitial fluid to its
emptying into the venous bloodstream.
Solution:
The lymph enters through lymphatic capillaries, and then into larger
lymphatic vessels. The lymph can only go in one direction due to
valves in the vessels. The larger lymphatics merge to form trunks that
enter into the blood via lymphatic ducts.
Glossary
adaptive immune response
relatively slow but very specific and effective immune response
controlled by lymphocytes
afferent lymphatic vessels
lead into a lymph node
antibody
antigen-specific protein secreted by plasma cells; immunoglobulin
antigen
molecule recognized by the receptors of B and T lymphocytes
barrier defenses
antipathogen defenses deriving from a barrier that physically prevents
pathogens from entering the body to establish an infection
B cells
lymphocytes that act by differentiating into an antibody-secreting
plasma cell
bone marrow
tissue found inside bones; the site of all blood cell differentiation and
maturation of B lymphocytes
bronchus-associated lymphoid tissue (BALT)
lymphoid nodule associated with the respiratory tract
chyle
lipid-rich lymph inside the lymphatic capillaries of the small intestine
cisterna chyli
bag-like vessel that forms the beginning of the thoracic duct
efferent lymphatic vessels
lead out of a lymph node
germinal centers
clusters of rapidly proliferating B cells found in secondary lymphoid
tissues
high endothelial venules
vessels containing unique endothelial cells specialized to allow
migration of lymphocytes from the blood to the lymph node
immune system
series of barriers, cells, and soluble mediators that combine to response
to infections of the body with pathogenic organisms
innate immune response
rapid but relatively nonspecific immune response
lymph
fluid contained within the lymphatic system
lymph node
one of the bean-shaped organs found associated with the lymphatic
vessels
lymphatic capillaries
smallest of the lymphatic vessels and the origin of lymph flow
lymphatic system
network of lymphatic vessels, lymph nodes, and ducts that carries
lymph from the tissues and back to the bloodstream.
lymphatic trunks
large lymphatics that collect lymph from smaller lymphatic vessels and
empties into the blood via lymphatic ducts
lymphocytes
white blood cells characterized by a large nucleus and small rim of
cytoplasm
lymphoid nodules
unencapsulated patches of lymphoid tissue found throughout the body
mucosa-associated lymphoid tissue (MALT)
lymphoid nodule associated with the mucosa
naive lymphocyte
mature B or T cell that has not yet encountered antigen for the first
time
natural killer cell (NK)
cytotoxic lymphocyte of innate immune response
plasma cell
differentiated B cell that is actively secreting antibody
primary lymphoid organ
site where lymphocytes mature and proliferate; red bone marrow and
thymus gland
right lymphatic duct
drains lymph fluid from the upper right side of body into the right
subclavian vein
secondary lymphoid organs
sites where lymphocytes mount adaptive immune responses; examples
include lymph nodes and spleen
spleen
secondary lymphoid organ that filters pathogens from the blood (white
pulp) and removes degenerating or damaged blood cells (red pulp)
T cell
lymphocyte that acts by secreting molecules that regulate the immune
system or by causing the destruction of foreign cells, viruses, and
cancer cells
thoracic duct
large duct that drains lymph from the lower limbs, left thorax, left
upper limb, and the left side of the head
thymocyte
immature T cell found in the thymus
thymus
primary lymphoid organ; where T lymphocytes proliferate and mature
tonsils
lymphoid nodules associated with the nasopharynx
Gross Anatomy of Urine Transport
By the end of this section, you will be able to:
e Identify the ureters, urinary bladder, and urethra, as well as their
location, structure, histology, and function
e Compare and contrast male and female urethras
e Describe the micturition reflex
e Describe voluntary and involuntary neural control of micturition
Rather than start with urine formation, this section will start with urine
excretion. Urine is a fluid of variable composition that requires specialized
structures to remove it from the body safely and efficiently. Blood is
filtered, and the filtrate is transformed into urine at a relatively constant rate
throughout the day. This processed liquid is stored until a convenient time
for excretion. All structures involved in the transport and storage of the
urine are large enough to be visible to the naked eye. This transport and
storage system not only stores the waste, but it protects the tissues from
damage due to the wide range of pH and osmolarity of the urine, prevents
infection by foreign organisms, and for the male, provides reproductive
functions.
Urethra
The urethra transports urine from the bladder to the outside of the body for
disposal. The urethra is the only urologic organ that shows any significant
anatomic difference between males and females; all other urine transport
structures are identical ([link]).
Female and Male Urethras
The urethra transports urine from the bladder to the outside of the
body. This image shows (a) a female urethra and (b) a male urethra.
Urinary bladder
Pubic bone Ureter
Ureter
Seminal
vesicle
Uterus —~_ Ductus
Urinary \A nN ~
bladder a .
Pubic bone
sas Feu bi | i
Clitoris ee 7 Vagina tactig ji:
Prostate
gland
Rectum
Anus
The urethra in both males and females begins inferior and central to the two
ureteral openings forming the three points of a triangular-shaped area at the
base of the bladder called the trigone (Greek tri- = “triangle” and the root
of the word “trigonometry”). The urethra tracks posterior and inferior to the
pubic symphysis (see [link ]a). In both males and females, the proximal
urethra is lined by transitional epithelium, whereas the terminal portion is a
nonkeratinized, stratified squamous epithelium. In the male,
pseudostratified columnar epithelium lines the urethra between these two
cell types. Voiding is regulated by an involuntary autonomic nervous
system-controlled internal urinary sphincter, consisting of smooth muscle
and voluntary skeletal muscle that forms the external urinary sphincter
below it.
Female Urethra
The external urethral orifice is embedded in the anterior vaginal wall
inferior to the clitoris, superior to the vaginal opening (introitus), and
medial to the labia minora. Its short length, about 4 cm, is less of a barrier
to fecal bacteria than the longer male urethra and the best explanation for
the greater incidence of UTI in women. Voluntary control of the external
urethral sphincter is a function of the pudendal nerve. It arises in the sacral
region of the spinal cord, traveling via the S2—S4 nerves of the sacral
plexus.
Male Urethra
The male urethra passes through the prostate gland immediately inferior to
the bladder before passing below the pubic symphysis (see [link]b). The
length of the male urethra varies between men but averages 20 cm in
length. It is divided into four regions: the preprostatic urethra, the prostatic
urethra, the membranous urethra, and the spongy or penile urethra. The
preprostatic urethra is very short and incorporated into the bladder wall.
The prostatic urethra passes through the prostate gland. During sexual
intercourse, it receives sperm via the ejaculatory ducts and secretions from
the seminal vesicles. Paired Cowper’s glands (bulbourethral glands)
produce and secrete mucus into the urethra to buffer urethral pH during
sexual stimulation. The mucus neutralizes the usually acidic environment
and lubricates the urethra, decreasing the resistance to ejaculation. The
membranous urethra passes through the deep muscles of the perineum,
where it is invested by the overlying urethral sphincters. The spongy urethra
exits at the tip (external urethral orifice) of the penis after passing through
the corpus spongiosum. Mucous glands are found along much of the length
of the urethra and protect the urethra from extremes of urine pH.
Innervation is the same in both males and females.
Bladder
The urinary bladder collects urine from both ureters ({link]). The bladder
lies anterior to the uterus in females, posterior to the pubic bone and
anterior to the rectum. During late pregnancy, its capacity is reduced due to
compression by the enlarging uterus, resulting in increased frequency of
urination. In males, the anatomy is similar, minus the uterus, and with the
addition of the prostate inferior to the bladder. The bladder is partially
retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered
“dome” projecting into the abdomen when the bladder is distended with
urine.
Bladder
Ureter
Peritoneum
Detrusor
muscle
Ureteral
, Transitional epithelium
openings
Lamina propria
Submucosa
Internal urethral
sphincter |
External urethral =a
sphincter
(a) (b)
(a) Anterior cross section of the bladder. (b) The detrusor
muscle of the bladder (source: monkey tissue) LM x
448. (Micrograph provided by the Regents of the
University of Michigan Medical School © 2012)
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
The bladder is a highly distensible organ comprised of irregular
crisscrossing bands of smooth muscle collectively called the detrusor
muscle. The interior surface is made of transitional cellular epithelium that
is structurally suited for the large volume fluctuations of the bladder. When
empty, it resembles columnar epithelia, but when stretched, it “transitions”
(hence the name) to a squamous appearance (see [link]). Volumes in adults
can range from nearly zero to 500-600 mL.
The detrusor muscle contracts with significant force in the young. The
bladder’s strength diminishes with age, but voluntary contractions of
abdominal skeletal muscles can increase intra-abdominal pressure to
promote more forceful bladder emptying. Such voluntary contraction is also
used in forceful defecation and childbirth.
Micturition Reflex
Micturition is a less-often used, but proper term for urination or voiding. It
results from an interplay of involuntary and voluntary actions by the
internal and external urethral sphincters. When bladder volume reaches
about 150 mL, an urge to void is sensed but is easily overridden. Voluntary
control of urination relies on consciously preventing relaxation of the
external urethral sphincter to maintain urinary continence. As the bladder
fills, subsequent urges become harder to ignore. Ultimately, voluntary
constraint fails with resulting incontinence, which will occur as bladder
volume approaches 300 to 400 mL.
Normal micturition is a result of stretch receptors in the bladder wall that
transmit nerve impulses to the sacral region of the spinal cord to generate a
spinal reflex. The resulting parasympathetic neural outflow causes
contraction of the detrusor muscle and relaxation of the involuntary internal
urethral sphincter. At the same time, the spinal cord inhibits somatic motor
neurons, resulting in the relaxation of the skeletal muscle of the external
urethral sphincter. The micturition reflex is active in infants but with
maturity, children learn to override the reflex by asserting external sphincter
control, thereby delaying voiding (potty training). This reflex may be
preserved even in the face of spinal cord injury that results in paraplegia or
quadriplegia. However, relaxation of the external sphincter may not be
possible in all cases, and therefore, periodic catheterization may be
necessary for bladder emptying.
Nerves involved in the control of urination include the hypogastric, pelvic,
and pudendal ([link]). Voluntary micturition requires an intact spinal cord
and functional pudendal nerve arising from the sacral micturition center.
Since the external urinary sphincter is voluntary skeletal muscle, actions by
cholinergic neurons maintain contraction (and thereby continence) during
filling of the bladder. At the same time, sympathetic nervous activity via the
hypogastric nerves suppresses contraction of the detrusor muscle. With
further bladder stretch, afferent signals traveling over sacral pelvic nerves
activate parasympathetic neurons. This activates efferent neurons to release
acetylcholine at the neuromuscular junctions, producing detrusor
contraction and bladder emptying.
Nerves Innervating the Urinary System
Sacrum
Uterus
Urinary bladder
Pubic bone
Sphincter
Clitoris
Pudendal
nerve
Labium minora Anus
Ureters
The kidneys and ureters are completely retroperitoneal, and the bladder has
a peritoneal covering only over the dome. As urine is formed, it drains into
the calyces of the kidney, which merge to form the funnel-shaped renal
pelvis in the hilum of each kidney. The renal pelvis narrows to become the
ureter of each kidney. As urine passes through the ureter, it does not
passively drain into the bladder but rather is propelled by waves of
peristalsis. As the ureters enter the pelvis, they sweep laterally, hugging the
pelvic walls. As they approach the bladder, they turn medially and pierce
the bladder wall obliquely. This is important because it creates an one-way
valve (a physiological sphincter rather than an anatomical sphincter) that
allows urine into the bladder but prevents reflux of urine from the bladder
back into the ureter. Children born lacking this oblique course of the ureter
through the bladder wall are susceptible to “vesicoureteral reflux,” which
dramatically increases their risk of serious UTI. Pregnancy also increases
the likelihood of reflux and UTI.
The ureters are approximately 30 cm long. The inner mucosa is lined with
transitional epithelium ([link]) and scattered goblet cells that secrete
protective mucus. The muscular layer of the ureter consists of longitudinal
and circular smooth muscles that create the peristaltic contractions to move
the urine into the bladder without the aid of gravity. Finally, a loose
adventitial layer composed of collagen and fat anchors the ureters between
the parietal peritoneum and the posterior abdominal wall.
Ureter
Peristaltic contractions help to move urine
through the lumen with contributions from fluid
pressure and gravity. LM x 128. (Micrograph
provided by the Regents of the University of
Michigan Medical School © 2012)
Chapter Review
The urethra is the only urinary structure that differs significantly between
males and females. This is due to the dual role of the male urethra in
transporting both urine and semen. The urethra arises from the trigone area
at the base of the bladder. Urination is controlled by an involuntary internal
sphincter of smooth muscle and a voluntary external sphincter of skeletal
muscle. The shorter female urethra contributes to the higher incidence of
bladder infections in females. The male urethra receives secretions from the
prostate gland, Cowper’s gland, and seminal vesicles as well as sperm. The
bladder is largely retroperitoneal and can hold up to 500-600 mL urine.
Micturition is the process of voiding the urine and involves both
involuntary and voluntary actions. Voluntary control of micturition requires
a mature and intact sacral micturition center. It also requires an intact spinal
cord. Loss of control of micturition is called incontinence and results in
voiding when the bladder contains about 250 mL urine. The ureters are
retroperitoneal and lead from the renal pelvis of the kidney to the trigone
area at the base of the bladder. A thick muscular wall consisting of
longitudinal and circular smooth muscle helps move urine toward the
bladder by way of peristaltic contractions.
Review Questions
Exercise:
Problem: Peristaltic contractions occur in the
a. urethra
b. bladder
c. ureters
d. urethra, bladder, and ureters
Solution:
C
Exercise:
Problem:
Somatic motor neurons must be to relax the external
urethral sphincter to allow urination.
a. stimulated
b. inhibited
Solution:
B
Exercise:
Problem:
Which part of the urinary system is not completely retroperitoneal?
a. kidneys
b. ureters
c. bladder
d. nephrons
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Why are females more likely to contract bladder infections than
males?
Solution:
The longer urethra of males means bacteria must travel farther to the
bladder to cause an infection.
Exercise:
Problem: Describe how forceful urination is accomplished.
Solution:
Forceful urination is accomplished by contraction of abdominal
muscles.
Glossary
anatomical sphincter
smooth or skeletal muscle surrounding the lumen of a vessel or hollow
organ that can restrict flow when contracted
detrusor muscle
smooth muscle in the bladder wall; fibers run in all directions to
reduce the size of the organ when emptying it of urine
external urinary sphincter
skeletal muscle; must be relaxed consciously to void urine
internal urinary sphincter
smooth muscle at the juncture of the bladder and urethra; relaxes as the
bladder fills to allow urine into the urethra
incontinence
loss of ability to control micturition
micturition
also called urination or voiding
physiological sphincter
sphincter consisting of circular smooth muscle indistinguishable from
adjacent muscle but possessing differential innervations, permitting its
function as a sphincter; structurally weak
retroperitoneal
outside the peritoneal cavity; in the case of the kidney and ureters,
between the parietal peritoneum and the abdominal wall
sacral micturition center
group of neurons in the sacral region of the spinal cord that controls
urination; acts reflexively unless its action is modified by higher brain
centers to allow voluntary urination
trigone
area at the base of the bladder marked by the two ureters in the
posterior—lateral aspect and the urethral orifice in the anterior aspect
oriented like points on a triangle
urethra
transports urine from the bladder to the outside environment
Gross Anatomy of the Kidney
By the end of this section, you will be able to:
¢ Describe the external structure of the kidney, including its location,
support structures, and covering
¢ Identify the major internal divisions and structures of the kidney
¢ Identify the major blood vessels associated with the kidney and trace
the path of blood through the kidney
e Compare and contrast the cortical and juxtamedullary nephrons
e Name structures found in the cortex and medulla
e Describe the physiological characteristics of the cortex and medulla
The kidneys lie on either side of the spine in the retroperitoneal space
between the parietal peritoneum and the posterior abdominal wall, well
protected by muscle, fat, and ribs. They are roughly the size of your fist,
and the male kidney is typically a bit larger than the female kidney. The
kidneys are well vascularized, receiving about 25 percent of the cardiac
output at rest.
Note:
—
mess OPenstax COLLEGE
There have never been sufficient kidney donations to provide a kidney to
each person needing one. Watch this video to learn about the TED
(Technology, Entertainment, Design) Conference held in March 2011. In
this video, Dr. Anthony Atala discusses a cutting-edge technique in which
anew kidney is “printed.” The successful utilization of this technology is
still several years in the future, but imagine a time when you can print a
replacement organ or tissue on demand.
External Anatomy
The left kidney is located at about the T12 to L3 vertebrae, whereas the
right is lower due to slight displacement by the liver. Upper portions of the
kidneys are somewhat protected by the eleventh and twelfth ribs ((link]).
Each kidney weighs about 125-175 g in males and 115-155 g in females.
They are about 11—14 cm in length, 6 cm wide, and 4 cm thick, and are
directly covered by a fibrous capsule composed of dense, irregular
connective tissue that helps to hold their shape and protect them. This
capsule is covered by a shock-absorbing layer of adipose tissue called the
renal fat pad, which in turn is encompassed by a tough renal fascia. The
fascia and, to a lesser extent, the overlying peritoneum serve to firmly
anchor the kidneys to the posterior abdominal wall in a retroperitoneal
position.
Kidneys
Liver
Kidney
12th rib
Ureter
The kidneys are slightly protected by the ribs
and are surrounded by fat for protection (not
shown).
On the superior aspect of each kidney is the adrenal gland. The adrenal
cortex directly influences renal function through the production of the
hormone aldosterone to stimulate sodium reabsorption.
Internal Anatomy
A frontal section through the kidney reveals an outer region called the renal
cortex and an inner region called the medulla (({link]). The renal columns
are connective tissue extensions that radiate downward from the cortex
through the medulla to separate the most characteristic features of the
medulla, the renal pyramids and renal papillae. The papillae are bundles
of collecting ducts that transport urine made by nephrons to the calyces of
the kidney for excretion. The renal columns also serve to divide the kidney
into 6-8 lobes and provide a supportive framework for vessels that enter
and exit the cortex. The pyramids and renal columns taken together
constitute the kidney lobes.
Left Kidney
Cortical ——_ jie
blood vessels _ Arcuate
y) blood vessels
| Interlobar ——_—_ ane
| blood vessels
Renal vein i) —_—— Major calyx
Minor calyx
aanel Renal pelvis
Renal ena
hilum nerve Pyramid
Renal artery
Papilla
Medulla
Renal column
Ureter
rt
Capsule saute
Renal Hilum
The renal hilum is the entry and exit site for structures servicing the
kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are
tucked into the sweeping convex outline of the cortex. Emerging from the
hilum is the renal pelvis, which is formed from the major and minor calyxes
in the kidney. The smooth muscle in the renal pelvis funnels urine via
peristalsis into the ureter. The renal arteries form directly from the
descending aorta, whereas the renal veins return cleansed blood directly to
the inferior vena cava. The artery, vein, and renal pelvis are arranged in an
anterior-to-posterior order.
Nephrons and Vessels
The renal artery first divides into segmental arteries, followed by further
branching to form interlobar arteries that pass through the renal columns to
reach the cortex ([link]). The interlobar arteries, in turn, branch into arcuate
arteries, cortical radiate arteries, and then into afferent arterioles. The
afferent arterioles service about 1.3 million nephrons in each kidney.
Blood Flow in the Kidney
Peritubular capillaries
Efferent
arteriole
Glomerulus
Afferent
arteriole
Cortical
radiate artery
Arcuate
artery
Ee ~
Interlobar
artery
Segmental
Interlobar
vein
Renal
artery
—— Renal vein
Nephrons are the “functional units” of the kidney; they cleanse the blood
and balance the constituents of the circulation. The afferent arterioles form
a tuft of high-pressure capillaries about 200 pm in diameter, the
glomerulus. The rest of the nephron consists of a continuous sophisticated
tubule whose proximal end surrounds the glomerulus in an intimate
embrace—this is Bowman’s capsule. The glomerulus and Bowman’s
capsule together form the renal corpuscle. As mentioned earlier, these
glomerular capillaries filter the blood based on particle size. After passing
through the renal corpuscle, the capillaries form a second arteriole, the
efferent arteriole ({link]). These will next form a capillary network around
the more distal portions of the nephron tubule, the peritubular capillaries
and vasa recta, before returning to the venous system. As the glomerular
filtrate progresses through the nephron, these capillary networks recover
most of the solutes and water, and return them to the circulation. Since a
capillary bed (the glomerulus) drains into a vessel that in turn forms a
second capillary bed, the definition of a portal system is met. This is the
only portal system in which an arteriole is found between the first and
second capillary beds. (Portal systems also link the hypothalamus to the
anterior pituitary, and the blood vessels of the digestive viscera to the liver.)
Blood Flow in the Nephron
Glomerular capsule
Efferent
arteriole
Afferent
arteriole
Proximal
convoluted tubule
Interlobular
artery
el
Loop of
A, the nephron
Peritubular
capillary
network
Urine flows into renal papilla
The two capillary beds are clearly shown
in this figure. The efferent arteriole is the
connecting vessel between the glomerulus
and the peritubular capillaries and vasa
recta.
Note:
a]
Visit this link to view an interactive tutorial of the flow of blood through
the kidney.
Cortex
In a dissected kidney, it is easy to identify the cortex; it appears lighter in
color compared to the rest of the kidney. All of the renal corpuscles as well
as both the proximal convoluted tubules (PCTs) and distal convoluted
tubules are found here. Some nephrons have a short loop of Henle that
does not dip beyond the cortex. These nephrons are called cortical
nephrons. About 15 percent of nephrons have long loops of Henle that
extend deep into the medulla and are called juxtamedullary nephrons.
Chapter Review
As noted previously, the structure of the kidney is divided into two principle
regions—the peripheral rim of cortex and the central medulla. The two
kidneys receive about 25 percent of cardiac output. They are protected in
the retroperitoneal space by the renal fat pad and overlying ribs and muscle.
Ureters, blood vessels, lymph vessels, and nerves enter and leave at the
renal hilum. The renal arteries arise directly from the aorta, and the renal
veins drain directly into the inferior vena cava. Kidney function is derived
from the actions of about 1.3 million nephrons per kidney; these are the
“functional units.” A capillary bed, the glomerulus, filters blood and the
filtrate is captured by Bowman’s capsule. A portal system is formed when
the blood flows through a second capillary bed surrounding the proximal
and distal convoluted tubules and the loop of Henle. Most water and solutes
are recovered by this second capillary bed. This filtrate is processed and
finally gathered by collecting ducts that drain into the minor calyces, which
merge to form major calyces; the filtrate then proceeds to the renal pelvis
and finally the ureters.
Review Questions
Exercise:
Problem:
The renal pyramids are separated from each other by extensions of the
renal cortex called
a. renal medulla
b. minor calyces
c. medullary cortices
d. renal columns
Solution:
D
Exercise:
Problem:
The primary structure found within the medulla is the
a. loop of Henle
b. minor calyces
c. portal system
d. ureter
Solution:
A
Exercise:
Problem:The right kidney is slightly lower because
a. it is displaced by the liver
b. it is displace by the heart
c. it is slightly smaller
d. it needs protection of the lower ribs
Solution:
A
Critical Thinking Questions
Exercise:
Problem: What anatomical structures provide protection to the kidney?
Solution:
Retroperitoneal anchoring, renal fat pads, and ribs provide protection
to the kidney.
Exercise:
Problem:
How does the renal portal system differ from the hypothalamo—
hypophyseal and digestive portal systems?
Solution:
The renal portal system has an artery between the first and second
capillary bed. The others have a vein.
Exercise:
Problem: Name the structures found in the renal hilum.
Solution:
The structures found in the renal hilum are arteries, veins, ureters,
lymphatics, and nerves.
Glossary
Bowman’s capsule
cup-shaped sack lined by a simple squamous epithelium (parietal
surface) and specialized cells called podocytes (visceral surface) that
participate in the filtration process; receives the filtrate which then
passes on to the PCTs
calyces
cup-like structures receiving urine from the collecting ducts where it
passes on to the renal pelvis and ureter
cortical nephrons
nephrons with loops of Henle that do not extend into the renal medulla
distal convoluted tubules
portions of the nephron distal to the loop of Henle that receive
hyposmotic filtrate from the loop of Henle and empty into collecting
ducts
efferent arteriole
arteriole carrying blood from the glomerulus to the capillary beds
around the convoluted tubules and loop of Henle; portion of the portal
system
glomerulus
tuft of capillaries surrounded by Bowman’s capsule; filters the blood
based on size
juxtamedullary nephrons
nephrons adjacent to the border of the cortex and medulla with loops
of Henle that extend into the renal medulla
loop of Henle
descending and ascending portions between the proximal and distal
convoluted tubules; those of cortical nephrons do not extend into the
medulla, whereas those of juxtamedullary nephrons do extend into the
medulla
nephrons
functional units of the kidney that carry out all filtration and
modification to produce urine; consist of renal corpuscles, proximal
and distal convoluted tubules, and descending and ascending loops of
Henle; drain into collecting ducts
medulla
inner region of kidney containing the renal pyramids
peritubular capillaries
second capillary bed of the renal portal system; surround the proximal
and distal convoluted tubules; associated with the vasa recta
proximal convoluted tubules (PCTs)
tortuous tubules receiving filtrate from Bowman’s capsule; most active
part of the nephron in reabsorption and secretion
renal columns
extensions of the renal cortex into the renal medulla; separates the
renal pyramids; contains blood vessels and connective tissues
renal corpuscle
consists of the glomerulus and Bowman’s capsule
renal cortex
outer part of kidney containing all of the nephrons; some nephrons
have loops of Henle extending into the medulla
renal fat pad
adipose tissue between the renal fascia and the renal capsule that
provides protective cushioning to the kidney
renal hilum
recessed medial area of the kidney through which the renal artery,
renal vein, ureters, lymphatics, and nerves pass
renal papillae
medullary area of the renal pyramids where collecting ducts empty
urine into the minor calyces
renal pyramids
six to eight cone-shaped tissues in the medulla of the kidney
containing collecting ducts and the loops of Henle of juxtamedullary
nephrons
vasa recta
branches of the efferent arterioles that parallel the course of the loops
of Henle and are continuous with the peritubular capillaries; with the
glomerulus, form a portal system
Microscopic Anatomy of the Kidney
By the end of this section, you will be able to:
e Distinguish the histological differences between the renal cortex and
medulla
e Describe the structure of the filtration membrane
e Identify the major structures and subdivisions of the renal corpuscles,
renal tubules, and renal capillaries
e Discuss the function of the peritubular capillaries and vasa recta
e Identify the location of the juxtaglomerular apparatus and describe the
cells that line it
e Describe the histology of the proximal convoluted tubule, loop of
Henle, distal convoluted tubule, and collecting ducts
The renal structures that conduct the essential work of the kidney cannot be
seen by the naked eye. Only a light or electron microscope can reveal these
structures. Even then, serial sections and computer reconstruction are
necessary to give us a comprehensive view of the functional anatomy of the
nephron and its associated blood vessels.
Nephrons: The Functional Unit
Nephrons take a simple filtrate of the blood and modify it into urine. Many
changes take place in the different parts of the nephron before urine is
created for disposal. The term forming urine will be used hereafter to
describe the filtrate as it is modified into true urine. The principle task of
the nephron population is to balance the plasma to homeostatic set points
and excrete potential toxins in the urine. They do this by accomplishing
three principle functions—filtration, reabsorption, and secretion. They also
have additional secondary functions that exert control in three areas: blood
pressure (via production of renin), red blood cell production (via the
hormone EPO), and calcium absorption (via conversion of calcidiol into
calcitriol, the active form of vitamin D).
Renal Corpuscle
As discussed earlier, the renal corpuscle consists of a tuft of capillaries
called the glomerulus that is largely surrounded by Bowman’s (glomerular)
capsule. The glomerulus is a high-pressure capillary bed between afferent
and efferent arterioles. Bowman’s capsule surrounds the glomerulus to form
a lumen, and captures and directs this filtrate to the PCT. The outermost
part of Bowman’s capsule, the parietal layer, is a simple squamous
epithelium. It transitions onto the glomerular capillaries in an intimate
embrace to form the visceral layer of the capsule. Here, the cells are not
squamous, but uniquely shaped cells (podocytes) extending finger-like
arms (pedicels) to cover the glomerular capillaries ((link]). These
projections interdigitate to form filtration slits, leaving small gaps between
the digits to form a sieve. As blood passes through the glomerulus, 10 to 20
percent of the plasma filters between these sieve-like fingers to be captured
by Bowman’s capsule and funneled to the PCT. Where the fenestrae
(windows) in the glomerular capillaries match the spaces between the
podocyte “fingers,” the only thing separating the capillary lumen and the
lumen of Bowman’s capsule is their shared basement membrane ([link]).
These three features comprise what is known as the filtration membrane.
This membrane permits very rapid movement of filtrate from capillary to
capsule though pores that are only 70 nm in diameter.
Podocytes
Cell
Filtration bodies
slits
Capillary
(b)
Podocytes interdigitate with structures called pedicels
and filter substances in a way similar to fenestrations.
In (a), the large cell body can be seen at the top right
comer, with branches extending from the cell body.
The smallest finger-like extensions are the pedicels.
Pedicels on one podocyte always interdigitate with the
pedicels of another podocyte. (b) This capillary has
three podocytes wrapped around it.
Fenestrated Capillary
Basement lle ae
Endothelium
Fenestrations
Fenestrations allow many substances
to diffuse from the blood based
primarily on size.
The fenestrations prevent filtration of blood cells or large proteins, but
allow most other constituents through. These substances cross readily if
they are less than 4 nm in size and most pass freely up to 8 nm in size. An
additional factor affecting the ability of substances to cross this barrier is
their electric charge. The proteins associated with these pores are negatively
charged, so they tend to repel negatively charged substances and allow
positively charged substances to pass more readily. The basement
membrane prevents filtration of medium-to-large proteins such as globulins.
There are also mesangial cells in the filtration membrane that can contract
to help regulate the rate of filtration of the glomerulus. Overall, filtration is
regulated by fenestrations in capillary endothelial cells, podocytes with
filtration slits, membrane charge, and the basement membrane between
capillary cells. The result is the creation of a filtrate that does not contain
cells or large proteins, and has a slight predominance of positively charged
substances.
Lying just outside Bowman’s capsule and the glomerulus is the
juxtaglomerular apparatus (JGA) ([link]). At the juncture where the
afferent and efferent arterioles enter and leave Bowman’s capsule, the initial
part of the distal convoluted tubule (DCT) comes into direct contact with
the arterioles. The wall of the DCT at that point forms a part of the JGA
known as the macula densa. This cluster of cuboidal epithelial cells
monitors the fluid composition of fluid flowing through the DCT. In
response to the concentration of Na” in the fluid flowing past them, these
cells release paracrine signals. They also have a single, nonmotile cilium
that responds to the rate of fluid movement in the tubule. The paracrine
signals released in response to changes in flow rate and Na* concentration
are adenosine triphosphate (ATP) and adenosine.
Juxtaglomerular Apparatus and Glomerulus
(a) The JGA allows specialized cells to monitor the composition of the
fluid in the DCT and adjust the glomerular filtration rate. (b) This
micrograph shows the glomerulus and surrounding structures. LM x
1540. (Micrograph provided by the Regents of University of Michigan
Medical School © 2012)
Podocyte ; Macula densa
Proximal
convoluted
tubule
Brush
border
Juxtaglomerular
Distal
cells
Proximal convoluted
convoluted tubule
Renal
tubule
nerve Basement
membrane
(a) (b)
Afferent arteriole
A second cell type in this apparatus is the juxtaglomerular cell. This is a
modified, smooth muscle cell lining the afferent arteriole that can contract
or relax in response to ATP or adenosine released by the macula densa.
Such contraction and relaxation regulate blood flow to the glomerulus. If
the osmolarity of the filtrate is too high (hyperosmotic), the juxtaglomerular
cells will contract, decreasing the glomerular filtration rate (GFR) so less
plasma is filtered, leading to less urine formation and greater retention of
fluid. This will ultimately decrease blood osmolarity toward the physiologic
norm. If the osmolarity of the filtrate is too low, the juxtaglomerular cells
will relax, increasing the GFR and enhancing the loss of water to the urine,
causing blood osmolarity to rise. In other words, when osmolarity goes up,
filtration and urine formation decrease and water is retained. When
osmolarity goes down, filtration and urine formation increase and water is
lost by way of the urine. The net result of these opposing actions is to keep
the rate of filtration relatively constant. A second function of the macula
densa cells is to regulate renin release from the juxtaglomerular cells of the
afferent arteriole ((link]). Active renin is a protein comprised of 304 amino
acids that cleaves several amino acids from angiotensinogen to produce
angiotensin I. Angiotensin I is not biologically active until converted to
angiotensin II by angiotensin-converting enzyme (ACE) from the lungs.
Angiotensin IT is a systemic vasoconstrictor that helps to regulate blood
pressure by increasing it. Angiotensin II also stimulates the release of the
steroid hormone aldosterone from the adrenal cortex. Aldosterone
stimulates Na* reabsorption by the kidney, which also results in water
retention and increased blood pressure.
Conversion of Angiotensin I to Angiotensin II
Macula densa
senses low fluid
flow or low Na*
concentration
Juxtaglomerular Angiotensin-converting
cells secrete renin enzyme (ACE) in
pulmonary blood
Widespread vasoconstriction
Kidney releases enzyme
renin into blood
Adrenal cortex
to secrete
Enzyme
Angiotensin | 3 Aldosterone
reaction
ersten Angiotensin II Stimulates
Liver releases
angiotensinogen
into blood
ADH causes aquaporins to
move to the collecting duct
plasma membrane, which
increases water reabsorption
>| Aldosterone stimulates Nat
uptake on the apical cell
membrane in the distal
convoluted tubule and
collecting ducts
The enzyme renin converts the pro-enzyme angiotensin I; the lung-
derived enzyme ACE converts angiotensin I into active angiotensin
i.
Proximal Convoluted Tubule (PCT)
Filtered fluid collected by Bowman’s capsule enters into the PCT. It is
called convoluted due to its tortuous path. Simple cuboidal cells form this
tubule with prominent microvilli on the luminal surface, forming a brush
border. These microvilli create a large surface area to maximize the
absorption and secretion of solutes (Na*, Cl’, glucose, etc.), the most
essential function of this portion of the nephron. These cells actively
transport ions across their membranes, so they possess a high concentration
of mitochondria in order to produce sufficient ATP.
Loop of Henle
The descending and ascending portions of the loop of Henle (sometimes
referred to as the nephron loop) are, of course, just continuations of the
same tubule. They run adjacent and parallel to each other after having made
a hairpin turn at the deepest point of their descent. The descending loop of
Henle consists of an initial short, thick portion and long, thin portion,
whereas the ascending loop consists of an initial short, thin portion
followed by a long, thick portion. The descending thick portion consists of
simple cuboidal epithelium similar to that of the PCT. The descending and
ascending thin portions consists of simple squamous epithelium. As you
will see later, these are important differences, since different portions of the
loop have different permeabilities for solutes and water. The ascending
thick portion consists of simple cuboidal epithelium similar to the DCT.
Distal Convoluted Tubule (DCT)
The DCT, like the PCT, is very tortuous and formed by simple cuboidal
epithelium, but it is shorter than the PCT. These cells are not as active as
those in the PCT; thus, there are fewer microvilli on the apical surface.
However, these cells must also pump ions against their concentration
gradient, so you will find of large numbers of mitochondria, although fewer
than in the PCT.
Collecting Ducts
The collecting ducts are continuous with the nephron but not technically
part of it. In fact, each duct collects filtrate from several nephrons for final
modification. Collecting ducts merge as they descend deeper in the medulla
to form about 30 terminal ducts, which empty at a papilla. They are lined
with simple squamous epithelium with receptors for ADH. When
stimulated by ADH, these cells will insert aquaporin channel proteins into
their membranes, which as their name suggests, allow water to pass from
the duct lumen through the cells and into the interstitial spaces to be
recovered by the vasa recta. This process allows for the recovery of large
amounts of water from the filtrate back into the blood. In the absence of
ADH, these channels are not inserted, resulting in the excretion of water in
the form of dilute urine. Most, if not all, cells of the body contain aquaporin
molecules, whose channels are so small that only water can pass. At least
10 types of aquaporins are known in humans, and six of those are found in
the kidney. The function of all aquaporins is to allow the movement of
water across the lipid-rich, hydrophobic cell membrane ([link]).
Aquaporin Water Channel
Water
channel
Cell membrane
Positive charges inside the channel prevent
the leakage of electrolytes across the cell
membrane, while allowing water to move
due to osmosis.
Chapter Review
The functional unit of the kidney, the nephron, consists of the renal
corpuscle, PCT, loop of Henle, and DCT. Cortical nephrons have short
loops of Henle, whereas juxtamedullary nephrons have long loops of Henle
extending into the medulla. About 15 percent of nephrons are
juxtamedullary. The glomerulus is a capillary bed that filters blood
principally based on particle size. The filtrate is captured by Bowman’s
capsule and directed to the PCT. A filtration membrane is formed by the
fused basement membranes of the podocytes and the capillary endothelial
cells that they embrace. Contractile mesangial cells further perform a role in
regulating the rate at which the blood is filtered. Specialized cells in the
JGA produce paracrine signals to regulate blood flow and filtration rates of
the glomerulus. Other JGA cells produce the enzyme renin, which plays a
central role in blood pressure regulation. The filtrate enters the PCT where
absorption and secretion of several substances occur. The descending and
ascending limbs of the loop of Henle consist of thick and thin segments.
Absorption and secretion continue in the DCT but to a lesser extent than in
the PCT. Each collecting duct collects forming urine from several nephrons
and responds to the posterior pituitary hormone ADH by inserting
aquaporin water channels into the cell membrane to fine tune water
recovery.
Review Questions
Exercise:
Problem:Blood filtrate is captured in the lumen of the
a. glomerulus
b. Bowman’s capsule
c. calyces
d. renal papillae
Solution:
B
Exercise:
Problem:
What are the names of the capillaries following the efferent arteriole?
a. arcuate and medullary
b. interlobar and interlobular
c. peritubular and vasa recta
d. peritubular and medullary
Solution:
C
Exercise:
Problem:The functional unit of the kidney is called
a. the renal hilus
b. the renal corpuscle
c. the nephron
d. Bowman’s capsule
Solution:
C
Critical Thinking Questions
Exercise:
Problem: Which structures make up the renal corpuscle?
Solution:
The structures that make up the renal corpuscle are the glomerulus,
Bowman’s capsule, and PCT.
Exercise:
Problem:
What are the major structures comprising the filtration membrane?
Solution:
The major structures comprising the filtration membrane are
fenestrations and podocyte fenestra, fused basement membrane, and
filtration slits.
Glossary
angiotensin-converting enzyme (ACE)
enzyme produced by the lungs that catalyzes the reaction of inactive
angiotensin I into active angiotensin II
angiotensin I
protein produced by the enzymatic action of renin on angiotensinogen;
inactive precursor of angiotensin II
angiotensin II
protein produced by the enzymatic action of ACE on inactive
angiotensin I; actively causes vasoconstriction and stimulates
aldosterone release by the adrenal cortex
angiotensinogen
inactive protein in the circulation produced by the liver; precursor of
angiotensin I; must be modified by the enzymes renin and ACE to be
activated
aquaporin
protein-forming water channels through the lipid bilayer of the cell;
allows water to cross; activation in the collecting ducts is under the
control of ADH
brush border
formed by microvilli on the surface of certain cuboidal cells; in the
kidney it is found in the PCT; increases surface area for absorption in
the kidney
fenestrations
small windows through a cell, allowing rapid filtration based on size;
formed in such a way as to allow substances to cross through a cell
without mixing with cell contents
filtration slits
formed by pedicels of podocytes; substances filter between the
pedicels based on size
forming urine
filtrate undergoing modifications through secretion and reabsorption
before true urine is produced
juxtaglomerular apparatus (JGA)
located at the juncture of the DCT and the afferent and efferent
arterioles of the glomerulus; plays a role in the regulation of renal
blood flow and GFR
juxtaglomerular cell
modified smooth muscle cells of the afferent arteriole; secretes renin in
response to a drop in blood pressure
macula densa
cells found in the part of the DCT forming the JGA; sense Na*
concentration in the forming urine
mesangial
contractile cells found in the glomerulus; can contract or relax to
regulate filtration rate
pedicels
finger-like projections of podocytes surrounding glomerular
capillaries; interdigitate to form a filtration membrane
podocytes
cells forming finger-like processes; form the visceral layer of
Bowman’s capsule; pedicels of the podocytes interdigitate to form a
filtration membrane
renin
enzyme produced by juxtaglomerular cells in response to decreased
blood pressure or sympathetic nervous activity; catalyzes the
conversion of angiotensinogen into angiotensin I
An Overview of the Endocrine System
By the end of this section, you will be able to:
e Distinguish the types of intercellular communication, their importance,
mechanisms, and effects
e Identify the major organs and tissues of the endocrine system and their
location in the body
Communication is a process in which a sender transmits signals to one or
more receivers to control and coordinate actions. In the human body, two
major organ systems participate in relatively “long distance”
communication: the nervous system and the endocrine system. Together,
these two systems are primarily responsible for maintaining homeostasis in
the body.
Neural and Endocrine Signaling
The nervous system uses two types of intercellular communication—
electrical and chemical signaling—either by the direct action of an
electrical potential, or in the latter case, through the action of chemical
neurotransmitters such as serotonin or norepinephrine. Neurotransmitters
act locally and rapidly. When an electrical signal in the form of an action
potential arrives at the synaptic terminal, they diffuse across the synaptic
cleft (the gap between a sending neuron and a receiving neuron or muscle
cell). Once the neurotransmitters interact (bind) with receptors on the
receiving (post-synaptic) cell, the receptor stimulation is transduced into a
response such as continued electrical signaling or modification of cellular
response. The target cell responds within milliseconds of receiving the
chemical “message”; this response then ceases very quickly once the neural
signaling ends. In this way, neural communication enables body functions
that involve quick, brief actions, such as movement, sensation, and
cognition.In contrast, the endocrine system uses just one method of
communication: chemical signaling. These signals are sent by the endocrine
organs, which secrete chemicals—the hormone— into the extracellular
fluid. Hormones are transported primarily via the bloodstream throughout
the body, where they bind to receptors on target cells, inducing a
characteristic response. As a result, endocrine signaling requires more time
than neural signaling to prompt a response in target cells, though the precise
amount of time varies with different hormones. For example, the hormones
released when you are confronted with a dangerous or frightening situation,
called the fight-or-flight response, occur by the release of adrenal hormones
—epinephrine and norepinephrine—within seconds. In contrast, it may take
up to 48 hours for target cells to respond to certain reproductive hormones.
fli: fm)
x Fr
=
openstax COLLEGE”
1
Visit this link to watch an animation of the events that occur when a
hormone binds to a cell membrane receptor. What is the secondary
messenger made by adenylyl cyclase during the activation of liver cells by
epinephrine?
In addition, endocrine signaling is typically less specific than neural
signaling. The same hormone may play a role in a variety of different
physiological processes depending on the target cells involved. For
example, the hormone oxytocin promotes uterine contractions in women in
labor. It is also important in breastfeeding, and may be involved in the
sexual response and in feelings of emotional attachment in both males and
females.
In general, the nervous system involves quick responses to rapid changes in
the external environment, and the endocrine system is usually slower acting
—taking care of the internal environment of the body, maintaining
homeostasis, and controlling reproduction ({link]). So how does the fight-
or-flight response that was mentioned earlier happen so quickly if hormones
are usually slower acting? It is because the two systems are connected. It is
the fast action of the nervous system in response to the danger in the
environment that stimulates the adrenal glands to secrete their hormones.
As a result, the nervous system can cause rapid endocrine responses to keep
up with sudden changes in both the external and internal environments
when necessary.
Endocrine and Nervous Systems
Endocrine
system
Siena Chemical
mechanism(s)
Primary chemical
: Hormones
signal
Distance traveled Long or short
Response time Fast or slow
Environment targeted Internal
Structures of the Endocrine System
Nervous system
Chemical/electrical
Neurotransmitters
Always short
Always fast
Internal and
external
The endocrine system consists of cells, tissues, and organs that secrete
hormones as a primary or secondary function. The endocrine gland is the
major player in this system. The primary function of these ductless glands is
to secrete their hormones directly into the surrounding fluid. The interstitial
fluid and the blood vessels then transport the hormones throughout the
body. The endocrine system includes the pituitary, thyroid, parathyroid,
adrenal, and pineal glands ({link]). Some of these glands have both
endocrine and non-endocrine functions. For example, the pancreas contains
cells that function in digestion as well as cells that secrete the hormones
insulin and glucagon, which regulate blood glucose levels. The
hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin,
female ovaries, and male testes are other organs that contain cells with
endocrine function. Moreover, adipose tissue has long been known to
produce hormones, and recent research has revealed that even bone tissue
has endocrine functions.
Endocrine System
Pineal gland
Thalamus
Pituitary gland
Thyroid cartilage
of the larynx
Thyroid gland
Thymus Parathyroid glands
(on posterior side
of thyroid)
Trachea
Adrenal glands
Pancreas
Uterus
Ovaries (female)
Testes (male)
Endocrine glands and cells are located throughout the body
and play an important role in homeostasis.
The ductless endocrine glands are not to be confused with the body’s
exocrine system, whose glands release their secretions through ducts.
Examples of exocrine glands include the sebaceous and sweat glands of the
skin. As just noted, the pancreas also has an exocrine function: most of its
cells secrete pancreatic juice through the pancreatic and accessory ducts to
the lumen of the small intestine.
Other Types of Chemical Signaling
In endocrine signaling, hormones secreted into the extracellular fluid
diffuse into the blood or lymph, and can then travel great distances
throughout the body. In contrast, autocrine signaling takes place within the
same cell. An autocrine (auto- = “self”) is a chemical that elicits a response
in the same cell that secreted it. Interleukin-1, or IL-1, is a signaling
molecule that plays an important role in inflammatory response. The cells
that secrete IL-1 have receptors on their cell surface that bind these
molecules, resulting in autocrine signaling.
Local intercellular communication is the province of the paracrine, also
called a paracrine factor, which is a chemical that induces a response in
neighboring cells. Although paracrines may enter the bloodstream, their
concentration is generally too low to elicit a response from distant tissues.
A familiar example to those with asthma is histamine, a paracrine that is
released by immune cells in the bronchial tree. Histamine causes the
smooth muscle cells of the bronchi to constrict, narrowing the airways.
Another example is the neurotransmitters of the nervous system, which act
only locally within the synaptic cleft.
Note:
Career Connections
Endocrinologist
Endocrinology is a specialty in the field of medicine that focuses on the
treatment of endocrine system disorders. Endocrinologists—medical
doctors who specialize in this field—are experts in treating diseases
associated with hormonal systems, ranging from thyroid disease to diabetes
mellitus. Endocrine surgeons treat endocrine disease through the removal,
or resection, of the affected endocrine gland.
Patients who are referred to endocrinologists may have signs and
symptoms or blood test results that suggest excessive or impaired
functioning of an endocrine gland or endocrine cells. The endocrinologist
may order additional blood tests to determine whether the patient’s
hormonal levels are abnormal, or they may stimulate or suppress the
function of the suspect endocrine gland and then have blood taken for
analysis. Treatment varies according to the diagnosis. Some endocrine
disorders, such as type 2 diabetes, may respond to lifestyle changes such as
modest weight loss, adoption of a healthy diet, and regular physical
activity. Other disorders may require medication, such as hormone
replacement, and routine monitoring by the endocrinologist. These include
disorders of the pituitary gland that can affect growth and disorders of the
thyroid gland that can result in a variety of metabolic problems.
Some patients experience health problems as a result of the normal decline
in hormones that can accompany aging. These patients can consult with an
endocrinologist to weigh the risks and benefits of hormone replacement
therapy intended to boost their natural levels of reproductive hormones.
In addition to treating patients, endocrinologists may be involved in
research to improve the understanding of endocrine system disorders and
develop new treatments for these diseases.
Chapter Review
The endocrine system consists of cells, tissues, and organs that secrete
hormones critical to homeostasis. The body coordinates its functions
through two major types of communication: neural and endocrine. Neural
communication includes both electrical and chemical signaling between
neurons and target cells. Endocrine communication involves chemical
signaling via the release of hormones into the extracellular fluid. From
there, hormones diffuse into the bloodstream and may travel to distant body
regions, where they elicit a response in target cells. Endocrine glands are
ductless glands that secrete hormones. Many organs of the body with other
primary functions—such as the heart, stomach, and kidneys—also have
hormone-secreting cells.
Interactive Link Questions
Exercise:
Problem:
Visit this link to watch an animation of the events that occur when a
hormone binds to a cell membrane receptor. What is the secondary
messenger made by adenylyl cyclase during the activation of liver
cells by epinephrine?
Solution:
cAMP
Review Questions
Exercise:
Problem:Endocrine glands
a. secrete hormones that travel through a duct to the target organs
b. release neurotransmitters into the synaptic cleft
c. secrete chemical messengers that travel in the bloodstream
d. include sebaceous glands and sweat glands
Solution:
G
Exercise:
Problem:
Chemical signaling that affects neighboring cells is called
a. autocrine
b. paracrine
c. endocrine
d. neuron
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Describe several main differences in the communication methods used
by the endocrine system and the nervous system.
Solution:
The endocrine system uses chemical signals called hormones to
convey information from one part of the body to a distant part of the
body. Hormones are released from the endocrine cell into the
extracellular environment, but then travel in the bloodstream to target
tissues. This communication and response can take seconds to days. In
contrast, neurons transmit electrical signals along their axons. At the
axon terminal, the electrical signal prompts the release of a chemical
signal called a neurotransmitter that carries the message across the
synaptic cleft to elicit a response in the neighboring cell. This method
of communication is nearly instantaneous, of very brief duration, and
is highly specific.
Exercise:
Problem:Compare and contrast endocrine and exocrine glands.
Solution:
Endocrine glands are ductless. They release their secretion into the
surrounding fluid, from which it enters the bloodstream or lymph to
travel to distant cells. Moreover, the secretions of endocrine glands are
hormones. Exocrine glands release their secretions through a duct that
delivers the secretion to the target location. Moreover, the secretions of
exocrine glands are not hormones, but compounds that have an
immediate physiologic function. For example, pancreatic juice
contains enzymes that help digest food.
Exercise:
Problem:
True or false: Neurotransmitters are a special class of paracrines.
Explain your answer.
Solution:
True. Neurotransmitters can be classified as paracrines because, upon
their release from a neuron’s axon terminals, they travel across a
microscopically small cleft to exert their effect on a nearby neuron or
muscle cell.
Glossary
autocrine
chemical signal that elicits a response in the same cell that secreted it
endocrine gland
tissue or organ that secretes hormones into the blood and lymph
without ducts such that they may be transported to organs distant from
the site of secretion
endocrine system
cells, tissues, and organs that secrete hormones as a primary or
secondary function and play an integral role in normal bodily
processes
exocrine system
cells, tissues, and organs that secrete substances directly to target
tissues via glandular ducts
hormone
secretion of an endocrine organ that travels via the bloodstream or
lymphatics to induce a response in target cells or tissues in another part
of the body
paracrine
chemical signal that elicits a response in neighboring cells; also called
paracrine factor
The Pituitary Gland and Hypothalamus
By the end of this section, you will be able to:
e Explain the interrelationships of the anatomy and functions of the
hypothalamus and the posterior and anterior lobes of the pituitary gland
e Identify the two hormones released from the posterior pituitary, their
target cells, and their principal actions
e Identify the six hormones produced by the anterior lobe of the pituitary
gland, their target cells, their principal actions, and their regulation by the
hypothalamus
The hypothalamus-—pituitary complex can be thought of as the “command
center” of the endocrine system. This complex secretes several hormones that
directly produce responses in target tissues, as well as hormones that regulate
the synthesis and secretion of hormones of other glands. In addition, the
hypothalamus-pituitary complex coordinates the messages of the endocrine
and nervous systems. In many cases, a stimulus received by the nervous system
must pass through the hypothalamus-pituitary complex to be translated into
hormones that can initiate a response.
The hypothalamus is a structure of the diencephalon of the brain located
anterior and inferior to the thalamus ({link]). It has both neural and endocrine
functions, producing and secreting many hormones. In addition, the
hypothalamus is anatomically and functionally related to the pituitary gland
(or hypophysis), a bean-sized organ suspended from it by a stem called the
infundibulum (or pituitary stalk). The pituitary gland is cradled within the
sellaturcica of the sphenoid bone of the skull. It consists of two lobes that arise
from distinct parts of embryonic tissue: the posterior pituitary
(neurohypophysis) is neural tissue, whereas the anterior pituitary (also known
as the adenohypophysis) is glandular tissue that develops from the primitive
digestive tract. The hormones secreted by the posterior and anterior pituitary,
and the intermediate zone between the lobes are summarized in [link].
Hypothalamus—Pituitary Complex
Thalamus
Hypothalamus
Infundibulum
Anterior pituitary
Posterior pituitary
Se
NE
The hypothalamus region lies inferior and anterior to the
thalamus. It connects to the pituitary gland by the stalk-like
infundibulum. The pituitary gland consists of an anterior
and posterior lobe, with each lobe secreting different
hormones in response to signals from the hypothalamus.
Pituitary Hormones
Pituitary Associated Chemical
lobe hormones class Effect
Promotes
Growth hormone
Anterior (GH) Protein growth of
body tissues
Pituitary Hormones
Pituitary
lobe
Anterior
Anterior
Anterior
Anterior
Anterior
Posterior
Associated
hormones
Prolactin (PRL)
Thyroid-stimulating
hormone (TSH)
Adrenocorticotropic
hormone (ACTH)
Follicle-stimulating
hormone (FSH)
Luteinizing
hormone (LH)
Antidiuretic
hormone (ADH)
Chemical
class
Peptide
Glycoprotein
Peptide
Glycoprotein
Glycoprotein
Peptide
Effect
Promotes
milk
production
from
mammary
glands
Stimulates
thyroid
hormone
release from
thyroid
Stimulates
hormone
release by
adrenal
cortex
Stimulates
gamete
production
in gonads
Stimulates
androgen
production
by gonads
Stimulates
water
reabsorption
by kidneys
Pituitary Hormones
Pituitary Associated Chemical
lobe hormones class
Posterior Oxytocin Peptide
Melanocyte-
Intermediate
stimulating Peptide
zone
hormone
Posterior Pituitary
Effect
Stimulates
uterine
contractions
during
childbirth
Stimulates
melanin
formation in
melanocytes
The posterior pituitary is actually an extension of the neurons of the
paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of
these regions rest in the hypothalamus, but their axons descend as the
hypothalamic—hypophyseal tract within the infundibulum, and end in axon
terminals that comprise the posterior pituitary ([Link]).
Posterior Pituitary
Neurosecretory cells of Neurosecretory cells of
paraventricular nucleus supraoptic nucleus
ADH release
Hypothalamus
Infundibulum
Hypothalamohypophyseal
tract
Posterior
pituitary
Pituitary
Anterior pituitary gland
Capillary plexus
Oitielease ADH release
Neurosecretory cells in the hypothalamus release oxytocin
(OT) or ADH into the posterior lobe of the pituitary gland.
These hormones are stored or released into the blood via
the capillary plexus.
The posterior pituitary gland does not produce hormones, but rather stores and
secretes hormones produced by the hypothalamus. The paraventricular nuclei
produce the hormone oxytocin, whereas the supraoptic nuclei produce ADH.
These hormones travel along the axons into storage sites in the axon terminals
of the posterior pituitary. In response to signals from the same hypothalamic
neurons, the hormones are released from the axon terminals into the
bloodstream.
Oxytocin
When fetal development is complete, the peptide-derived hormone oxytocin
(tocia- = “childbirth”) stimulates uterine contractions and dilation of the cervix.
Throughout most of pregnancy, oxytocin hormone receptors are not expressed
at high levels in the uterus. Toward the end of pregnancy, the synthesis of
oxytocin receptors in the uterus increases, and the smooth muscle cells of the
uterus become more sensitive to its effects. Oxytocin is continually released
throughout childbirth through a positive feedback mechanism. As noted earlier,
oxytocin prompts uterine contractions that push the fetal head toward the
cervix. In response, cervical stretching stimulates additional oxytocin to be
synthesized by the hypothalamus and released from the pituitary. This increases
the intensity and effectiveness of uterine contractions and prompts additional
dilation of the cervix. The feedback loop continues until birth.
Although the mother’s high blood levels of oxytocin begin to decrease
immediately following birth, oxytocin continues to play a role in maternal and
newborn health. First, oxytocin is necessary for the milk ejection reflex
(commonly referred to as “let-down”) in breastfeeding women. As the newborn
begins suckling, sensory receptors in the nipples transmit signals to the
hypothalamus. In response, oxytocin is secreted and released into the
bloodstream. Within seconds, cells in the mother’s milk ducts contract, ejecting
milk into the infant’s mouth. Secondly, in both males and females, oxytocin is
thought to contribute to parent-newborn bonding, known as attachment.
Oxytocin is also thought to be involved in feelings of love and closeness, as
well as in the sexual response.
Antidiuretic Hormone (ADH)
The solute concentration of the blood, or blood osmolarity, may change in
response to the consumption of certain foods and fluids, as well as in response
to disease, injury, medications, or other factors. Blood osmolarity is constantly
monitored by osmoreceptors—specialized cells within the hypothalamus that
are particularly sensitive to the concentration of sodium ions and other solutes.
In response to high blood osmolarity, which can occur during dehydration or
following a very salty meal, the osmoreceptors signal the posterior pituitary to
release antidiuretic hormone (ADH). The target cells of ADH are located in
the tubular cells of the kidneys. Its effect is to increase epithelial permeability
to water, allowing increased water reabsorption. The more water reabsorbed
from the filtrate, the greater the amount of water that is returned to the blood
and the less that is excreted in the urine. A greater concentration of water
results in a reduced concentration of solutes. ADH is also known as vasopressin
because, in very high concentrations, it causes constriction of blood vessels,
which increases blood pressure by increasing peripheral resistance. The release
of ADH is controlled by a negative feedback loop. As blood osmolarity
decreases, the hypothalamic osmoreceptors sense the change and prompt a
corresponding decrease in the secretion of ADH. As a result, less water is
reabsorbed from the urine filtrate.
Interestingly, drugs can affect the secretion of ADH. For example, alcohol
consumption inhibits the release of ADH, resulting in increased urine
production that can eventually lead to dehydration and a hangover. A disease
called diabetes insipidus is characterized by chronic underproduction of ADH
that causes chronic dehydration. Because little ADH is produced and secreted,
not enough water is reabsorbed by the kidneys. Although patients feel thirsty,
and increase their fluid consumption, this doesn’t effectively decrease the
solute concentration in their blood because ADH levels are not high enough to
trigger water reabsorption in the kidneys. Electrolyte imbalances can occur in
severe cases of diabetes insipidus.
Anterior Pituitary
The anterior pituitary originates from the digestive tract in the embryo and
migrates toward the brain during fetal development. There are three regions:
the pars distalis is the most anterior, the pars intermedia is adjacent to the
posterior pituitary, and the pars tuberalis is a slender “tube” that wraps the
infundibulum.
Recall that the posterior pituitary does not synthesize hormones, but merely
stores them. In contrast, the anterior pituitary does manufacture hormones.
However, the secretion of hormones from the anterior pituitary is regulated by
two classes of hormones. These hormones—secreted by the hypothalamus—are
the releasing hormones that stimulate the secretion of hormones from the
anterior pituitary and the inhibiting hormones that inhibit secretion.
Hypothalamic hormones are secreted by neurons, but enter the anterior
pituitary through blood vessels ([{link]). Within the infundibulum is a bridge of
capillaries that connects the hypothalamus to the anterior pituitary. This
network, called the hypophyseal portal system, allows hypothalamic
hormones to be transported to the anterior pituitary without first entering the
systemic circulation. The system originates from the superior hypophyseal
artery, which branches off the carotid arteries and transports blood to the
hypothalamus. The branches of the superior hypophyseal artery form the
hypophyseal portal system (see [link]). Hypothalamic releasing and inhibiting
hormones travel through a primary capillary plexus to the portal veins, which
carry them into the anterior pituitary. Hormones produced by the anterior
pituitary (in response to releasing hormones) enter a secondary capillary
plexus, and from there drain into the circulation.
Anterior Pituitary
@) Hypothalamus
releases hormone
Superior
Hypothalamus hypophyseal
Neurosecretory
cells
Infundibulum : WSs Primary capillary
Hypophyseal y plexus of hypophyseal
portal veins r portal system
Posterior pituitary
Anterior
Pituitary gland
Secondary capillary
plexus of hypophyseal
portal system
@) Anterior pituitary @) Hypothalamus hormone stimulates
hormone pituitary to release hormones
The anterior pituitary manufactures seven hormones. The
hypothalamus produces separate hormones that stimulate or
inhibit hormone production in the anterior pituitary.
Hormones from the hypothalamus reach the anterior
pituitary via the hypophyseal portal system.
The anterior pituitary produces seven hormones. These are the growth hormone
(GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone
(ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), beta
endorphin, and prolactin. Of the hormones of the anterior pituitary, TSH,
ACTH, FSH, and LH are collectively referred to as tropic hormones (trope- =
“turning”) because they turn on or off the function of other endocrine glands.
Growth Hormone
The endocrine system regulates the growth of the human body, protein
synthesis, and cellular replication. A major hormone involved in this process is
growth hormone (GH), also called somatotropin—a protein hormone
produced and secreted by the anterior pituitary gland. Its primary function is
anabolic; it promotes protein synthesis and tissue building through direct and
indirect mechanisms ([link]). GH levels are controlled by the release of GHRH
and GHIH (also known as somatostatin) from the hypothalamus.
Hormonal Regulation of Growth
1) Release of growth hormone: GHRH release 3) Inhibition of growth hormone: GHIH release
¢ Hypothalamus releases * High IGF-1 levels perceived by
growth hormone-releasing hypothalamus
hormone (GHRH) ra fy * Growth hormone-—inhibiting
¢ GHRH stimulates the anterior we YY hormone (GHIH) is released to
pituitary to release growth YY inhibit GH release
hormone (GH) * GHIH inhibits GH release in the
anterior pituitary
GH release Sa GH }Gase
2a) Glucose-sparing effect: 2b) Growth effects: 2c) Diabetogenic effect:
¢ Stimulates adipose cells ¢ Increases uptake of amino * GH stimulates liver to
to break down stored fat, acids from the blood break down glycogen
fueling growth effects ¢ Enhances cellular proliferation into glucose, fueling
and reduces apoptosis growth effects
Targets:
Liver releases
IGF-1, further
Adipose cells stimulating
growth effects
a oe” Bone cells
Muscle cells
IGF-1 release
, Nervous system
’ cells
Immune system
cells
Growth hormone (GH) directly accelerates the rate of protein
synthesis in skeletal muscle and bones. Insulin-like growth
factor 1 (IGF-1) is activated by growth hormone and indirectly
supports the formation of new proteins in muscle cells and
bone.
A glucose-sparing effect occurs when GH stimulates lipolysis, or the
breakdown of adipose tissue, releasing fatty acids into the blood. As a result,
many tissues switch from glucose to fatty acids as their main energy source,
which means that less glucose is taken up from the bloodstream.
GH also initiates the diabetogenic effect in which GH stimulates the liver to
break down glycogen to glucose, which is then deposited into the blood. The
name “diabetogenic” is derived from the similarity in elevated blood glucose
levels observed between individuals with untreated diabetes mellitus and
individuals experiencing GH excess. Blood glucose levels rise as the result of a
combination of glucose-sparing and diabetogenic effects.
GH indirectly mediates growth and protein synthesis by triggering the liver and
other tissues to produce a group of proteins called insulin-like growth factors
(IGFs). These proteins enhance cellular proliferation and inhibit apoptosis, or
programmed cell death. IGFs stimulate cells to increase their uptake of amino
acids from the blood for protein synthesis. Skeletal muscle and cartilage cells
are particularly sensitive to stimulation from IGFs.
Dysfunction of the endocrine system’s control of growth can result in several
disorders. For example, gigantism is a disorder in children that is caused by the
secretion of abnormally large amounts of GH, resulting in excessive growth. A
similar condition in adults is acromegaly, a disorder that results in the growth
of bones in the face, hands, and feet in response to excessive levels of GH in
individuals who have stopped growing. Abnormally low levels of GH in
children can cause growth impairment—a disorder called pituitary dwarfism
(also known as growth hormone deficiency).
Thyroid-Stimulating Hormone
The activity of the thyroid gland is regulated by thyroid-stimulating hormone
(TSH), also called thyrotropin. TSH is released from the anterior pituitary in
response to thyrotropin-releasing hormone (TRH) from the hypothalamus. As
discussed shortly, it triggers the secretion of thyroid hormones by the thyroid
gland. In a classic negative feedback loop, elevated levels of thyroid hormones
in the bloodstream then trigger a drop in production of TRH and subsequently
SEL.
Adrenocorticotropic Hormone
The adrenocorticotropic hormone (ACTH), also called corticotropin,
stimulates the adrenal cortex (the more superficial “bark” of the adrenal glands)
to secrete corticosteroid hormones such as cortisol. ACTH come from a
precursor molecule known as pro-opiomelanotropin (POMC) which produces
several biologically active molecules when cleaved, including ACTH,
melanocyte-stimulating hormone, and the brain opioid peptides known as
endorphins.
The release of ACTH is regulated by the corticotropin-releasing hormone
(CRH) from the hypothalamus in response to normal physiologic rhythms. A
variety of stressors can also influence its release, and the role of ACTH in the
stress response is discussed later in this chapter.
Follicle-Stimulating Hormone and Luteinizing Hormone
The endocrine glands secrete a variety of hormones that control the
development and regulation of the reproductive system (these glands include
the anterior pituitary, the adrenal cortex, and the gonads—the testes in males
and the ovaries in females). Much of the development of the reproductive
system occurs during puberty and is marked by the development of sex-specific
characteristics in both male and female adolescents. Puberty is initiated by
gonadotropin-releasing hormone (GnRH), a hormone produced and secreted by
the hypothalamus. GnRH stimulates the anterior pituitary to secrete
gonadotropins—hormones that regulate the function of the gonads. The levels
of GnRH are regulated through a negative feedback loop; high levels of
reproductive hormones inhibit the release of GnRH. Throughout life,
gonadotropins regulate reproductive function and, in the case of women, the
onset and cessation of reproductive capacity.
The gonadotropins include two glycoprotein hormones: follicle-stimulating
hormone (FSH) stimulates the production and maturation of sex cells, or
gametes, including ova in women and sperm in men. FSH also promotes
follicular growth; these follicles then release estrogens in the female ovaries.
Luteinizing hormone (LH) triggers ovulation in women, as well as the
production of estrogens and progesterone by the ovaries. LH stimulates
production of testosterone by the male testes.
Prolactin
As its name implies, prolactin (PRL) promotes lactation (milk production) in
women. During pregnancy, it contributes to development of the mammary
glands, and after birth, it stimulates the mammary glands to produce breast
milk. However, the effects of prolactin depend heavily upon the permissive
effects of estrogens, progesterone, and other hormones. And as noted earlier,
the let-down of milk occurs in response to stimulation from oxytocin.
In a non-pregnant woman, prolactin secretion is inhibited by prolactin-
inhibiting hormone (PIH), which is actually the neurotransmitter dopamine, and
is released from neurons in the hypothalamus. Only during pregnancy do
prolactin levels rise in response to prolactin-releasing hormone (PRH) from the
hypothalamus.
Intermediate Pituitary: Melanocyte-Stimulating Hormone
The cells in the zone between the pituitary lobes secrete a hormone known as
melanocyte-stimulating hormone (MSH) that is formed by cleavage of the pro-
opiomelanocortin (POMC) precursor protein. Local production of MSH in the
skin is responsible for melanin production in response to UV light exposure.
The role of MSH made by the pituitary is more complicated. For instance,
people with lighter skin generally have the same amount of MSH as people
with darker skin. Nevertheless, this hormone is capable of darkening of the skin
by inducing melanin production in the skin’s melanocytes. Women also show
increased MSH production during pregnancy; in combination with estrogens, it
can lead to darker skin pigmentation, especially the skin of the areolas and
labia minora. [link] is a summary of the pituitary hormones and their principal
effects.
Major Pituitary Hormones
Posterior Pituitary Hormones
Releasing hormone Pituitary
(hypothalamus) hormone Target Effects
ADH Stores ————» Kidneys, ——_ Water balance
ADH sweat glands,
circulatory
system
- OT ——+» Female ——~ Triggers uterine
reproductive contractions during
system childbirth
Anterior Pituitary Hormones
Releasing hormone Pituitary
(hypothalamus) hormone Target Effects
GnRH = ——e» LH ——+* Reproductive ———® Stimulates production
system of sex hormones by
gonads
GnRH ——» FSH ——® Ffeproductive ———» Stimulates production
system of sperm and eggs
TRH —> TSH — Thyroid gland ———* Stimulates the release
of thyroid hormone
(TH). TH regulates
metabolism.
PRH —> +~=PRL — Mammary ——_ Promotes milk
(inhibited glands production
by PIH)
GHRH mY GH — Liver,bone, ——— Induces targets to
(inhibited muscles produce insulin-like
by GHIH) growth factors (IGF).
IGFs stimulate body
growth and a higher
metabolic rate.
CRH ——» ACTH ——+* Adrenal ——> Induces targets to
glands produce glucocorticoids,
which regulate
metabolism and the
stress response
Major pituitary hormones and their target organs.
Note:
Sch
Visit this link to watch an animation showing the role of the hypothalamus and
the pituitary gland. Which hormone is released by the pituitary to stimulate the
thyroid gland?
Chapter Review
The hypothalamus—pituitary complex is located in the diencephalon of the
brain. The hypothalamus and the pituitary gland are connected by a structure
called the infundibulum, which contains vasculature and nerve axons. The
pituitary gland is divided into two distinct structures with different embryonic
origins. The posterior lobe houses the axon terminals of hypothalamic neurons.
It stores and releases into the bloodstream two hypothalamic hormones:
oxytocin and antidiuretic hormone (ADH). The anterior lobe is connected to
the hypothalamus by vasculature in the infundibulum and produces and
secretes six hormones. Their secretion is regulated, however, by releasing and
inhibiting hormones from the hypothalamus. The six anterior pituitary
hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH),
adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH),
luteinizing hormone (LH), and prolactin (PRL).
Interactive Link Questions
Exercise:
Problem:
Visit this link to watch an animation showing the role of the hypothalamus
and the pituitary gland. Which hormone is released by the pituitary to
stimulate the thyroid gland?
Solution:
Thyroid-stimulating hormone.
Review Questions
Exercise:
Problem:
The hypothalamus is functionally and anatomically connected to the
posterior pituitary lobe by a bridge of
a. blood vessels
b. nerve axons
c. cartilage
d. bone
Solution:
B
Exercise:
Problem: Which of the following is an anterior pituitary hormone?
a. ADH
b. oxytocin
c. TSH
d. cortisol
Solution:
C
Exercise:
Problem:How many hormones are produced by the posterior pituitary?
a. O
ans
DMN
Solution:
A
Exercise:
Problem:
Which of the following hormones contributes to the regulation of the
body’s fluid and electrolyte balance?
a. adrenocorticotropic hormone
b. antidiuretic hormone
c. luteinizing hormone
d. all of the above
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Compare and contrast the anatomical relationship of the anterior and
posterior lobes of the pituitary gland to the hypothalamus.
Solution:
The anterior lobe of the pituitary gland is connected to the hypothalamus
by vasculature, which allows regulating hormones from the hypothalamus
to travel to the anterior pituitary. In contrast, the posterior lobe is
connected to the hypothalamus by a bridge of nerve axons called the
hypothalamic—hypophyseal tract, along which the hypothalamus sends
hormones produced by hypothalamic nerve cell bodies to the posterior
pituitary for storage and release into the circulation.
Exercise:
Problem: Name the target tissues for prolactin.
Solution:
The mammary glands are the target tissues for prolactin.
Glossary
acromegaly
disorder in adults caused when abnormally high levels of GH trigger
growth of bones in the face, hands, and feet
adrenocorticotropic hormone (ACTH)
anterior pituitary hormone that stimulates the adrenal cortex to secrete
corticosteroid hormones (also called corticotropin)
antidiuretic hormone (ADH)
hypothalamic hormone that is stored by the posterior pituitary and that
signals the kidneys to reabsorb water
follicle-stimulating hormone (FSH)
anterior pituitary hormone that stimulates the production and maturation
of sex cells
gigantism
disorder in children caused when abnormally high levels of GH prompt
excessive growth
gonadotropins
hormones that regulate the function of the gonads
growth hormone (GH)
anterior pituitary hormone that promotes tissue building and influences
nutrient metabolism (also called somatotropin)
hypophyseal portal system
network of blood vessels that enables hypothalamic hormones to travel
into the anterior lobe of the pituitary without entering the systemic
circulation
hypothalamus
region of the diencephalon inferior to the thalamus that functions in neural
and endocrine signaling
infundibulum
stalk containing vasculature and neural tissue that connects the pituitary
gland to the hypothalamus (also called the pituitary stalk)
insulin-like growth factors (IGF)
protein that enhances cellular proliferation, inhibits apoptosis, and
stimulates the cellular uptake of amino acids for protein synthesis
luteinizing hormone (LH)
anterior pituitary hormone that triggers ovulation and the production of
ovarian hormones in females, and the production of testosterone in males
osmoreceptor
hypothalamic sensory receptor that is stimulated by changes in solute
concentration (osmotic pressure) in the blood
oxytocin
hypothalamic hormone stored in the posterior pituitary gland and
important in stimulating uterine contractions in labor, milk ejection during
breastfeeding, and feelings of attachment (also produced in males)
pituitary dwarfism
disorder in children caused when abnormally low levels of GH result in
growth retardation
pituitary gland
bean-sized organ suspended from the hypothalamus that produces, stores,
and secretes hormones in response to hypothalamic stimulation (also
called hypophysis)
prolactin (PRL)
anterior pituitary hormone that promotes development of the mammary
glands and the production of breast milk
thyroid-stimulating hormone (TSH)
anterior pituitary hormone that triggers secretion of thyroid hormones by
the thyroid gland (also called thyrotropin)
The Thyroid Gland
By the end of this section, you will be able to:
¢ Describe the location and anatomy of the thyroid gland
e Discuss the synthesis of triiodothyronine and thyroxine
e Explain the role of thyroid hormones in the regulation of basal
metabolism
e Identify the hormone produced by the parafollicular cells of the
thyroid
A butterfly-shaped organ, the thyroid gland is located anterior to the
trachea, just inferior to the larynx ([link]). The medial region, called the
isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid
lobes are embedded with parathyroid glands, primarily on their posterior
surfaces. The tissue of the thyroid gland is composed mostly of thyroid
follicles. The follicles are made up of a central cavity filled with a sticky
fluid called colloid. Surrounded by a wall of epithelial follicle cells, the
colloid is the center of thyroid hormone production, and that production is
dependent on the hormones’ essential and unique component: iodine.
Thyroid Gland
Hyoid bone
Thyroid cartilage
Superior thyroid
artery
Isthmus of the
thyroid
Common carotid
arteries Trachea
Hyoid bone
Thyroid cartilage
Cricoid cartilage
Right parathyroid
Left parathyroid glands
glands
Left inferior thyroid
artery
From left subclavian _, * From right subclavian
artery artery
b) Posterior view
Right inferior thyroid
artery
Parafollicular cell
Colloid-containing
follicle
Follicle cells
(cuboidal epithelium)
c) Thyroid follicle cells
The thyroid gland is located in the neck
where it wraps around the trachea. (a)
Anterior view of the thyroid gland. (b)
Posterior view of the thyroid gland. (c)
The glandular tissue is composed
primarily of thyroid follicles. The larger
parafollicular cells often appear within the
matrix of follicle cells. LM x 1332.
(Micrograph provided by the Regents of
University of Michigan Medical School ©
2012)
Synthesis and Release of Thyroid Hormones
Hormones are produced in the colloid when atoms of the mineral iodine
attach to a glycoprotein, called thyroglobulin, that is secreted into the
colloid by the follicle cells. The following steps outline the hormones’
assembly:
1. Binding of TSH to its receptors in the follicle cells of the thyroid gland
causes the cells to actively transport iodide ions (I-) across their cell
membrane, from the bloodstream into the cytosol. As a result, the
concentration of iodide ions “trapped” in the follicular cells is many
times higher than the concentration in the bloodstream.
2. Iodide ions then move to the lumen of the follicle cells that border the
colloid. There, the ions undergo oxidation (their negatively charged
electrons are removed). The oxidation of two iodide ions (2 I") results
in iodine (Ip), which passes through the follicle cell membrane into the
colloid.
3. In the colloid, peroxidase enzymes link the iodine to the tyrosine
amino acids in thyroglobulin to produce two intermediaries: a tyrosine
attached to one iodine and a tyrosine attached to two iodines. When
one of each of these intermediaries is linked by covalent bonds, the
resulting compound is triiodothyronine (T3), a thyroid hormone with
three iodines. Much more commonly, two copies of the second
intermediary bond, forming tetraiodothyronine, also known as
thyroxine (T,), a thyroid hormone with four iodines.
These hormones remain in the colloid center of the thyroid follicles until
TSH stimulates endocytosis of colloid back into the follicle cells. There,
lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3
and Ty, which diffuse across the follicle cell membrane and enter the
bloodstream.
In the bloodstream, less than one percent of the circulating T3 and T,
remains unbound. This free T3 and T, can cross the lipid bilayer of cell
membranes and be taken up by cells. The remaining 99 percent of
circulating T3 and Ty is bound to specialized transport proteins called
thyroxine-binding globulins (TBGs), to albumin, or to other plasma
proteins. This “packaging” prevents their free diffusion into body cells.
When blood levels of T3 and Ty begin to decline, bound T3 and Ty are
released from these plasma proteins and readily cross the membrane of
target cells. T3 is more potent than Ty, and many cells convert T, to T3
through the removal of an iodine atom.
Regulation of TH Synthesis
The release of T3 and T, from the thyroid gland is regulated by thyroid-
stimulating hormone (TSH). As shown in [link], low blood levels of T3 and
T, stimulate the release of thyrotropin-releasing hormone (TRH) from the
hypothalamus, which triggers secretion of TSH from the anterior pituitary.
In turn, TSH stimulates the thyroid gland to secrete T3 and Ty. The levels of
TRH, TSH, T3, and Ty are regulated by a negative feedback system in
which increasing levels of T3 and T, decrease the production and secretion
or TSH,
Classic Negative Feedback Loop
1) Metabolic rate and/or T; and T,4
concentration in blood...
Low?
* Hypothalamus releases TRH. :
This triggers TSH release by High?
the pituitary. * Hypothalamus stops TRH 4) Negative feedback:
<——
release ¢ Elevated T3 and T, levels
inhibit release of TRH and TSH
——
¢ Anterior pituitary stops
TSH release
Thyroid follicle
T, release
T, release
3) Effects of T, and T, release:
¢ Increased basal metabolic
2) Effects of TSH release: rate of body cells
* Triggers release of T3 and T4 * Rise in body temperature
by thyroid follicle cells (calorigenic effect)
A classic negative feedback loop controls the regulation of
thyroid hormone levels.
Functions of Thyroid Hormones
The thyroid hormones, T3 and Ty, are often referred to as metabolic
hormones because their levels influence the body’s basal metabolic rate, the
amount of energy used by the body at rest. When T3 and Ty bind to
intracellular receptors located on the mitochondria, they cause an increase
in nutrient breakdown and the use of oxygen to produce ATP. In addition,
T3 and T, initiate the transcription of genes involved in glucose oxidation.
Although these mechanisms prompt cells to produce more ATP, the process
is inefficient, and an abnormally increased level of heat is released as a
byproduct of these reactions. This so-called calorigenic effect (calor- =
“heat”) raises body temperature.
Adequate levels of thyroid hormones are also required for protein synthesis
and for fetal and childhood tissue development and growth. They are
especially critical for normal development of the nervous system both in
utero and in early childhood, and they continue to support neurological
function in adults. As noted earlier, these thyroid hormones have a complex
interrelationship with reproductive hormones, and deficiencies can
influence libido, fertility, and other aspects of reproductive function.
Finally, thyroid hormones increase the body’s sensitivity to catecholamines
(epinephrine and norepinephrine) from the adrenal medulla by upregulation
of receptors in the blood vessels. When levels of T3 and T, hormones are
excessive, this effect accelerates the heart rate, strengthens the heartbeat,
and increases blood pressure. Because thyroid hormones regulate
metabolism, heat production, protein synthesis, and many other body
functions, thyroid disorders can have severe and widespread consequences.
Note:
Disorders of the...
Endocrine System: Iodine Deficiency, Hypothyroidism, and
Hyperthyroidism
As discussed above, dietary iodine is required for the synthesis of T3 and
Ty. But for much of the world’s population, foods do not provide adequate
levels of this mineral, because the amount varies according to the level in
the soil in which the food was grown, as well as the irrigation and
fertilizers used. Marine fish and shrimp tend to have high levels because
they concentrate iodine from seawater, but many people in landlocked
regions lack access to seafood. Thus, the primary source of dietary iodine
in many countries is iodized salt. Fortification of salt with iodine began in
the United States in 1924, and international efforts to iodize salt in the
world’s poorest nations continue today.
Dietary iodine deficiency can result in the impaired ability to synthesize T3
and Ty, leading to a variety of severe disorders. When T3 and T,4 cannot be
produced, TSH is secreted in increasing amounts. As a result of this
hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles,
increasing their deposits of colloid. The accumulation of colloid increases
the overall size of the thyroid gland, a condition called a goiter ([link]). A
goiter is only a visible indication of the deficiency. Other iodine deficiency
disorders include impaired growth and development, decreased fertility,
and prenatal and infant death. Moreover, iodine deficiency is the primary
cause of preventable mental retardation worldwide. Neonatal
hypothyroidism (cretinism) is characterized by cognitive deficits, short
stature, and sometimes deafness and muteness in children and adults born
to mothers who were iodine-deficient during pregnancy.
Goiter
(credit: “Almazi”/Wikimedia Commons)
In areas of the world with access to iodized salt, dietary deficiency is rare.
Instead, inflammation of the thyroid gland is the more common cause of
low blood levels of thyroid hormones. Called hypothyroidism, the
condition is characterized by a low metabolic rate, weight gain, cold
extremities, constipation, reduced libido, menstrual irregularities, and
reduced mental activity. In contrast, hyperthyroidism—an abnormally
elevated blood level of thyroid hormones—is often caused by a pituitary or
thyroid tumor. In Graves’ disease, the hyperthyroid state results from an
autoimmune reaction in which antibodies overstimulate the follicle cells of
the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate,
excessive body heat and sweating, diarrhea, weight loss, tremors, and
increased heart rate. The person’s eyes may bulge (called exophthalmos) as
antibodies produce inflammation in the soft tissues of the orbits. The
person may also develop a goiter.
Calcitonin
The thyroid gland also secretes a hormone called calcitonin that is
produced by the parafollicular cells (also called C cells) that stud the tissue
between distinct follicles. Calcitonin is released in response to a rise in
blood calcium levels. It appears to have a function in decreasing blood
calcium concentrations by:
e Inhibiting the activity of osteoclasts, bone cells that release calcium
into the circulation by degrading bone matrix
e Increasing osteoblastic activity
e Decreasing calcium absorption in the intestines
e Increasing calcium loss in the urine
However, these functions are usually not significant in maintaining calcium
homeostasis, so the importance of calcitonin is not entirely understood.
Pharmaceutical preparations of calcitonin are sometimes prescribed to
reduce osteoclast activity in people with osteoporosis and to reduce the
degradation of cartilage in people with osteoarthritis. The hormones
secreted by thyroid are summarized in [link].
Thyroid Hormones
Chemical
Associated hormones class Effect
Thyroxine (Ta), melee Stimulate basal
triiodothyronine (T3) metabolic rate
2+
Calcitonin Peptide Reduces blood Ca
levels
Of course, calcium is critical for many other biological processes. It is a
second messenger in many signaling pathways, and is essential for muscle
contraction, nerve impulse transmission, and blood clotting. Given these
roles, it is not surprising that blood calcium levels are tightly regulated by
the endocrine system. The organs involved in the regulation are the
parathyroid glands.
Chapter Review
The thyroid gland is a butterfly-shaped organ located in the neck anterior to
the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient
metabolism, the production of ATP, and calcium homeostasis. They also
contribute to protein synthesis and the normal growth and development of
body tissues, including maturation of the nervous system, and they increase
the body’s sensitivity to catecholamines. The thyroid hormones
triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the
thyroid gland in response to thyroid-stimulating hormone (TSH) from the
anterior pituitary. Synthesis of the amino acid—derived T3 and T, hormones
requires iodine. Insufficient amounts of iodine in the diet can lead to goiter,
cretinism, and many other disorders.
Review Questions
Exercise:
Problem:
Which of the following statements about the thyroid gland is true?
a. It is located anterior to the trachea and inferior to the larynx.
b. The parathyroid glands are embedded within it.
c. It manufactures three hormones.
d. all of the above
Solution:
D
Exercise:
Problem:
The secretion of thyroid hormones is controlled by
a. TSH from the hypothalamus
b. TSH from the anterior pituitary
c. thyroxine from the anterior pituitary
d. thyroglobulin from the thyroid’s parafollicular cells
Solution:
B
Exercise:
Problem:The development of a goiter indicates that
a. the anterior pituitary is abnormally enlarged
b. there is hypertrophy of the thyroid’s follicle cells
c. there is an excessive accumulation of colloid in the thyroid
follicles
d. the anterior pituitary is secreting excessive growth hormone
Solution:
C
Exercise:
Problem:
Iodide ions cross from the bloodstream into follicle cells via
a. simple diffusion
b. facilitated diffusion
c. active transport
d. osmosis
Solution:
C
Critical Thinking Questions
Exercise:
Problem:
Explain why maternal iodine deficiency might lead to neurological
impairment in the fetus.
Solution:
Iodine deficiency in a pregnant woman would also deprive the fetus.
Iodine is required for the synthesis of thyroid hormones, which
contribute to fetal growth and development, including maturation of
the nervous system. Insufficient amounts would impair these functions.
Exercise:
Problem:
Define hyperthyroidism and explain why one of its symptoms is
weight loss.
Solution:
Hyperthyroidism is an abnormally elevated blood level of thyroid
hormones due to an overproduction of T3 and T,. An individual with
hyperthyroidism is likely to lose weight because one of the primary
roles of thyroid hormones is to increase the body’s basal metabolic
rate, increasing the breakdown of nutrients and the production of ATP.
Glossary
calcitonin
peptide hormone produced and secreted by the parafollicular cells (C
cells) of the thyroid gland that functions to decrease blood calcium
levels
colloid
viscous fluid in the central cavity of thyroid follicles, containing the
glycoprotein thyroglobulin
goiter
enlargement of the thyroid gland either as a result of iodine deficiency
or hyperthyroidism
hyperthyroidism
clinically abnormal, elevated level of thyroid hormone in the blood;
characterized by an increased metabolic rate, excess body heat,
sweating, diarrhea, weight loss, and increased heart rate
hypothyroidism
clinically abnormal, low level of thyroid hormone in the blood;
characterized by low metabolic rate, weight gain, cold extremities,
constipation, and reduced mental activity
neonatal hypothyroidism
condition characterized by cognitive deficits, short stature, and other
signs and symptoms in people born to women who were iodine-
deficient during pregnancy
thyroid gland
large endocrine gland responsible for the synthesis of thyroid
hormones
thyroxine
(also, tetraiodothyronine, T4) amino acid—derived thyroid hormone that
is more abundant but less potent than T3 and often converted to T3 by
target cells
triiodothyronine
(also, T3) amino acid—derived thyroid hormone that is less abundant
but more potent than T,
The Parathyroid Glands
By the end of this section, you will be able to:
¢ Describe the location and structure of the parathyroid glands
e Describe the hormonal control of blood calcium levels
e Discuss the physiological response of parathyroid dysfunction
The parathyroid glands are tiny, round structures usually found embedded
in the posterior surface of the thyroid gland ([link]). A thick connective
tissue capsule separates the glands from the thyroid tissue. Most people
have four parathyroid glands, but occasionally there are more in tissues of
the neck or chest. The function of one type of parathyroid cells, the oxyphil
cells, is not clear. The primary functional cells of the parathyroid glands are
the chief cells. These epithelial cells produce and secrete the parathyroid
hormone (PTH), the major hormone involved in the regulation of blood
calcium levels.
Parathyroid Glands
pa
Hyoid bone oa Ly
Thyroid y| 4
cartilage
Oxyphil cells
Cricoid
i Blood vessel
cartilage
- Wit P 7 ; y: OT ed. Oe = 1 Parathyroid
parathyroid
parathyroid
glands
glands
a) Thyroid gland, posterior view b) Micrograph of parathyroid tissue
The small parathyroid glands are embedded in the posterior
surface of the thyroid gland. LM x 760. (Micrograph provided
by the Regents of University of Michigan Medical School ©
2012)
Note:
mss" OPENStax COLLEGE
F : ca
[aS
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
The parathyroid glands produce and secrete PTH, a peptide hormone, in
response to low blood calcium levels ({link]). PTH secretion causes the
release of calcium from the bones by stimulating osteoclasts, which secrete
enzymes that degrade bone and release calcium into the interstitial fluid.
PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby
sparing blood calcium. PTH causes increased reabsorption of calcium (and
magnesium) in the kidney tubules from the urine filtrate. In addition, PTH
initiates the production of the steroid hormone calcitriol (also known as
1,25-dihydroxyvitamin D), which is the active form of vitamin Ds, in the
kidneys. Calcitriol then stimulates increased absorption of dietary calcium
by the intestines. A negative feedback loop regulates the levels of PTH,
with rising blood calcium levels inhibiting further release of PTH.
Parathyroid Hormone in Maintaining Blood Calcium Homeostasis
\ 2s Osteoclasts
Ss :
1) Blood calcium Compact bone
concentration drops
6) Effects of calcitonin on bone:
+ Stimulates osteoblasts
+ Inhibits osteoclasts
+ Calcium is removed from blood and used to build bone
2) Release of PTH: 5) Calcitonin release:
+ Chief cells of the parathyroid gland release parathyroid
hormone (PTH).
+ High concentrations of calcium stimulate parafollicular cells
in the thyroid to release calcitonin.
i 4 C
¥ rr
4 2 TA EAT vy
om
4) Blood calcium levels increase
3a) Effects of PTH on bone: 3b) Effects of PTH on kidneys: 3c) Effects of calcitriol on intestine:
+ Inhibits osteoblasts + PTH stimulates kidney tubule cells to + Stimulates intestines to absorb
+ Stimulates osteoclasts recover waste calcium from the urine. calcium from digesting food
+ Bone is broken down, releasing + PTH stimulates kidney tubule cells
calcium ions into bloodstream to release calcitriol.
i at Intestinal lumen
Kidney _ Interstitial
Urine tubule cells fluid
+ Food
— . Intestinal cells
Compact / a A
bone =
| Intestinal
IF - : connective tissue
Osteoclasts = | with blood supply
PT
Osteoblasts
Parathyroid hormone increases blood calcium levels when they
drop too low. Conversely, calcitonin, which is released from
the thyroid gland, decreases blood calcium levels when they
become too high. These two mechanisms constantly maintain
blood calcium concentration at homeostasis.
Abnormally high activity of the parathyroid gland can cause
hyperparathyroidism, a disorder caused by an overproduction of PTH that
results in excessive calcium reabsorption from bone. Hyperparathyroidism
can significantly decrease bone density, leading to spontaneous fractures or
deformities. As blood calcium levels rise, cell membrane permeability to
sodium is decreased, and the responsiveness of the nervous system is
reduced. At the same time, calcium deposits may collect in the body’s
tissues and organs, impairing their functioning.
In contrast, abnormally low blood calcium levels may be caused by
parathyroid hormone deficiency, called hypoparathyroidism, which may
develop following injury or surgery involving the thyroid gland. Low blood
calcium increases membrane permeability to sodium, resulting in muscle
twitching, cramping, spasms, or convulsions. Severe deficits can paralyze
muscles, including those involved in breathing, and can be fatal.
When blood calcium levels are high, calcitonin is produced and secreted by
the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin
inhibits the activity of osteoclasts, reduces the absorption of dietary calcium
in the intestine, and signals the kidneys to reabsorb less calcium, resulting
in larger amounts of calcium excreted in the urine.
Chapter Review
Calcium is required for a variety of important physiologic processes,
including neuromuscular functioning; thus, blood calcium levels are closely
regulated. The parathyroid glands are small structures located on the
posterior thyroid gland that produce parathyroid hormone (PTH), which
regulates blood calcium levels. Low blood calcium levels cause the
production and secretion of PTH. In contrast, elevated blood calcium levels
inhibit secretion of PTH and trigger secretion of the thyroid hormone
calcitonin. Underproduction of PTH can result in hypoparathyroidism. In
contrast, overproduction of PTH can result in hyperparathyroidism.
Review Questions
Exercise:
Problem:
When blood calcium levels are low, PTH stimulates
a. urinary excretion of calcium by the kidneys
b. a reduction in calcium absorption from the intestines
c. the activity of osteoblasts
d. the activity of osteoclasts
Solution:
D
Exercise:
Problem:
Which of the following can result from hyperparathyroidism?
a. increased bone deposition
b. fractures
c. convulsions
d. all of the above
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Describe the role of negative feedback in the function of the
parathyroid gland.
Solution:
The production and secretion of PTH is regulated by a negative
feedback loop. Low blood calcium levels initiate the production and
secretion of PTH. PTH increases bone resorption, calcium absorption
from the intestines, and calcium reabsorption by the kidneys. As a
result, blood calcium levels begin to rise. This, in turn, inhibits the
further production and secretion of PTH.
Exercise:
Problem:
Explain why someone with a parathyroid gland tumor might develop
kidney stones.
Solution:
A parathyroid gland tumor can prompt hypersecretion of PTH. This
can raise blood calcium levels so excessively that calcium deposits
begin to accumulate throughout the body, including in the kidney
tubules, where they are referred to as kidney stones.
Glossary
hyperparathyroidism
disorder caused by overproduction of PTH that results in abnormally
elevated blood calcium
hypoparathyroidism
disorder caused by underproduction of PTH that results in abnormally
low blood calcium
parathyroid glands
small, round glands embedded in the posterior thyroid gland that
produce parathyroid hormone (PTH)
parathyroid hormone (PTH)
peptide hormone produced and secreted by the parathyroid glands in
response to low blood calcium levels
The Adrenal Glands
By the end of this section, you will be able to:
¢ Describe the location and structure of the adrenal glands
e Identify the hormones produced by the adrenal cortex and adrenal
medulla, and summarize their target cells and effects
The adrenal glands are wedges of glandular and neuroendocrine tissue
adhering to the top of the kidneys by a fibrous capsule ({link]). The adrenal
glands have a rich blood supply and experience one of the highest rates of
blood flow in the body. They are served by several arteries branching off
the aorta, including the suprarenal and renal arteries. Blood flows to each
adrenal gland at the adrenal cortex and then drains into the adrenal medulla.
Adrenal hormones are released into the circulation via the left and right
suprarenal veins.
Adrenal Glands
Connective tissue |
capsule
Tissue area Hormones released Examples
‘+ Zona glomerulosa ——*> Mineralcorticoids
eee f eh ‘ Se hs cortex) (regulate mineral
4
pes - oe 4}
Aldosterone
balance)
Zona fasciculata ———® Glucocorticoids Cortisol
(adrenal cortex) (regulate glucose Corticosterone
metabolism) Cortisone
Zona reticularis ———- Androgens A
a Dehydroepian-
(adrenal cortex) (stimulate rap sabe
masculinization)
Adrenal medulla ——— Stress hormones Epinephrine
(stimulate Norepinephrine
sympathetic ANS)
Adrenal gland
Superior surface of
kidney
Both adrenal glands sit atop the kidneys and are composed of
an outer cortex and an inner medulla, all surrounded by a
connective tissue capsule. The cortex can be subdivided into
additional zones, all of which produce different types of
hormones. LM x 204. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)
Note:
od
es
—_s
mss’ OPENStax COLLEGE
— , 7
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
The adrenal gland consists of an outer cortex of glandular tissue and an
inner medulla of nervous tissue. The cortex itself is divided into three
zones: the zona glomerulosa, the zona fasciculata, and the zona
reticularis. Each region secretes its own set of hormones.
The adrenal cortex, as a component of the hypothalamic-pituitary-adrenal
(HPA) axis, secretes steroid hormones important for the regulation of the
long-term stress response, blood pressure and blood volume, nutrient uptake
and storage, fluid and electrolyte balance, and inflammation. The HPA axis
involves the stimulation of hormone release of adrenocorticotropic hormone
(ACTH) from the pituitary by the hypothalamus. ACTH then stimulates the
adrenal cortex to produce the hormone cortisol. This pathway will be
discussed in more detail below.
The adrenal medulla is neuroendocrine tissue composed of postganglionic
sympathetic nervous system (SNS) neurons. It is really an extension of the
autonomic nervous system, which regulates homeostasis in the body. The
sympathomedullary (SAM) pathway involves the stimulation of the
medulla by impulses from the hypothalamus via neurons from the thoracic
spinal cord. The medulla is stimulated to secrete the amine hormones
epinephrine and norepinephrine.
One of the major functions of the adrenal gland is to respond to stress.
Stress can be either physical or psychological or both. Physical stresses
include exposing the body to injury, walking outside in cold and wet
conditions without a coat on, or malnutrition. Psychological stresses include
the perception of a physical threat, a fight with a loved one, or just a bad
day at school.
The body responds in different ways to short-term stress and long-term
stress following a pattern known as the general adaptation syndrome
(GAS). Stage one of GAS is called the alarm reaction. This is short-term
stress, the fight-or-flight response, mediated by the hormones epinephrine
and norepinephrine from the adrenal medulla via the SAM pathway. Their
function is to prepare the body for extreme physical exertion. Once this
stress is relieved, the body quickly returns to normal. The section on the
adrenal medulla covers this response in more detail.
If the stress is not soon relieved, the body adapts to the stress in the second
stage called the stage of resistance. If a person is starving for example, the
body may send signals to the gastrointestinal tract to maximize the
absorption of nutrients from food.
If the stress continues for a longer term however, the body responds with
symptoms quite different than the fight-or-flight response. During the stage
of exhaustion, individuals may begin to suffer depression, the suppression
of their immune response, severe fatigue, or even a fatal heart attack. These
symptoms are mediated by the hormones of the adrenal cortex, especially
cortisol, released as a result of signals from the HPA axis.
Adrenal hormones also have several non-stress-related functions, including
the increase of blood sodium and glucose levels, which will be described in
detail below.
Adrenal Cortex
The adrenal cortex consists of multiple layers of lipid-storing cells that
occur in three structurally distinct regions. Each of these regions produces
different hormones.
Note:
. or
mss’ OPENStAX COLLEGE
Visit this link to view an animation describing the location and function of
the adrenal glands. Which hormone produced by the adrenal glands is
responsible for the mobilization of energy stores?
Hormones of the Zona Glomerulosa
The most superficial region of the adrenal cortex is the zona glomerulosa,
which produces a group of hormones collectively referred to as
mineralocorticoids because of their effect on body minerals, especially
sodium and potassium. These hormones are essential for fluid and
electrolyte balance.
Aldosterone is the major mineralocorticoid. It is important in the regulation
of the concentration of sodium and potassium ions in urine, sweat, and
saliva. For example, it is released in response to elevated blood K™, low
blood Na‘, low blood pressure, or low blood volume. In response,
aldosterone increases the excretion of K” and the retention of Na‘, which in
turn increases blood volume and blood pressure. Its secretion is prompted
when CRH from the hypothalamus triggers ACTH release from the anterior
pituitary.
Aldosterone is also a key component of the renin-angiotensin-aldosterone
system (RAAS) in which specialized cells of the kidneys secrete the
enzyme renin in response to low blood volume or low blood pressure.
Renin then catalyzes the conversion of the blood protein angiotensinogen,
produced by the liver, to the hormone angiotensin I. Angiotensin I is
converted in the lungs to angiotensin II by angiotensin-converting enzyme
(ACE). Angiotensin II has three major functions:
1. Initiating vasoconstriction of the arterioles, decreasing blood flow
2. Stimulating kidney tubules to reabsorb NaCl and water, increasing
blood volume
3. Signaling the adrenal cortex to secrete aldosterone, the effects of
which further contribute to fluid retention, restoring blood pressure
and blood volume
For individuals with hypertension, or high blood pressure, drugs are
available that block the production of angiotensin IT. These drugs, known as
ACE inhibitors, block the ACE enzyme from converting angiotensin I to
angiotensin II, thus mitigating the latter’s ability to increase blood pressure.
Hormones of the Zona Fasciculata
The intermediate region of the adrenal cortex is the zona fasciculata, named
as such because the cells form small fascicles (bundles) separated by tiny
blood vessels. The cells of the zona fasciculata produce hormones called
glucocorticoids because of their role in glucose metabolism. The most
important of these is cortisol, some of which the liver converts to cortisone.
A glucocorticoid produced in much smaller amounts is corticosterone. In
response to long-term stressors, the hypothalamus secretes CRH, which in
turn triggers the release of ACTH by the anterior pituitary. ACTH triggers
the release of the glucocorticoids. Their overall effect is to inhibit tissue
building while stimulating the breakdown of stored nutrients to maintain
adequate fuel supplies. In conditions of long-term stress, for example,
cortisol promotes the catabolism of glycogen to glucose, the catabolism of
stored triglycerides into fatty acids and glycerol, and the catabolism of
muscle proteins into amino acids. These raw materials can then be used to
synthesize additional glucose and ketones for use as body fuels. The
hippocampus, which is part of the temporal lobe of the cerebral cortices and
important in memory formation, is highly sensitive to stress levels because
of its many glucocorticoid receptors.
You are probably familiar with prescription and over-the-counter
medications containing glucocorticoids, such as cortisone injections into
inflamed joints, prednisone tablets and steroid-based inhalers used to
manage severe asthma, and hydrocortisone creams applied to relieve itchy
skin rashes. These drugs reflect another role of cortisol—the
downregulation of the immune system, which inhibits the inflammatory
response.
Hormones of the Zona Reticularis
The deepest region of the adrenal cortex is the zona reticularis, which
produces small amounts of a class of steroid sex hormones called
androgens. During puberty and most of adulthood, androgens are produced
in the gonads. The androgens produced in the zona reticularis supplement
the gonadal androgens. They are produced in response to ACTH from the
anterior pituitary and are converted in the tissues to testosterone or
estrogens. In adult women, they may contribute to the sex drive, but their
function in adult men is not well understood. In post-menopausal women,
as the functions of the ovaries decline, the main source of estrogens
becomes the androgens produced by the zona reticularis.
Adrenal Medulla
As noted earlier, the adrenal cortex releases glucocorticoids in response to
long-term stress such as severe illness. In contrast, the adrenal medulla
releases its hormones in response to acute, short-term stress mediated by the
sympathetic nervous system (SNS).
The medullary tissue is composed of unique postganglionic SNS neurons
called chromaffin cells, which are large and irregularly shaped, and
produce the neurotransmitters epinephrine (also called adrenaline) and
norepinephrine (or noradrenaline). Epinephrine is produced in greater
quantities—approximately a 4 to 1 ratio with norepinephrine—and is the
more powerful hormone. Because the chromaffin cells release epinephrine
and norepinephrine into the systemic circulation, where they travel widely
and exert effects on distant cells, they are considered hormones. Derived
from the amino acid tyrosine, they are chemically classified as
catecholamines.
The secretion of medullary epinephrine and norepinephrine is controlled by
a neural pathway that originates from the hypothalamus in response to
danger or stress (the SAM pathway). Both epinephrine and norepinephrine
signal the liver and skeletal muscle cells to convert glycogen into glucose,
resulting in increased blood glucose levels. These hormones increase the
heart rate, pulse, and blood pressure to prepare the body to fight the
perceived threat or flee from it. In addition, the pathway dilates the airways,
raising blood oxygen levels. It also prompts vasodilation, further increasing
the oxygenation of important organs such as the lungs, brain, heart, and
skeletal muscle. At the same time, it triggers vasoconstriction to blood
vessels serving less essential organs such as the gastrointestinal tract,
kidneys, and skin, and downregulates some components of the immune
system. Other effects include a dry mouth, loss of appetite, pupil dilation,
and a loss of peripheral vision. The major hormones of the adrenal glands
are summarized in [link].
Hormones of the Adrenal Glands
Adrenal Associated Chemical
gland hormones class Effect
Adrenal iaeiaiane Sirota Dice’ blood
cortex Na’ levels
Adrenal Cortisol, ; Increase blood
corticosterone, Steroid
cortex ; glucose levels
cortisone
Stimulate fight-
Amine or-flight
response
Adrenal Epinephrine,
medulla norepinephrine
Disorders Involving the Adrenal Glands
Several disorders are caused by the dysregulation of the hormones produced
by the adrenal glands. For example, Cushing’s disease is a disorder
characterized by high blood glucose levels and the accumulation of lipid
deposits on the face and neck. It is caused by hypersecretion of cortisol. The
most common source of Cushing’s disease is a pituitary tumor that secretes
cortisol or ACTH in abnormally high amounts. Other common signs of
Cushing’s disease include the development of a moon-shaped face, a
buffalo hump on the back of the neck, rapid weight gain, and hair loss.
Chronically elevated glucose levels are also associated with an elevated risk
of developing type 2 diabetes. In addition to hyperglycemia, chronically
elevated glucocorticoids compromise immunity, resistance to infection, and
memory, and can result in rapid weight gain and hair loss.
In contrast, the hyposecretion of corticosteroids can result in Addison’s
disease, a rare disorder that causes low blood glucose levels and low blood
sodium levels. The signs and symptoms of Addison’s disease are vague and
are typical of other disorders as well, making diagnosis difficult. They may
include general weakness, abdominal pain, weight loss, nausea, vomiting,
sweating, and cravings for salty food.
Chapter Review
The adrenal glands, located superior to each kidney, consist of two regions:
the adrenal cortex and adrenal medulla. The adrenal cortex—the outer layer
of the gland—produces mineralocorticoids, glucocorticoids, and androgens.
The adrenal medulla at the core of the gland produces epinephrine and
norepinephrine.
The adrenal glands mediate a short-term stress response and a long-term
stress response. A perceived threat results in the secretion of epinephrine
and norepinephrine from the adrenal medulla, which mediate the fight-or-
flight response. The long-term stress response is mediated by the secretion
of CRH from the hypothalamus, which triggers ACTH, which in turn
stimulates the secretion of corticosteroids from the adrenal cortex. The
mineralocorticoids, chiefly aldosterone, cause sodium and fluid retention,
which increases blood volume and blood pressure.
Interactive Link Questions
Exercise:
Problem:
Visit this link to view an animation describing the location and
function of the adrenal glands. Which hormone produced by the
adrenal glands is responsible for mobilization of energy stores?
Solution:
Cortisol.
Review Questions
Exercise:
Problem:The adrenal glands are attached superiorly to which organ?
a. thyroid
b. liver
c. kidneys
d. hypothalamus
Solution:
C
Exercise:
Problem: What secretory cell type is found in the adrenal medulla?
a. chromaffin cells
b. neuroglial cells
c. follicle cells
d. oxyphil cells
Solution:
A
Exercise:
Problem: Cushing’s disease is a disorder caused by
a. abnormally low levels of cortisol
b. abnormally high levels of cortisol
c. abnormally low levels of aldosterone
d. abnormally high levels of aldosterone
Solution:
B
Exercise:
Problem:
Which of the following responses s not part of the fight-or-flight
response?
a. pupil dilation
b. increased oxygen supply to the lungs
c. suppressed digestion
d. reduced mental activity
Solution:
D
Critical Thinking Questions
Exercise:
Problem:
What are the three regions of the adrenal cortex and what hormones do
they produce?
Solution:
The outer region is the zona glomerulosa, which produces
mineralocorticoids such as aldosterone; the next region is the zona
fasciculata, which produces glucocorticoids such as cortisol; the inner
region is the zona reticularis, which produces androgens.
Exercise:
Problem:
If innervation to the adrenal medulla were disrupted, what would be
the physiological outcome?
Solution:
Damage to the innervation of the adrenal medulla would prevent the
adrenal glands from responding to the hypothalamus during the fight-
or-flight response. Therefore, the response would be reduced.
Exercise:
Problem:
Compare and contrast the short-term and long-term stress response.
Solution:
The short-term stress response involves the hormones epinephrine and
norepinephrine, which work to increase the oxygen supply to organs
important for extreme muscular action such as the brain, lungs, and
muscles. In the long-term stress response, the hormone cortisol is
involved in catabolism of glycogen stores, proteins, and triglycerides,
glucose and ketone synthesis, and downregulation of the immune
system.
Glossary
adrenal cortex
outer region of the adrenal glands consisting of multiple layers of
epithelial cells and capillary networks that produces mineralocorticoids
and glucocorticoids
adrenal glands
endocrine glands located at the top of each kidney that are important
for the regulation of the stress response, blood pressure and blood
volume, water homeostasis, and electrolyte levels
adrenal medulla
inner layer of the adrenal glands that plays an important role in the
stress response by producing epinephrine and norepinephrine
angiotensin-converting enzyme
the enzyme that converts angiotensin I to angiotensin IT
alarm reaction
the short-term stress, or the fight-or-flight response, of stage one of the
general adaptation syndrome mediated by the hormones epinephrine
and norepinephrine
aldosterone
hormone produced and secreted by the adrenal cortex that stimulates
sodium and fluid retention and increases blood volume and blood
pressure
chromaffin
neuroendocrine cells of the adrenal medulla
cortisol
glucocorticoid important in gluconeogenesis, the catabolism of
glycogen, and downregulation of the immune system
epinephrine
primary and most potent catecholamine hormone secreted by the
adrenal medulla in response to short-term stress; also called adrenaline
general adaptation syndrome (GAS)
the human body’s three-stage response pattern to short- and long-term
stress
glucocorticoids
hormones produced by the zona fasciculata of the adrenal cortex that
influence glucose metabolism
mineralocorticoids
hormones produced by the zona glomerulosa cells of the adrenal
cortex that influence fluid and electrolyte balance
norepinephrine
secondary catecholamine hormone secreted by the adrenal medulla in
response to short-term stress; also called noradrenaline
stage of exhaustion
stage three of the general adaptation syndrome; the body’s long-term
response to stress mediated by the hormones of the adrenal cortex
stage of resistance
stage two of the general adaptation syndrome; the body’s continued
response to stress after stage one diminishes
zona fasciculata
intermediate region of the adrenal cortex that produce hormones called
glucocorticoids
zona glomerulosa
most superficial region of the adrenal cortex, which produces the
hormones collectively referred to as mineralocorticoids
zona reticularis
deepest region of the adrenal cortex, which produces the steroid sex
hormones called androgens
The Pineal Gland
By the end of this section, you will be able to:
e Describe the location and structure of the pineal gland
e Discuss the function of melatonin
Recall that the hypothalamus, part of the diencephalon of the brain, sits
inferior and somewhat anterior to the thalamus. Inferior but somewhat
posterior to the thalamus is the pineal gland, a tiny endocrine gland whose
functions are not entirely clear. The pinealocyte cells that make up the
pineal gland are known to produce and secrete the amine hormone
melatonin, which is derived from serotonin.
The secretion of melatonin varies according to the level of light received
from the environment. When photons of light stimulate the retinas of the
eyes, a nerve impulse is sent to a region of the hypothalamus called the
suprachiasmatic nucleus (SCN), which is important in regulating biological
rhythms. From the SCN, the nerve signal is carried to the spinal cord and
eventually to the pineal gland, where the production of melatonin is
inhibited. As a result, blood levels of melatonin fall, promoting
wakefulness. In contrast, as light levels decline—such as during the evening
—melatonin production increases, boosting blood levels and causing
drowsiness.
Note:
openstax COLLEGE”
:
2
Visit this link to view an animation describing the function of the hormone
melatonin. What should you avoid doing in the middle of your sleep cycle
that would lower melatonin?
The secretion of melatonin may influence the body’s circadian rhythms, the
dark-light fluctuations that affect not only sleepiness and wakefulness, but
also appetite and body temperature. Interestingly, children have higher
melatonin levels than adults, which may prevent the release of
gonadotropins from the anterior pituitary, thereby inhibiting the onset of
puberty. Finally, an antioxidant role of melatonin is the subject of current
research.
Jet lag occurs when a person travels across several time zones and feels
sleepy during the day or wakeful at night. Traveling across multiple time
zones significantly disturbs the light-dark cycle regulated by melatonin. It
can take up to several days for melatonin synthesis to adjust to the light-
dark patterns in the new environment, resulting in jet lag. Some air travelers
take melatonin supplements to induce sleep.
Chapter Review
The pineal gland is an endocrine structure of the diencephalon of the brain,
and is located inferior and posterior to the thalamus. It is made up of
pinealocytes. These cells produce and secrete the hormone melatonin in
response to low light levels. High blood levels of melatonin induce
drowsiness. Jet lag, caused by traveling across several time zones, occurs
because melatonin synthesis takes several days to readjust to the light-dark
patterns in the new environment.
Interactive Link Questions
Exercise:
Problem:
Visit this link to view an animation describing the function of the
hormone melatonin. What should you avoid doing in the middle of
your sleep cycle that would lower melatonin?
Solution:
Turning on the lights.
Review Questions
Exercise:
Problem: What cells secrete melatonin?
a. melanocytes
b. pinealocytes
c. suprachiasmatic nucleus cells
d. retinal cells
Solution:
B
Exercise:
Problem: The production of melatonin is inhibited by
a. declining levels of light
b. exposure to bright light
c. the secretion of serotonin
d. the activity of pinealocytes
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
Seasonal affective disorder (SAD) is a mood disorder characterized by,
among other symptoms, increased appetite, sluggishness, and
increased sleepiness. It occurs most commonly during the winter
months, especially in regions with long winter nights. Propose a role
for melatonin in SAD and a possible non-drug therapy.
Solution:
SAD is thought to occur in part because low levels and duration of
sunlight allow excessive and prolonged secretion of melatonin. Light
therapy—daytime exposure to very bright lighting—is one common
therapy.
Exercise:
Problem:
Retinitis pigmentosa (RP) is a disease that causes deterioration of the
retinas of the eyes. Describe the impact RP would have on melatonin
levels.
Solution:
The retina is important for melatonin production because it senses
light. Bright light inhibits the production of melatonin, whereas low
light levels promote the production of melatonin. Therefore,
deterioration of the retinas would most likely disturb the sleep-wake
pattern because melatonin production would be elevated.
Glossary
melatonin
amino acid—derived hormone that is secreted in response to low light
and causes drowsiness
pineal gland
endocrine gland that secretes melatonin, which is important in
regulating the sleep-wake cycle
pinealocyte
cell of the pineal gland that produces and secretes the hormone
melatonin
Gonadal and Placental Hormones
By the end of this section, you will be able to:
e Identify the most important hormones produced by the testes and
ovaries
e Name the hormones produced by the placenta and state their functions
This section briefly discusses the hormonal role of the gonads—the male
testes and female ovaries—which produce the sex cells (sperm and ova) and
secrete the gonadal hormones. The roles of the gonadotropins released from
the anterior pituitary (FSH and LH) were discussed earlier.
The primary hormone produced by the male testes is testosterone, a steroid
hormone important in the development of the male reproductive system, the
maturation of sperm cells, and the development of male secondary sex
characteristics such as a deepened voice, body hair, and increased muscle
mass. Interestingly, testosterone is also produced in the female ovaries, but
at a much reduced level. In addition, the testes produce the peptide hormone
inhibin, which inhibits the secretion of FSH from the anterior pituitary
gland. FSH stimulates spermatogenesis.
The primary hormones produced by the ovaries are estrogens, which
include estradiol, estriol, and estrone. Estrogens play an important role in a
larger number of physiological processes, including the development of the
female reproductive system, regulation of the menstrual cycle, the
development of female secondary sex characteristics such as increased
adipose tissue and the development of breast tissue, and the maintenance of
pregnancy. Another significant ovarian hormone is progesterone, which
contributes to regulation of the menstrual cycle and is important in
preparing the body for pregnancy as well as maintaining pregnancy. In
addition, the granulosa cells of the ovarian follicles produce inhibin, which
—as in males—inhibits the secretion of FSH.During the initial stages of
pregnancy, an organ called the placenta develops within the uterus. The
placenta supplies oxygen and nutrients to the fetus, excretes waste products,
and produces and secretes estrogens and progesterone. The placenta
produces human chorionic gonadotropin (hCG) as well. The hCG hormone
promotes progesterone synthesis and reduces the mother’s immune function
to protect the fetus from immune rejection. It also secretes human placental
lactogen (hPL), which plays a role in preparing the breasts for lactation, and
relaxin, which is thought to help soften and widen the pubic symphysis in
preparation for childbirth. The hormones controlling reproduction are
summarized in [link].
Reproductive Hormones
Associated
Gonad hormones
Testes Testosterone
Testes Inhibin
Estrogens
Ovaries and
progesterone
Human
Placenta chorionic
gonadotropin
Chemical
class
Steroid
Protein
Steroid
Protein
Effect
Stimulates
development of male
secondary sex
characteristics and
sperm production
Inhibits FSH release
from pituitary
Stimulate
development of
female secondary sex
characteristics and
prepare the body for
childbirth
Promotes
progesterone
synthesis during
pregnancy and
inhibits immune
response against fetus
Note:
Everyday Connections
Anabolic Steroids
The endocrine system can be exploited for illegal or unethical purposes. A
prominent example of this is the use of steroid drugs by professional
athletes.
Commonly used for performance enhancement, anabolic steroids are
synthetic versions of the male sex hormone, testosterone. By boosting
natural levels of this hormone, athletes experience increased muscle mass.
Synthetic versions of human growth hormone are also used to build muscle
mass.
The use of performance-enhancing drugs is banned by all major collegiate
and professional sports organizations in the United States because they
impart an unfair advantage to athletes who take them. In addition, the
drugs can cause significant and dangerous side effects. For example,
anabolic steroid use can increase cholesterol levels, raise blood pressure,
and damage the liver. Altered testosterone levels (both too low or too high)
have been implicated in causing structural damage to the heart, and
increasing the risk for cardiac arrhythmias, heart attacks, congestive heart
failure, and sudden death. Paradoxically, steroids can have a feminizing
effect in males, including shriveled testicles and enlarged breast tissue. In
females, their use can cause masculinizing effects such as an enlarged
clitoris and growth of facial hair. In both sexes, their use can promote
increased aggression (commonly known as “roid-rage”), depression, sleep
disturbances, severe acne, and infertility.
Chapter Review
The male and female reproductive system is regulated by follicle-
stimulating hormone (FSH) and luteinizing hormone (LH) produced by the
anterior lobe of the pituitary gland in response to gonadotropin-releasing
hormone (GnRH) from the hypothalamus. In males, FSH stimulates sperm
maturation, which is inhibited by the hormone inhibin. The steroid hormone
testosterone, a type of androgen, is released in response to LH and is
responsible for the maturation and maintenance of the male reproductive
system, as well as the development of male secondary sex characteristics. In
females, FSH promotes egg maturation and LH signals the secretion of the
female sex hormones, the estrogens and progesterone. Both of these
hormones are important in the development and maintenance of the female
reproductive system, as well as maintaining pregnancy. The placenta
develops during early pregnancy, and secretes several hormones important
for maintaining the pregnancy.
Review Questions
Exercise:
Problem:The gonads produce what class of hormones?
a. amine hormones
b. peptide hormones
c. steroid hormones
d. catecholamines
Solution:
G
Exercise:
Problem:
The production of FSH by the anterior pituitary is reduced by which
hormone?
a. estrogens
b. progesterone
c. relaxin
d. inhibin
Solution:
D
Exercise:
Problem:
The function of the placental hormone human placental lactogen (hPL)
is to
a. prepare the breasts for lactation
b. nourish the placenta
c. regulate the menstrual cycle
d. all of the above
Solution:
A
Critical Thinking Questions
Exercise:
Problem:Compare and contrast the role of estrogens and progesterone.
Solution:
Both estrogens and progesterone are steroid hormones produced by the
ovaries that help regulate the menstrual cycle. Estrogens play an
important role in the development of the female reproductive tract and
secondary sex characteristics. They also help maintain pregnancy.
Progesterone prepares the body for pregnancy and helps maintain
pregnancy.
Exercise:
Problem:
Describe the role of placental secretion of relaxin in preparation for
childbirth.
Solution:
Relaxin produced by the placenta is thought to soften and widen the
pubic symphysis. This increases the size of the pelvic outlet, the birth
canal through which the fetus passes during vaginal childbirth.
Glossary
estrogens
class of predominantly female sex hormones important for the
development and growth of the female reproductive tract, secondary
sex characteristics, the female reproductive cycle, and the maintenance
of pregnancy
inhibin
hormone secreted by the male and female gonads that inhibits FSH
production by the anterior pituitary
progesterone
predominantly female sex hormone important in regulating the female
reproductive cycle and the maintenance of pregnancy
testosterone
steroid hormone secreted by the male testes and important in the
maturation of sperm cells, growth and development of the male
reproductive system, and the development of male secondary sex
characteristics
The Endocrine Pancreas
By the end of this section, you will be able to:
e Describe the location and structure of the pancreas, and the
morphology and function of the pancreatic islets
¢ Compare and contrast the functions of insulin and glucagon
The pancreas is a long, slender organ, most of which is located posterior to
the bottom half of the stomach ([link]). Although it is primarily an exocrine
gland, secreting a variety of digestive enzymes, the pancreas has an
endocrine function. Its pancreatic islets—clusters of cells formerly known
as the islets of Langerhans—secrete the hormones glucagon, insulin,
somatostatin, and pancreatic polypeptide (PP).
Pancreas
Splenic artery
Pancreatic
hormones:
¢ Insulin
¢ Glucagon
Spleen
Pancreatic islets
Bile duct (from
gall bladder)
Common bile duct
Duodenum of
small intestine
Acinar cells
secrete digestive
enzymes
Pancreatic duct
} ~~ Exocrine acinus
The pancreatic exocrine function involves the acinar
cells secreting digestive enzymes that are transported
into the small intestine by the pancreatic duct. Its
endocrine function involves the secretion of insulin
(produced by beta cells) and glucagon (produced by
alpha cells) within the pancreatic islets. These two
hormones regulate the rate of glucose metabolism in the
body. The micrograph reveals pancreatic islets. LM x
760. (Micrograph provided by the Regents of University
of Michigan Medical School © 2012)
Note:
|
[=]
1
- -
mss’ OPENStax COLLEGE
pot
Data
View the University of Michigan WebScope to explore the tissue sample in
greater detail.
Cells and Secretions of the Pancreatic Islets
The pancreatic islets each contain four varieties of cells:
The alpha cell produces the hormone glucagon and makes up
approximately 20 percent of each islet. Glucagon plays an important
role in blood glucose regulation; low blood glucose levels stimulate its
release.
The beta cell produces the hormone insulin and makes up
approximately 75 percent of each islet. Elevated blood glucose levels
stimulate the release of insulin.
The delta cell accounts for four percent of the islet cells and secretes
the peptide hormone somatostatin. Recall that somatostatin is also
released by the hypothalamus (as GHIH), and the stomach and
intestines also secrete it. An inhibiting hormone, pancreatic
somatostatin inhibits the release of both glucagon and insulin.
The PP cell accounts for about one percent of islet cells and secretes
the pancreatic polypeptide hormone. It is thought to play a role in
appetite, as well as in the regulation of pancreatic exocrine and
endocrine secretions. Pancreatic polypeptide released following a meal
may reduce further food consumption; however, it is also released in
response to fasting.
Regulation of Blood Glucose Levels by Insulin and Glucagon
Glucose is required for cellular respiration and is the preferred fuel for all
body cells. The body derives glucose from the breakdown of the
carbohydrate-containing foods and drinks we consume. Glucose not
immediately taken up by cells for fuel can be stored by the liver and
muscles as glycogen, or converted to triglycerides and stored in the adipose
tissue. Hormones regulate both the storage and the utilization of glucose as
required. Receptors located in the pancreas sense blood glucose levels, and
subsequently the pancreatic cells secrete glucagon or insulin to maintain
normal levels.
Glucagon
Receptors in the pancreas can sense the decline in blood glucose levels,
such as during periods of fasting or during prolonged labor or exercise
({link]). In response, the alpha cells of the pancreas secrete the hormone
glucagon, which has several effects:
e It stimulates the liver to convert its stores of glycogen back into
glucose. This response is known as glycogenolysis. The glucose is
then released into the circulation for use by body cells.
e It stimulates the liver to take up amino acids from the blood and
convert them into glucose. This response is known as gluconeogenesis.
e It stimulates lipolysis, the breakdown of stored triglycerides into free
fatty acids and glycerol. Some of the free glycerol released into the
bloodstream travels to the liver, which converts it into glucose. This is
also a form of gluconeogenesis.
Taken together, these actions increase blood glucose levels. The activity of
glucagon is regulated through a negative feedback mechanism; rising blood
glucose levels inhibit further glucagon production and secretion.
Homeostatic Regulation of Blood Glucose Levels
Insulin release: Insulin effects:
¢ Beta cells of pancreas
release insulin * Triggers body cells to take up
glucose from the blood and
utilize it in cellular respiration
Splenic artery
¢ Inhibits glycogenolysis
— glucose is removed from the
blood and stored as glycogen
in the liver
¢ Inhibits gluconeogenesis Rough ER
- amino acids and free glycerol
are NOT converted to
glucose in the ER
Smooth ER
Blood glucose
concentration
decreases
Hyperglycemia
(elevated blood glucose)
Hypoglycemia
(low blood glucose) yh
START: Homeostasis
(70-110 mg/dL) ee
Blood glucose
concentration
increases
Glucagon effects:
Inhibits body cells from taking
up glucose from the blood and
Glucagon release: utilizing it in cellular respiration
¢ Alpha cells of pancreas
| |
nee ee Splenic artery * Stimulates glycogenolysis
— glycogen in the liver is
broken down into glucose
and released into the blood
¢ Stimulates gluconeogenesis
— amino acids and free glycerol Rough ER
are converted to glucose in
the ER and released into the
blood
Smooth ER
Blood glucose concentration is tightly maintained between 70
mg/dL and 110 mg/dL. If blood glucose concentration rises
above this range, insulin is released, which stimulates body
cells to remove glucose from the blood. If blood glucose
concentration drops below this range, glucagon is released,
which stimulates body cells to release glucose into the blood.
Insulin
The primary function of insulin is to facilitate the uptake of glucose into
body cells. Red blood cells, as well as cells of the brain, liver, kidneys, and
the lining of the small intestine, do not have insulin receptors on their cell
membranes and do not require insulin for glucose uptake. Although all
other body cells do require insulin if they are to take glucose from the
bloodstream, skeletal muscle cells and adipose cells are the primary targets
of insulin.
The presence of food in the intestine triggers the release of gastrointestinal
tract hormones such as glucose-dependent insulinotropic peptide
(previously known as gastric inhibitory peptide). This is in turn the initial
trigger for insulin production and secretion by the beta cells of the pancreas.
Once nutrient absorption occurs, the resulting surge in blood glucose levels
further stimulates insulin secretion.
Precisely how insulin facilitates glucose uptake is not entirely clear.
However, insulin appears to activate a tyrosine kinase receptor, triggering
the phosphorylation of many substrates within the cell. These multiple
biochemical reactions converge to support the movement of intracellular
vesicles containing facilitative glucose transporters to the cell membrane. In
the absence of insulin, these transport proteins are normally recycled slowly
between the cell membrane and cell interior. Insulin triggers the rapid
movement of a pool of glucose transporter vesicles to the cell membrane,
where they fuse and expose the glucose transporters to the extracellular
fluid. The transporters then move glucose by facilitated diffusion into the
cell interior.
Visit this link to view an animation describing the location and function of
the pancreas. What goes wrong in the function of insulin in type 2
diabetes?
Insulin also reduces blood glucose levels by stimulating glycolysis, the
metabolism of glucose for generation of ATP. Moreover, it stimulates the
liver to convert excess glucose into glycogen for storage, and it inhibits
enzymes involved in glycogenolysis and gluconeogenesis. Finally, insulin
promotes triglyceride and protein synthesis. The secretion of insulin is
regulated through a negative feedback mechanism. As blood glucose levels
decrease, further insulin release is inhibited. The pancreatic hormones are
summarized in [link].
Hormones of the Pancreas
Chemical
Associated hormones class Effect
Insulin (beta cells) Protein Reduces blood glucose
levels
Hormones of the Pancreas
Chemical
Associated hormones class Effect
Glucagon (alpha cells) Protein Tacreases DiOOG eileos:
levels
Somatostatin (delta Peavieia Inhibits insulin and
cells) glucagon release
Palicrealle/poly pepude Protein Role in appetite
(PP cells)
Note:
Disorders of the...
Endocrine System: Diabetes Mellitus
Dysfunction of insulin production and secretion, as well as the target cells’
responsiveness to insulin, can lead to a condition called diabetes mellitus.
An increasingly common disease, diabetes mellitus has been diagnosed in
more than 18 million adults in the United States, and more than 200,000
children. It is estimated that up to 7 million more adults have the condition
but have not been diagnosed. In addition, approximately 79 million people
in the US are estimated to have pre-diabetes, a condition in which blood
glucose levels are abnormally high, but not yet high enough to be classified
as diabetes.
There are two main forms of diabetes mellitus. Type 1 diabetes is an
autoimmune disease affecting the beta cells of the pancreas. Certain genes
are recognized to increase susceptibility. The beta cells of people with type
1 diabetes do not produce insulin; thus, synthetic insulin must be
administered by injection or infusion. This form of diabetes accounts for
less than five percent of all diabetes cases.
Type 2 diabetes accounts for approximately 95 percent of all cases. It is
acquired, and lifestyle factors such as poor diet, inactivity, and the presence
of pre-diabetes greatly increase a person’s risk. About 80 to 90 percent of
people with type 2 diabetes are overweight or obese. In type 2 diabetes,
cells become resistant to the effects of insulin. In response, the pancreas
increases its insulin secretion, but over time, the beta cells become
exhausted. In many cases, type 2 diabetes can be reversed by moderate
weight loss, regular physical activity, and consumption of a healthy diet;
however, if blood glucose levels cannot be controlled, the diabetic will
eventually require insulin.
Two of the early manifestations of diabetes are excessive urination and
excessive thirst. They demonstrate how the out-of-control levels of glucose
in the blood affect kidney function. The kidneys are responsible for
filtering glucose from the blood. Excessive blood glucose draws water into
the urine, and as a result the person eliminates an abnormally large quantity
of sweet urine. The use of body water to dilute the urine leaves the body
dehydrated, and so the person is unusually and continually thirsty. The
person may also experience persistent hunger because the body cells are
unable to access the glucose in the bloodstream.
Over time, persistently high levels of glucose in the blood injure tissues
throughout the body, especially those of the blood vessels and nerves.
Inflammation and injury of the lining of arteries lead to atherosclerosis and
an increased risk of heart attack and stroke. Damage to the microscopic
blood vessels of the kidney impairs kidney function and can lead to kidney
failure. Damage to blood vessels that serve the eyes can lead to blindness.
Blood vessel damage also reduces circulation to the limbs, whereas nerve
damage leads to a loss of sensation, called neuropathy, particularly in the
hands and feet. Together, these changes increase the risk of injury,
infection, and tissue death (necrosis), contributing to a high rate of toe,
foot, and lower leg amputations in people with diabetes. Uncontrolled
diabetes can also lead to a dangerous form of metabolic acidosis called
ketoacidosis. Deprived of glucose, cells increasingly rely on fat stores for
fuel. However, in a glucose-deficient state, the liver is forced to use an
alternative lipid metabolism pathway that results in the increased
production of ketone bodies (or ketones), which are acidic. The build-up of
ketones in the blood causes ketoacidosis, which—if left untreated—may
lead to a life-threatening “diabetic coma.” Together, these complications
make diabetes the seventh leading cause of death in the United States.
Diabetes is diagnosed when lab tests reveal that blood glucose levels are
higher than normal, a condition called hyperglycemia. The treatment of
diabetes depends on the type, the severity of the condition, and the ability
of the patient to make lifestyle changes. As noted earlier, moderate weight
loss, regular physical activity, and consumption of a healthful diet can
reduce blood glucose levels. Some patients with type 2 diabetes may be
unable to control their disease with these lifestyle changes, and will require
medication. Historically, the first-line treatment of type 2 diabetes was
insulin. Research advances have resulted in alternative options, including
medications that enhance pancreatic function.
Note:
[=] [=
—
meee, OPENStAX COLLEGE
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Visit this link to view an animation describing the role of insulin and the
pancreas in diabetes.
Chapter Review
The pancreas has both exocrine and endocrine functions. The pancreatic
islet cell types include alpha cells, which produce glucagon; beta cells,
which produce insulin; delta cells, which produce somatostatin; and PP
cells, which produce pancreatic polypeptide. Insulin and glucagon are
involved in the regulation of glucose metabolism. Insulin is produced by the
beta cells in response to high blood glucose levels. It enhances glucose
uptake and utilization by target cells, as well as the storage of excess
glucose for later use. Dysfunction of the production of insulin or target cell
resistance to the effects of insulin causes diabetes mellitus, a disorder
characterized by high blood glucose levels. The hormone glucagon is
produced and secreted by the alpha cells of the pancreas in response to low
blood glucose levels. Glucagon stimulates mechanisms that increase blood
glucose levels, such as the catabolism of glycogen into glucose.
Interactive Link Questions
Exercise:
Problem:
Visit this link to view an animation describing the location and
function of the pancreas. What goes wrong in the function of insulin in
type 2 diabetes?
Solution:
Insulin is overproduced.
Review Questions
Exercise:
Problem:
If an autoimmune disorder targets the alpha cells, production of which
hormone would be directly affected?
a. somatostatin
b. pancreatic polypeptide
c. insulin
d. glucagon
Solution:
D
Exercise:
Problem: Which of the following statements about insulin is true?
a. Insulin acts as a transport protein, carrying glucose across the cell
membrane.
b. Insulin facilitates the movement of intracellular glucose
transporters to the cell membrane.
c. Insulin stimulates the breakdown of stored glycogen into glucose.
d. Insulin stimulates the kidneys to reabsorb glucose into the
bloodstream.
Solution:
B
Critical Thinking Questions
Exercise:
Problem:
What would be the physiological consequence of a disease that
destroyed the beta cells of the pancreas?
Solution:
The beta cells produce the hormone insulin, which is important in the
regulation of blood glucose levels. All insulin-dependent cells of the
body require insulin in order to take up glucose from the bloodstream.
Destruction of the beta cells would result in an inability to produce and
secrete insulin, leading to abnormally high blood glucose levels and
the disease called type 1 diabetes mellitus.
Exercise:
Problem:
Why is foot care extremely important for people with diabetes
mellitus?
Solution:
Excessive blood glucose levels damage the blood vessels and nerves of
the body’s extremities, increasing the risk for injury, infection, and
tissue death. Loss of sensation to the feet means that a diabetic patient
will not be able to feel foot trauma, such as from ill-fitting shoes. Even
minor injuries commonly lead to infection, which , can progress to
tissue death without proper care, requiring amputation.
Glossary
alpha cell
pancreatic islet cell type that produces the hormone glucagon
beta cell
pancreatic islet cell type that produces the hormone insulin
delta cell
minor cell type in the pancreas that secretes the hormone somatostatin
diabetes mellitus
condition caused by destruction or dysfunction of the beta cells of the
pancreas or cellular resistance to insulin that results in abnormally high
blood glucose levels
glucagon
pancreatic hormone that stimulates the catabolism of glycogen to
glucose, thereby increasing blood glucose levels
hyperglycemia
abnormally high blood glucose levels
insulin
pancreatic hormone that enhances the cellular uptake and utilization of
glucose, thereby decreasing blood glucose levels
pancreas
organ with both exocrine and endocrine functions located posterior to
the stomach that is important for digestion and the regulation of blood
glucose
pancreatic islets
specialized clusters of pancreatic cells that have endocrine functions;
also called islets of Langerhans
PP cell
minor cell type in the pancreas that secretes the hormone pancreatic
polypeptide
Organs with Secondary Endocrine Functions
By the end of this section, you will be able to:
e Identify the organs with a secondary endocrine function, the hormone
they produce, and its effects
In your study of anatomy and physiology, you have already encountered a
few of the many organs of the body that have secondary endocrine
functions. Here, you will learn about the hormone-producing activities of
the heart, gastrointestinal tract, kidneys, skeleton, adipose tissue, skin, and
thymus.
Heart
When the body experiences an increase in blood volume or pressure, the
cells of the heart’s atrial wall stretch. In response, specialized cells in the
wall of the atria produce and secrete the peptide hormone atrial natriuretic
peptide (ANP). ANP signals the kidneys to reduce sodium reabsorption,
thereby decreasing the amount of water reabsorbed from the urine filtrate
and reducing blood volume. Other actions of ANP include the inhibition of
renin secretion and the initiation of the renin-angiotensin-aldosterone
system (RAAS) and vasodilation. Therefore, ANP aids in decreasing blood
pressure, blood volume, and blood sodium levels.
Gastrointestinal Tract
The endocrine cells of the GI tract are located in the mucosa of the stomach
and small intestine. Some of these hormones are secreted in response to
eating a meal and aid in digestion. An example of a hormone secreted by
the stomach cells is gastrin, a peptide hormone secreted in response to
stomach distention that stimulates the release of hydrochloric acid. Secretin
is a peptide hormone secreted by the small intestine as acidic chyme
(partially digested food and fluid) moves from the stomach. It stimulates the
release of bicarbonate from the pancreas, which buffers the acidic chyme,
and inhibits the further secretion of hydrochloric acid by the stomach.
Cholecystokinin (CCK) is another peptide hormone released from the small
intestine. It promotes the secretion of pancreatic enzymes and the release of
bile from the gallbladder, both of which facilitate digestion. Other
hormones produced by the intestinal cells aid in glucose metabolism, such
as by stimulating the pancreatic beta cells to secrete insulin, reducing
glucagon secretion from the alpha cells, or enhancing cellular sensitivity to
insulin.
Kidneys
The kidneys participate in several complex endocrine pathways and
produce certain hormones. A decline in blood flow to the kidneys
stimulates them to release the enzyme renin, triggering the renin-
angiotensin-aldosterone (RAAS) system, and stimulating the reabsorption
of sodium and water. The reabsorption increases blood flow and blood
pressure. The kidneys also play a role in regulating blood calcium levels
through the production of calcitriol from vitamin D3, which is released in
response to the secretion of parathyroid hormone (PTH). In addition, the
kidneys produce the hormone erythropoietin (EPO) in response to low
oxygen levels. EPO stimulates the production of red blood cells
(erythrocytes) in the bone marrow, thereby increasing oxygen delivery to
tissues. You may have heard of EPO as a performance-enhancing drug (in a
synthetic form).
Skeleton
Although bone has long been recognized as a target for hormones, only
recently have researchers recognized that the skeleton itself produces at
least two hormones. Fibroblast growth factor 23 (FGF23) is produced by
bone cells in response to increased blood levels of vitamin D3 or phosphate.
It triggers the kidneys to inhibit the formation of calcitriol from vitamin D3
and to increase phosphorus excretion. Osteocalcin, produced by osteoblasts,
stimulates the pancreatic beta cells to increase insulin production. It also
acts on peripheral tissues to increase their sensitivity to insulin and their
utilization of glucose.
Adipose Tissue
Adipose tissue produces and secretes several hormones involved in lipid
metabolism and storage. One important example is leptin, a protein
manufactured by adipose cells that circulates in amounts directly
proportional to levels of body fat. Leptin is released in response to food
consumption and acts by binding to brain neurons involved in energy intake
and expenditure. Binding of leptin produces a feeling of satiety after a meal,
thereby reducing appetite. It also appears that the binding of leptin to brain
receptors triggers the sympathetic nervous system to regulate bone
metabolism, increasing deposition of cortical bone. Adiponectin—another
hormone synthesized by adipose cells—appears to reduce cellular insulin
resistance and to protect blood vessels from inflammation and
atherosclerosis. Its levels are lower in people who are obese, and rise
following weight loss.
Skin
The skin functions as an endocrine organ in the production of the inactive
form of vitamin D3, cholecalciferol. When cholesterol present in the
epidermis is exposed to ultraviolet radiation, it is converted to
cholecalciferol, which then enters the blood. In the liver, cholecalciferol is
converted to an intermediate that travels to the kidneys and is further
converted to calcitriol, the active form of vitamin D3. Vitamin D is
important in a variety of physiological processes, including intestinal
calcium absorption and immune system function. In some studies, low
levels of vitamin D have been associated with increased risks of cancer,
severe asthma, and multiple sclerosis. Vitamin D deficiency in children
causes rickets, and in adults, osteomalacia—both of which are characterized
by bone deterioration.
Thymus
The thymus is an organ of the immune system that is larger and more
active during infancy and early childhood, and begins to atrophy as we age.
Its endocrine function is the production of a group of hormones called
thymosins that contribute to the development and differentiation of T
lymphocytes, which are immune cells. Although the role of thymosins is
not yet well understood, it is clear that they contribute to the immune
response. Thymosins have been found in tissues other than the thymus and
have a wide variety of functions, so the thymosins cannot be strictly
categorized as thymic hormones.
Liver
The liver is responsible for secreting at least four important hormones or
hormone precursors: insulin-like growth factor (somatomedin),
angiotensinogen, thrombopoetin, and hepcidin. Insulin-like growth factor-1
is the immediate stimulus for growth in the body, especially of the bones.
Angiotensinogen is the precursor to angiotensin, mentioned earlier, which
increases blood pressure. Thrombopoetin stimulates the production of the
blood’s platelets. Hepcidins block the release of iron from cells in the body,
helping to regulate iron homeostasis in our body fluids. The major
hormones of these other organs are summarized in [Link].
Organs with Secondary Endocrine Functions and Their Major
Hormones
Organ Major hormones Effects
Reduces blood
Heart Atrial natriuretic volume, blood
peptide (ANP) pressure, and Na*
concentration
Gastrointestinal Gastrin, secretin, and a digestion oad
ae and buffering of
tract cholecystokinin
stomach acids
Organs with Secondary Endocrine Functions and Their Major
Hormones
Organ
Gastrointestinal
tract
Kidneys
Kidneys
Kidneys
Skeleton
Skeleton
Adipose tissue
Adipose tissue
Major hormones
Glucose-dependent
insulinotropic peptide
(GIP) and glucagon-
like peptide 1 (GLP-
1)
Renin
Calcitriol
Erythropoietin
FGF23
Osteocalcin
Leptin
Adiponectin
Effects
Stimulate beta cells
of the pancreas to
release insulin
Stimulates release of
aldosterone
Aids in the
absorption of Ca**
Triggers the
formation of red
blood cells in the
bone marrow
Inhibits production of
calcitriol and
increases phosphate
excretion
Increases insulin
production
Promotes satiety
signals in the brain
Reduces insulin
resistance
Organs with Secondary Endocrine Functions and Their Major
Hormones
Organ
Skin
Thymus (and
other organs)
Liver
Liver
Liver
Liver
Chapter Review
Major hormones
Cholecalciferol
Thymosins
Insulin-like growth
factor-1
Angiotensinogen
Thrombopoetin
Hepcidin
Effects
Modified to form
vitamin D
Among other things,
aids in the
development of T
lymphocytes of the
immune system
Stimulates bodily
growth
Raises blood pressure
Causes increase in
platelets
Blocks release of iron
into body fluids
Some organs have a secondary endocrine function. For example, the walls
of the atria of the heart produce the hormone atrial natriuretic peptide
(ANP), the gastrointestinal tract produces the hormones gastrin, secretin,
and cholecystokinin, which aid in digestion, and the kidneys produce
erythropoietin (EPO), which stimulates the formation of red blood cells.
Even bone, adipose tissue, and the skin have secondary endocrine functions.
Review Questions
Exercise:
Problem:The walls of the atria produce which hormone?
a. cholecystokinin
b. atrial natriuretic peptide
c. renin
d. calcitriol
Solution:
B
Exercise:
Problem:The end result of the RAAS is to
a. reduce blood volume
b. increase blood glucose
c. reduce blood pressure
d. increase blood pressure
Solution:
D
Exercise:
Problem: Athletes may take synthetic EPO to boost their
a. blood calcium levels
b. secretion of growth hormone
c. blood oxygen levels
d. muscle mass
Solution:
C
Exercise:
Problem:
Hormones produced by the thymus play a role in the
a. development of T cells
b. preparation of the body for childbirth
c. regulation of appetite
d. release of hydrochloric acid in the stomach
Solution:
A
Critical Thinking Questions
Exercise:
Problem:Summarize the role of GI tract hormones following a meal.
Solution:
The presence of food in the GI tract stimulates the release of hormones
that aid in digestion. For example, gastrin is secreted in response to
stomach distention and causes the release of hydrochloric acid in the
stomach. Secretin is secreted when acidic chyme enters the small
intestine, and stimulates the release of pancreatic bicarbonate. In the
presence of fat and protein in the duodenum, CCK stimulates the
release of pancreatic digestive enzymes and bile from the gallbladder.
Other GI tract hormones aid in glucose metabolism and other
functions.
Exercise:
Problem:
Compare and contrast the thymus gland in infancy and adulthood.
Solution:
The thymus gland is important for the development and maturation of
T cells. During infancy and early childhood, the thymus gland is large
and very active, as the immune system is still developing. During
adulthood, the thymus gland atrophies because the immune system is
already developed.
Glossary
atrial natriuretic peptide (ANP)
peptide hormone produced by the walls of the atria in response to high
blood pressure, blood volume, or blood sodium that reduces the
reabsorption of sodium and water in the kidneys and promotes
vasodilation
erythropoietin (EPO)
protein hormone secreted in response to low oxygen levels that
triggers the bone marrow to produce red blood cells
leptin
protein hormone secreted by adipose tissues in response to food
consumption that promotes satiety
thymosins
hormones produced and secreted by the thymus that play an important
role in the development and differentiation of T cells
thymus
organ that is involved in the development and maturation of T-cells
and is particularly active during infancy and childhood
Male Anatomy
By the end of this section, you will be able to:
e Describe the structure and function of the organs of the male
reproductive system
¢ Describe the structure and function of the sperm cell
e Explain the events during spermatogenesis that produce haploid sperm
from diploid cells
e Identify the importance of testosterone in male reproductive function
Unique for its role in human reproduction, a gamete is a specialized sex
cell carrying 23 chromosomes—one half the number in body cells. At
fertilization, the chromosomes in one male gamete, called a sperm (or
spermatozoon), combine with the chromosomes in one female gamete,
called an oocyte. The function of the male reproductive system ((link]) is to
produce sperm and transfer them to the female reproductive tract. The
paired testes are a crucial component in this process, as they produce both
sperm and androgens, the hormones that support male reproductive
physiology. In humans, the most important male androgen is testosterone.
Several accessory organs and ducts aid the process of sperm maturation and
transport the sperm and other seminal components to the penis, which
delivers sperm to the female reproductive tract. In this section, we examine
each of these different structures, and discuss the process of sperm
production and transport.
Male Reproductive System
(a) Uncircumcised penis (b) Circumcised penis
Penis
Prepuce
(foreskin)
(c) Male Reproductive System: lateral view
i
Ductus (vas) deferens
Suspensory ligament of penis
Ampulla of ductus deferens
a : aml | a) | Seminal vesicle
Pubic symphysis ME; if
Ejaculatory duct
Prostatic urethra
Deep muscles of perineum
Bulbourethral gland
ie Muscles of perineum
Corpus cavernosum —— } surrounding anus
= Membranous urethra
Spongy urethra
Testis
Epididymis
Corpus spongiosum Scrotum
External urethral opening
The structures of the male reproductive system include the
testes, the epididymides, the penis, and the ducts and glands
that produce and carry semen. Sperm exit the scrotum through
the ductus deferens, which is bundled in the spermatic cord.
The seminal vesicles and prostate gland add fluids to the sperm
to create semen.
Scrotum
The testes are located in a skin-covered, highly pigmented, muscular sack
called the scrotum that extends from the body behind the penis (see [Link]).
This location is important in sperm production, which occurs within the
testes, and proceeds more efficiently when the testes are kept 2 to 4°C
below core body temperature.
The dartos muscle makes up the subcutaneous muscle layer of the scrotum
({link]). It continues internally to make up the scrotal septum, a wall that
divides the scrotum into two compartments, each housing one testis.
Descending from the internal oblique muscle of the abdominal wall are the
two cremaster muscles, which cover each testis like a muscular net. By
contracting simultaneously, the dartos and cremaster muscles can elevate
the testes in cold weather (or water), moving the testes closer to the body
and decreasing the surface area of the scrotum to retain heat. Alternatively,
as the environmental temperature increases, the scrotum relaxes, moving
the testes farther from the body core and increasing scrotal surface area,
which promotes heat loss. Externally, the scrotum has a raised medial
thickening on the surface called the raphae.
The Scrotum and Testes
External view of scrotum Muscle layer Deep tissues
Ductus Spermatic
deferens cord
Testicular
artery
Autonomic
nerve
Lymphatic
vessel
|> Testis
Y
Cremaster
muscles muscles
This anterior view shows the structures of the scrotum and
testes.
Testes
The testes (singular = testis) are the male gonads—that is, the male
reproductive organs. They produce both sperm and androgens, such as
testosterone, and are active throughout the reproductive lifespan of the
male.
Paired ovals, the testes are each approximately 4 to 5 cm in length and are
housed within the scrotum (see [link]). They are surrounded by two distinct
layers of protective connective tissue ({link]). The outer tunica vaginalis is a
serous membrane that has both a parietal and a thin visceral layer. Beneath
the tunica vaginalis is the tunica albuginea, a tough, white, dense
connective tissue layer covering the testis itself. Not only does the tunica
albuginea cover the outside of the testis, it also invaginates to form septa
that divide the testis into 300 to 400 structures called lobules. Within the
lobules, sperm develop in structures called seminiferous tubules. During the
seventh month of the developmental period of a male fetus, each testis
moves through the abdominal musculature to descend into the scrotal
cavity. This is called the “descent of the testis.” Cryptorchidism is the
clinical term used when one or both of the testes fail to descend into the
scrotum prior to birth.
Anatomy of the Testis
} Into inguinal canal
Spermatic cord __a
Cremaster
muscle
Efferent Tunica vaginalis
ductule
Body of Head of»
epididymis / epididymis
Ductus | Seminiferous
deferens tubule lobules
' nC = Septa (tunica
Rete w4 FP BOD oars albuginea)
testis ——s * “4
Tunica albuginea
Straight
tubule
Tail of epididymis
This sagittal view shows the seminiferous tubules,
the site of sperm production. Formed sperm are
transferred to the epididymis, where they mature.
They leave the epididymis during an ejaculation
via the ductus deferens.
The tightly coiled seminiferous tubules form the bulk of each testis. They
are composed of developing sperm cells surrounding a lumen, the hollow
center of the tubule, where formed sperm are released into the duct system
of the testis. Specifically, from the lumens of the seminiferous tubules,
sperm move into the straight tubules (or tubuli recti), and from there into a
fine meshwork of tubules called the rete testes. Sperm leave the rete testes,
and the testis itself, through the 15 to 20 efferent ductules that cross the
tunica albuginea.
Inside the seminiferous tubules are six different cell types. These include
supporting cells called sustentacular cells, as well as five types of
developing sperm cells called germ cells. Germ cell development
progresses from the basement membrane—at the perimeter of the tubule—
toward the lumen. Let’s look more closely at these cell types.
Sertoli Cells
Surrounding all stages of the developing sperm cells are elongate,
branching Sertoli cells. Sertoli cells are a type of supporting cell called a
sustentacular cell, or sustentocyte, that are typically found in epithelial
tissue. Sertoli cells secrete signaling molecules that promote sperm
production and can control whether germ cells live or die. They extend
physically around the germ cells from the peripheral basement membrane
of the seminiferous tubules to the lumen. Tight junctions between these
sustentacular cells create the blood—testis barrier, which keeps bloodborne
substances from reaching the germ cells and, at the same time, keeps
surface antigens on developing germ cells from escaping into the
bloodstream and prompting an autoimmune response.
Germ Cells
The least mature cells, the spermatogonia (singular = spermatogonium),
line the basement membrane inside the tubule. Spermatogonia are the stem
cells of the testis, which means that they are still able to differentiate into a
variety of different cell types throughout adulthood. Spermatogonia divide
to produce primary and secondary spermatocytes, then spermatids, which
finally produce formed sperm. The process that begins with spermatogonia
and concludes with the production of sperm is called spermatogenesis.
Spermatogenesis
As just noted, spermatogenesis occurs in the seminiferous tubules that form
the bulk of each testis (see [link]). The process begins at puberty, after
which time sperm are produced constantly throughout a man’s life. One
production cycle, from spermatogonia through formed sperm, takes
approximately 64 days. A new cycle starts approximately every 16 days,
although this timing is not synchronous across the seminiferous tubules.
Sperm counts—the total number of sperm a man produces—slowly decline
after age 35, and some studies suggest that smoking can lower sperm counts
irrespective of age.
The process of spermatogenesis begins with mitosis of the diploid
spermatogonia ([link]). Because these cells are diploid (2n), they each have
a complete copy of the father’s genetic material, or 46 chromosomes.
However, mature gametes are haploid (1n), containing 23 chromosomes—
meaning that daughter cells of spermatogonia must undergo a second
cellular division through the process of meiosis.
Spermatogenesis
(a) Spermatogenesis
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| Spermiogenesis 7 '
(interstitial) cells
Spermatozoa — peritubular
(sperm) capillary
(a) Mitosis of a spermatogonial stem cell involves a single cell
division that results in two identical, diploid daughter cells
(spermatogonia to primary spermatocyte). Meiosis has two
rounds of cell division: primary spermatocyte to secondary
spermatocyte, and then secondary spermatocyte to spermatid.
This produces four haploid daughter cells (spermatids). (b) In
this electron micrograph of a cross-section of a seminiferous
tubule from a rat, the lumen is the light-shaded area in the
center of the image. The location of the primary spermatocytes
is near the basement membrane, and the early spermatids are
approaching the lumen (tissue source: rat). EM x 900.
(Micrograph provided by the Regents of University of
Michigan Medical School © 2012)
Two identical diploid cells result from spermatogonia mitosis. One of these
cells remains a spermatogonium, and the other becomes a primary
spermatocyte, the next stage in the process of spermatogenesis. As in
mitosis, DNA is replicated in a primary spermatocyte, before it undergoes a
cell division called meiosis I. During meiosis I each of the 23 pairs of
chromosomes separates. This results in two cells, called secondary
spermatocytes, each with only half the number of chromosomes. Now a
second round of cell division (meiosis IT) occurs in both of the secondary
spermatocytes. During meiosis II each of the 23 replicated chromosomes
divides, similar to what happens during mitosis. Thus, meiosis results in
separating the chromosome pairs. This second meiotic division results in a
total of four cells with only half of the number of chromosomes. Each of
these new cells is a spermatid. Although haploid, early spermatids look
very similar to cells in the earlier stages of spermatogenesis, with a round
shape, central nucleus, and large amount of cytoplasm. A process called
spermiogenesis transforms these early spermatids, reducing the cytoplasm,
and beginning the formation of the parts of a true sperm. The fifth stage of
germ cell formation—spermatozoa, or formed sperm—is the end result of
this process, which occurs in the portion of the tubule nearest the lumen.
Eventually, the sperm are released into the lumen and are moved along a
series of ducts in the testis toward a structure called the epididymis for the
next step of sperm maturation.
Structure of Formed Sperm
Sperm are smaller than most cells in the body; in fact, the volume of a
sperm cell is 85,000 times less than that of the female gamete.
Approximately 100 to 300 million sperm are produced each day, whereas
women typically ovulate only one oocyte per month. As is true for most
cells in the body, the structure of sperm cells speaks to their function.
Sperm have a distinctive head, mid-piece, and tail region ([link]). The head
of the sperm contains the extremely compact haploid nucleus with very
little cytoplasm. These qualities contribute to the overall small size of the
sperm (the head is only 5 pm long). A structure called the acrosome covers
most of the head of the sperm cell as a “cap” that is filled with lysosomal
enzymes important for preparing sperm to participate in fertilization.
Tightly packed mitochondria fill the mid-piece of the sperm. ATP produced
by these mitochondria will power the flagellum, which extends from the
neck and the mid-piece through the tail of the sperm, enabling it to move
the entire sperm cell. The central strand of the flagellum, the axial filament,
is formed from one centriole inside the maturing sperm cell during the final
stages of spermatogenesis.
Structure of Sperm
Acrosome Axial filament
Plasma membrane
Nucleus Mitochondria
Centriole
Flagellum
Head Mid-piece Tail End
Sperm cells are divided into a head, containing DNA; a mid-
piece, containing mitochondria; and a tail, providing motility.
The acrosome is oval and somewhat flattened.
Sperm Transport
To fertilize an egg, sperm must be moved from the seminiferous tubules in
the testes, through the epididymis, and—later during ejaculation—along the
length of the penis and out into the female reproductive tract.
Role of the Epididymis
From the lumen of the seminiferous tubules, the immotile sperm are
surrounded by testicular fluid and moved to the epididymis (plural =
epididymides), a coiled tube attached to the testis where newly formed
sperm continue to mature (see [link]). Though the epididymis does not take
up much room in its tightly coiled state, it would be approximately 6 m (20
feet) long if straightened. It takes an average of 12 days for sperm to move
through the coils of the epididymis, with the shortest recorded transit time
in humans being one day. Sperm enter the head of the epididymis and are
moved along predominantly by the contraction of smooth muscles lining
the epididymal tubes. As they are moved along the length of the
epididymis, the sperm further mature and acquire the ability to move under
their own power. Once inside the female reproductive tract, they will use
this ability to move independently toward the unfertilized egg. The more
mature sperm are then stored in the tail of the epididymis (the final section)
until ejaculation occurs.
Duct System
During ejaculation, sperm exit the tail of the epididymis and are pushed by
smooth muscle contraction to the ductus deferens (also called the vas
deferens). The ductus deferens is a thick, muscular tube that is bundled
together inside the scrotum with connective tissue, blood vessels, and
nerves into a structure called the spermatic cord (see [link] and [link]).
Because the ductus deferens is physically accessible within the scrotum,
surgical sterilization to interrupt sperm delivery can be performed by
cutting and sealing a small section of the ductus (vas) deferens. This
procedure is called a vasectomy, and it is an effective form of male birth
control. Although it may be possible to reverse a vasectomy, clinicians
consider the procedure permanent, and advise men to undergo it only if they
are certain they no longer wish to father children.
Note:
Interactive Link Feature
eas
— openstax COLLEGE”
aun
Watch this video to learn about a vasectomy. As described in this video, a
vasectomy is a procedure in which a small section of the ductus (vas)
deferens is removed from the scrotum. This interrupts the path taken by
sperm through the ductus deferens. If sperm do not exit through the vas,
either because the man has had a vasectomy or has not ejaculated, in what
region of the testis do they remain?
From each epididymis, each ductus deferens extends superiorly into the
abdominal cavity through the inguinal canal in the abdominal wall. From
here, the ductus deferens continues posteriorly to the pelvic cavity, ending
posterior to the bladder where it dilates in a region called the ampulla
(meaning “flask”).
Sperm make up only 5 percent of the final volume of semen, the thick,
milky fluid that the male ejaculates. The bulk of semen is produced by three
critical accessory glands of the male reproductive system: the seminal
vesicles, the prostate, and the bulbourethral glands.
Seminal Vesicles
As sperm pass through the ampulla of the ductus deferens at ejaculation,
they mix with fluid from the associated seminal vesicle (see [link]). The
paired seminal vesicles are glands that contribute approximately 60 percent
of the semen volume. Seminal vesicle fluid contains large amounts of
fructose, which is used by the sperm mitochondria to generate ATP to allow
movement through the female reproductive tract.
The fluid, now containing both sperm and seminal vesicle secretions, next
moves into the associated ejaculatory duct, a short structure formed from
the ampulla of the ductus deferens and the duct of the seminal vesicle. The
paired ejaculatory ducts transport the seminal fluid into the next structure,
the prostate gland.
Prostate Gland
As shown in [link], the centrally located prostate gland sits anterior to the
rectum at the base of the bladder surrounding the prostatic urethra (the
portion of the urethra that runs within the prostate). About the size of a
walnut, the prostate is formed of both muscular and glandular tissues. It
excretes an alkaline, milky fluid to the passing seminal fluid—now called
semen—that is critical to first coagulate and then decoagulate the semen
following ejaculation. The temporary thickening of semen helps retain it
within the female reproductive tract, providing time for sperm to utilize the
fructose provided by seminal vesicle secretions. When the semen regains its
fluid state, sperm can then pass farther into the female reproductive tract.
The prostate normally doubles in size during puberty. At approximately age
25, it gradually begins to enlarge again. This enlargement does not usually
cause problems; however, abnormal growth of the prostate, or benign
prostatic hyperplasia (BPH), can cause constriction of the urethra as it
passes through the middle of the prostate gland, leading to a number of
lower urinary tract symptoms, such as a frequent and intense urge to
urinate, a weak stream, and a sensation that the bladder has not emptied
completely. By age 60, approximately 40 percent of men have some degree
of BPH. By age 80, the number of affected individuals has jumped to as
many as 80 percent. Treatments for BPH attempt to relieve the pressure on
the urethra so that urine can flow more normally. Mild to moderate
symptoms are treated with medication, whereas severe enlargement of the
prostate is treated by surgery in which a portion of the prostate tissue is
removed.
Another common disorder involving the prostate is prostate cancer.
According to the Centers for Disease Control and Prevention (CDC),
prostate cancer is the second most common cancer in men. However, some
forms of prostate cancer grow very slowly and thus may not ever require
treatment. Aggressive forms of prostate cancer, in contrast, involve
metastasis to vulnerable organs like the lungs and brain. There is no link
between BPH and prostate cancer, but the symptoms are similar. Prostate
cancer is detected by a medical history, a blood test, and a rectal exam that
allows physicians to palpate the prostate and check for unusual masses. If a
mass is detected, the cancer diagnosis is confirmed by biopsy of the cells.
Bulbourethral Glands
The final addition to semen is made by two bulbourethral glands (or
Cowper’s glands) that release a thick, salty fluid that lubricates the end of
the urethra and the vagina, and helps to clean urine residues from the penile
urethra. The fluid from these accessory glands is released after the male
becomes sexually aroused, and shortly before the release of the semen. It is
therefore sometimes called pre-ejaculate. It is important to note that, in
addition to the lubricating proteins, it is possible for bulbourethral fluid to
pick up sperm already present in the urethra, and therefore it may be able to
cause pregnancy.
Note:
Interactive Link Feature
Watch this video to explore the structures of the male reproductive system
and the path of sperm, which starts in the testes and ends as the sperm
leave the penis through the urethra. Where are sperm deposited after they
leave the ejaculatory duct?
The Penis
The penis is the male organ of copulation (sexual intercourse). It is flaccid
for non-sexual actions, such as urination, and turgid and rod-like with
sexual arousal. When erect, the stiffness of the organ allows it to penetrate
into the vagina and deposit semen into the female reproductive tract.
Cross-Sectional Anatomy of the Penis
Flaccid: Lateral view Flaccid: Transverse view
Penile venules
(uncompressed)
Deep dorsal vein
Corpora cavernosa
Cavernosal arteries
Spongy urethra
Prepuce
Corpus spongiosum
Erect: Transverse view
Erect: Lateral view
(4) Cavernosal arteries dilate,
engorging corporal tissue
with blood
@ Engorging causes corporal tissue
to swell, erecting the penis
Engorged corporal tissue compresses penile
veins and venules, maintaining erection
Three columns of erectile tissue make up most of the volume
of the penis.
The shaft of the penis surrounds the urethra ([link]). The shaft is composed
of three column-like chambers of erectile tissue that span the length of the
shaft. Each of the two larger lateral chambers is called a corpus
cavernosum (plural = corpora cavernosa). Together, these make up the bulk
of the penis. The corpus spongiosum, which can be felt as a raised ridge on
the erect penis, is a smaller chamber that surrounds the spongy, or penile,
urethra. The end of the penis, called the glans penis, has a high
concentration of nerve endings, resulting in very sensitive skin that
influences the likelihood of ejaculation (see [link]). The skin from the shaft
extends down over the glans and forms a collar called the prepuce (or
foreskin). The foreskin also contains a dense concentration of nerve
endings, and both lubricate and protect the sensitive skin of the glans penis.
A surgical procedure called circumcision, often performed for religious or
social reasons, removes the prepuce, typically within days of birth.
Both sexual arousal and REM sleep (during which dreaming occurs) can
induce an erection. Penile erections are the result of vasocongestion, or
engorgement of the tissues because of more arterial blood flowing into the
penis than is leaving in the veins. During sexual arousal, nitric oxide (NO)
is released from nerve endings near blood vessels within the corpora
cavernosa and spongiosum. Release of NO activates a signaling pathway
that results in relaxation of the smooth muscles that surround the penile
arteries, causing them to dilate. This dilation increases the amount of blood
that can enter the penis and induces the endothelial cells in the penile
arterial walls to also secrete NO and perpetuate the vasodilation. The rapid
increase in blood volume fills the erectile chambers, and the increased
pressure of the filled chambers compresses the thin-walled penile venules,
preventing venous drainage of the penis. The result of this increased blood
flow to the penis and reduced blood return from the penis is erection.
Depending on the flaccid dimensions of a penis, it can increase in size
slightly or greatly during erection, with the average length of an erect penis
measuring approximately 15 cm.
Note:
Disorders of the... Feature
Male Reproductive System
Erectile dysfunction (ED) is a condition in which a man has difficulty
either initiating or maintaining an erection. The combined prevalence of
minimal, moderate, and complete ED is approximately 40 percent in men
at age 40, and reaches nearly 70 percent by 70 years of age. In addition to
aging, ED is associated with diabetes, vascular disease, psychiatric
disorders, prostate disorders, the use of some drugs such as certain
antidepressants, and problems with the testes resulting in low testosterone
concentrations. These physical and emotional conditions can lead to
interruptions in the vasodilation pathway and result in an inability to
achieve an erection.
Recall that the release of NO induces relaxation of the smooth muscles that
surround the penile arteries, leading to the vasodilation necessary to
achieve an erection. To reverse the process of vasodilation, an enzyme
called phosphodiesterase (PDE) degrades a key component of the NO
signaling pathway called cGMP. There are several different forms of this
enzyme, and PDE type 5 is the type of PDE found in the tissues of the
penis. Scientists discovered that inhibiting PDE5 increases blood flow, and
allows vasodilation of the penis to occur.
PDEs and the vasodilation signaling pathway are found in the vasculature
in other parts of the body. In the 1990s, clinical trials of a PDES inhibitor
called sildenafil were initiated to treat hypertension and angina pectoris
(chest pain caused by poor blood flow through the heart). The trial showed
that the drug was not effective at treating heart conditions, but many men
experienced erection and priapism (erection lasting longer than 4 hours).
Because of this, a clinical trial was started to investigate the ability of
sildenafil to promote erections in men suffering from ED. In 1998, the
FDA approved the drug, marketed as Viagra®. Since approval of the drug,
sildenafil and similar PDE inhibitors now generate over a billion dollars a
year in sales, and are reported to be effective in treating approximately 70
to 85 percent of cases of ED. Importantly, men with health problems—
especially those with cardiac disease taking nitrates—should avoid Viagra
or talk to their physician to find out if they are a candidate for the use of
this drug, as deaths have been reported for at-risk users.
Testosterone
Testosterone, an androgen, is a steroid hormone produced by Leydig cells.
The alternate term for Leydig cells, interstitial cells, reflects their location
between the seminiferous tubules in the testes. In male embryos,
testosterone is secreted by Leydig cells by the seventh week of
development, with peak concentrations reached in the second trimester.
This early release of testosterone results in the anatomical differentiation of
the male sexual organs. In childhood, testosterone concentrations are low.
They increase during puberty, activating characteristic physical changes and
initiating spermatogenesis.
Functions of Testosterone
The continued presence of testosterone is necessary to keep the male
reproductive system working properly, and Leydig cells produce
approximately 6 to 7 mg of testosterone per day. Testicular steroidogenesis
(the manufacture of androgens, including testosterone) results in
testosterone concentrations that are 100 times higher in the testes than in the
circulation. Maintaining these normal concentrations of testosterone
promotes spermatogenesis, whereas low levels of testosterone can lead to
infertility. In addition to intratesticular secretion, testosterone is also
released into the systemic circulation and plays an important role in muscle
development, bone growth, the development of secondary sex
characteristics, and maintaining libido (sex drive) in both males and
females. In females, the ovaries secrete small amounts of testosterone,
although most is converted to estradiol. A small amount of testosterone is
also secreted by the adrenal glands in both sexes.
Control of Testosterone
The regulation of testosterone concentrations throughout the body is critical
for male reproductive function. The intricate interplay between the
endocrine system and the reproductive system is shown in [link].
Regulation of Testosterone Production
(1) Hypothalamus releases GnRH.
GnRH stimulates the anterior pituitary
to release FSH and LH.
(8) inhibin negatively
feeds back to anterior
pituitary, inhibiting
further release of FSH.
Testosterone negatively
feeds back to the
hypothalamus and
pituitary, inhibiting
further release of
GnRH, FSH, and LH.
FSH release
LH release
@) LH stimulates
the Leydig cells to
release testosterone.
Leydig (interstitial) cells
— FSH stimulates the
3 Sertoli cells to release
ABP. ABP binds to
testosterone, keeping
the latter at a high
concentration.
Ne rails
Inhibin release
JOD)
“f= ~ = Seminiferous
— tubule
Sertoli cell
Androgen-binding Testosterone
protein (ABP) release release
The hypothalamus and pituitary gland regulate the production
of testosterone and the cells that assist in spermatogenesis.
GnRH activates the anterior pituitary to produce LH and FSH,
which in turn stimulate Leydig cells and Sertoli cells,
respectively. The system is a negative feedback loop because
the end products of the pathway, testosterone and inhibin,
interact with the activity of GnRH to inhibit their own
production.
The regulation of Leydig cell production of testosterone begins outside of
the testes. The hypothalamus and the pituitary gland in the brain integrate
external and internal signals to control testosterone synthesis and secretion.
The regulation begins in the hypothalamus. Pulsatile release of a hormone
called gonadotropin-releasing hormone (GnRH) from the hypothalamus
stimulates the endocrine release of hormones from the pituitary gland.
Binding of GnRH to its receptors on the anterior pituitary gland stimulates
release of the two gonadotropins: luteinizing hormone (LH) and follicle-
stimulating hormone (FSH). These two hormones are critical for
reproductive function in both men and women. In men, FSH binds
predominantly to the Sertoli cells within the seminiferous tubules to
promote spermatogenesis. FSH also stimulates the Sertoli cells to produce
hormones called inhibins, which function to inhibit FSH release from the
pituitary, thus reducing testosterone secretion. These polypeptide hormones
correlate directly with Sertoli cell function and sperm number; inhibin B
can be used as a marker of spermatogenic activity. In men, LH binds to
receptors on Leydig cells in the testes and upregulates the production of
testosterone.
A negative feedback loop predominantly controls the synthesis and
secretion of both FSH and LH. Low blood concentrations of testosterone
stimulate the hypothalamic release of GnRH. GnRH then stimulates the
anterior pituitary to secrete LH into the bloodstream. In the testis, LH binds
to LH receptors on Leydig cells and stimulates the release of testosterone.
When concentrations of testosterone in the blood reach a critical threshold,
testosterone itself will bind to androgen receptors on both the hypothalamus
and the anterior pituitary, inhibiting the synthesis and secretion of GnRH
and LH, respectively. When the blood concentrations of testosterone once
again decline, testosterone no longer interacts with the receptors to the same
degree and GnRH and LH are once again secreted, stimulating more
testosterone production. This same process occurs with FSH and inhibin to
control spermatogenesis.
Note:
Aging and the... Feature
Male Reproductive System
Declines in Leydig cell activity can occur in men beginning at 40 to 50
years of age. The resulting reduction in circulating testosterone
concentrations can lead to symptoms of andropause, also known as male
menopause. While the reduction in sex steroids in men is akin to female
menopause, there is no clear sign—such as a lack of a menstrual period—
to denote the initiation of andropause. Instead, men report feelings of
fatigue, reduced muscle mass, depression, anxiety, irritability, loss of
libido, and insomnia. A reduction in spermatogenesis resulting in lowered
fertility is also reported, and sexual dysfunction can also be associated with
andropausal symptoms.
Whereas some researchers believe that certain aspects of andropause are
difficult to tease apart from aging in general, testosterone replacement is
sometimes prescribed to alleviate some symptoms. Recent studies have
shown a benefit from androgen replacement therapy on the new onset of
depression in elderly men; however, other studies caution against
testosterone replacement for long-term treatment of andropause symptoms,
showing that high doses can sharply increase the risk of both heart disease
and prostate cancer.
Chapter Review
Gametes are the reproductive cells that combine to form offspring. Organs
called gonads produce the gametes, along with the hormones that regulate
human reproduction. The male gametes are called sperm. Spermatogenesis,
the production of sperm, occurs within the seminiferous tubules that make
up most of the testis. The scrotum is the muscular sac that holds the testes
outside of the body cavity.
Spermatogenesis begins with mitotic division of spermatogonia (stem cells)
to produce primary spermatocytes that undergo the two divisions of meiosis
to become secondary spermatocytes, then the haploid spermatids. During
spermiogenesis, spermatids are transformed into spermatozoa (formed
sperm). Upon release from the seminiferous tubules, sperm are moved to
the epididymis where they continue to mature. During ejaculation, sperm
exit the epididymis through the ductus deferens, a duct in the spermatic
cord that leaves the scrotum. The ampulla of the ductus deferens meets the
seminal vesicle, a gland that contributes fructose and proteins, at the
ejaculatory duct. The fluid continues through the prostatic urethra, where
secretions from the prostate are added to form semen. These secretions help
the sperm to travel through the urethra and into the female reproductive
tract. Secretions from the bulbourethral glands protect sperm and cleanse
and lubricate the penile (spongy) urethra.
The penis is the male organ of copulation. Columns of erectile tissue called
the corpora cavernosa and corpus spongiosum fill with blood when sexual
arousal activates vasodilatation in the blood vessels of the penis.
Testosterone regulates and maintains the sex organs and sex drive, and
induces the physical changes of puberty. Interplay between the testes and
the endocrine system precisely control the production of testosterone with a
negative feedback loop.
Interactive Link Questions
Exercise:
Problem:
Watch this video to learn about vasectomy. As described in this video,
a vasectomy is a procedure in which a small section of the ductus (vas)
deferens is removed from the scrotum. This interrupts the path taken
by sperm through the ductus deferens. If sperm do not exit through the
vas, either because the man has had a vasectomy or has not ejaculated,
in what region of the testis do they remain?
Solution:
Sperm remain in the epididymis until they degenerate.
Exercise:
Problem:
Watch this video to explore the structures of the male reproductive
system and the path of sperm that starts in the testes and ends as the
sperm leave the penis through the urethra. Where are sperm deposited
after they leave the ejaculatory duct?
Solution:
Sperm enter the prostate.
Review Questions
Exercise:
Problem: What are male gametes called?
a. OVa
b. sperm
c. testes
d. testosterone
Solution:
b
Exercise:
Problem: Leydig cells
a. secrete testosterone
b. activate the sperm flagellum
c. support spermatogenesis
d. secrete seminal fluid
Solution:
A
Exercise:
Problem:
Which hypothalamic hormone contributes to the regulation of the male
reproductive system?
a. luteinizing hormone
b. gonadotropin-releasing hormone
c. follicle-stimulating hormone
d. androgens
Solution:
b
Exercise:
Problem: What is the function of the epididymis?
a. sperm maturation and storage
b. produces the bulk of seminal fluid
c. provides nitric oxide needed for erections
d. spermatogenesis
Solution:
a
Exercise:
Problem: Spermatogenesis takes place in the
a. prostate gland
b. glans penis
c. seminiferous tubules
d. ejaculatory duct
Solution:
Critical Thinking Questions
Exercise:
Problem:
Briefly explain why mature gametes carry only one set of
chromosomes.
Solution:
A single gamete must combine with a gamete from an individual of the
opposite sex to produce a fertilized egg, which has a complete set of
chromosomes and is the first cell of a new individual.
Exercise:
Problem:
What special features are evident in sperm cells but not in somatic
cells, and how do these specializations function?
Solution:
Unlike somatic cells, sperm are haploid. They also have very little
cytoplasm. They have a head with a compact nucleus covered by an
acrosome filled with enzymes, and a mid-piece filled with
mitochondria that power their movement. They are motile because of
their tail, a structure containing a flagellum, which is specialized for
movement.
Exercise:
Problem:
What do each of the three male accessory glands contribute to the
semen?
Solution:
The three accessory glands make the following contributions to semen:
the seminal vesicle contributes about 60 percent of the semen volume,
with fluid that contains large amounts of fructose to power the
movement of sperm; the prostate gland contributes substances critical
to sperm maturation; and the bulbourethral glands contribute a thick
fluid that lubricates the ends of the urethra and the vagina and helps to
clean urine residues from the urethra.
Exercise:
Problem: Describe how penile erection occurs.
Solution:
During sexual arousal, nitric oxide (NO) is released from nerve
endings near blood vessels within the corpora cavernosa and corpus
spongiosum. The release of NO activates a signaling pathway that
results in relaxation of the smooth muscles that surround the penile
arteries, causing them to dilate. This dilation increases the amount of
blood that can enter the penis, and induces the endothelial cells in the
penile arterial walls to secrete NO, perpetuating the vasodilation. The
rapid increase in blood volume fills the erectile chambers, and the
increased pressure of the filled chambers compresses the thin-walled
penile venules, preventing venous drainage of the penis. An erection is
the result of this increased blood flow to the penis and reduced blood
return from the penis.
Exercise:
Problem:
While anabolic steroids (synthetic testosterone) bulk up muscles, they
can also affect testosterone production in the testis. Using what you
know about negative feedback, describe what would happen to
testosterone production in the testis if a male takes large amounts of
synthetic testosterone.
Solution:
Testosterone production by the body would be reduced if a male were
taking anabolic steroids. This is because the hypothalamus responds to
rising testosterone levels by reducing its secretion of GnRH, which
would in turn reduce the anterior pituitary’s release of LH, finally
reducing the manufacture of testosterone in the testes.
Glossary
blood-testis barrier
tight junctions between Sertoli cells that prevent bloodborne pathogens
from gaining access to later stages of spermatogenesis and prevent the
potential for an autoimmune reaction to haploid sperm
bulbourethral glands
(also, Cowper’s glands) glands that secrete a lubricating mucus that
cleans and lubricates the urethra prior to and during ejaculation
corpus cavernosum
either of two columns of erectile tissue in the penis that fill with blood
during an erection
corpus spongiosum
(plural = corpora cavernosa) column of erectile tissue in the penis that
fills with blood during an erection and surrounds the penile urethra on
the ventral portion of the penis
ductus deferens
(also, vas deferens) duct that transports sperm from the epididymis
through the spermatic cord and into the ejaculatory duct; also referred
as the vas deferens
ejaculatory duct
duct that connects the ampulla of the ductus deferens with the duct of
the seminal vesicle at the prostatic urethra
epididymis
(plural = epididymides) coiled tubular structure in which sperm start to
mature and are stored until ejaculation
gamete
haploid reproductive cell that contributes genetic material to form an
offspring
glans penis
bulbous end of the penis that contains a large number of nerve endings
gonadotropin-releasing hormone (GnRH)
hormone released by the hypothalamus that regulates the production of
follicle-stimulating hormone and luteinizing hormone from the
pituitary gland
gonads
reproductive organs (testes in men and ovaries in women) that produce
gametes and reproductive hormones
inguinal canal
opening in abdominal wall that connects the testes to the abdominal
cavity
Leydig cells
cells between the seminiferous tubules of the testes that produce
testosterone; a type of interstitial cell
penis
male organ of copulation
prepuce
(also, foreskin) flap of skin that forms a collar around, and thus
protects and lubricates, the glans penis; also referred as the foreskin
prostate gland
doughnut-shaped gland at the base of the bladder surrounding the
urethra and contributing fluid to semen during ejaculation
scrotum
external pouch of skin and muscle that houses the testes
semen
ejaculatory fluid composed of sperm and secretions from the seminal
vesicles, prostate, and bulbourethral glands
seminal vesicle
gland that produces seminal fluid, which contributes to semen
seminiferous tubules
tube structures within the testes where spermatogenesis occurs
Sertoli cells
cells that support germ cells through the process of spermatogenesis; a
type of sustentacular cell
sperm
(also, spermatozoon) male gamete
spermatic cord
bundle of nerves and blood vessels that supplies the testes; contains
ductus deferens
spermatid
immature sperm cells produced by meiosis II of secondary
spermatocytes
spermatocyte
cell that results from the division of spermatogonium and undergoes
meiosis I and meiosis IT to form spermatids
spermatogenesis
formation of new sperm, occurs in the seminiferous tubules of the
testes
spermatogonia
(singular = spermatogonium) diploid precursor cells that become
sperm
spermiogenesis
transformation of spermatids to spermatozoa during spermatogenesis
testes
(singular = testis) male gonads
Female Anatomy
By the end of this section, you will be able to:
¢ Describe the structure and function of the organs of the female
reproductive system
e List the steps of oogenesis
e Describe the hormonal changes that occur during the ovarian and
menstrual cycles
e Trace the path of an oocyte from ovary to fertilization
The female reproductive system functions to produce gametes and
reproductive hormones, just like the male reproductive system; however, it
also has the additional task of supporting the developing fetus and
delivering it to the outside world. Unlike its male counterpart, the female
reproductive system is located primarily inside the pelvic cavity ([link]).
Recall that the ovaries are the female gonads. The gamete they produce is
called an oocyte. We’|I discuss the production of oocytes in detail shortly.
First, let’s look at some of the structures of the female reproductive system.
Female Reproductive System
Uterus
Ovary
Bladder
Pubic sym is Fornix of uterus
Mons pubis Cervix
Urethra Rectum
Clitoris Vagina
Labium minora
Anus
Labium majora
(a) Human female reproductive system: lateral view
Ovary
Fimbriae
Ovarian
earns ligament
=s_4 5 Broad
= = SS ‘ ligament
(oviduct)
Cervix
Vagina
(b) Human female reproductive system: anterior view
The major organs of the female
reproductive system are located inside the
pelvic cavity.
External Female Genitals
The external female reproductive structures are referred to collectively as
the vulva ({link]). The mons pubis is a pad of fat that is located at the
anterior, over the pubic bone. After puberty, it becomes covered in pubic
hair. The labia majora (labia = “lips”; majora = “larger”) are folds of hair-
covered skin that begin just posterior to the mons pubis. The thinner and
more pigmented labia minora (labia = “lips”; minora = “smaller”) extend
medial to the labia majora. Although they naturally vary in shape and size
from woman to woman, the labia minora serve to protect the female urethra
and the entrance to the female reproductive tract.
The superior, anterior portions of the labia minora come together to encircle
the clitoris (or glans clitoris), an organ that originates from the same cells
as the glans penis and has abundant nerves that make it important in sexual
sensation and orgasm. The hymen is a thin membrane that sometimes
partially covers the entrance to the vagina. An intact hymen cannot be used
as an indication of “virginity”; even at birth, this is only a partial
membrane, as menstrual fluid and other secretions must be able to exit the
body, regardless of penile—vaginal intercourse. The vaginal opening is
located between the opening of the urethra and the anus. It is flanked by
outlets to the Bartholin’s glands (or greater vestibular glands).
The Vulva
Corpus cavernosum
J
ot
—____"
- Urethral opening — + hs
= Labia majora ii ‘a
: 2 } 7
FL ‘
3 '
Bulb of vestibule
Opening of right Ba . y ee
Bartholin's gland
/
A= Anus
AN Bartholin’s glands
Vulva: External anterior view Vulva: Internal anteriolateral view
The external female genitalia are referred to collectively as the
vulva.
Vagina
The vagina, shown at the bottom of [link] and [link], is a muscular canal
(approximately 10 cm long) that serves as the entrance to the reproductive
tract. It also serves as the exit from the uterus during menses and childbirth.
The outer walls of the anterior and posterior vagina are formed into
longitudinal columns, or ridges, and the superior portion of the vagina—
called the fornix—meets the protruding uterine cervix. The walls of the
vagina are lined with an outer, fibrous adventitia; a middle layer of smooth
muscle; and an inner mucous membrane with transverse folds called rugae.
Together, the middle and inner layers allow the expansion of the vagina to
accommodate intercourse and childbirth. The thin, perforated hymen can
partially surround the opening to the vaginal orifice. The hymen can be
ruptured with strenuous physical exercise, penile—vaginal intercourse, and
childbirth. The Bartholin’s glands and the lesser vestibular glands (located
near the clitoris) secrete mucus, which keeps the vestibular area moist.
The vagina is home to a normal population of microorganisms that help to
protect against infection by pathogenic bacteria, yeast, or other organisms
that can enter the vagina. In a healthy woman, the most predominant type of
vaginal bacteria is from the genus Lactobacillus. This family of beneficial
bacterial flora secretes lactic acid, and thus protects the vagina by
maintaining an acidic pH (below 4.5). Potential pathogens are less likely to
survive in these acidic conditions. Lactic acid, in combination with other
vaginal secretions, makes the vagina a self-cleansing organ. However,
douching—or washing out the vagina with fluid—can disrupt the normal
balance of healthy microorganisms, and actually increase a woman’s risk
for infections and irritation. Indeed, the American College of Obstetricians
and Gynecologists recommend that women do not douche, and that they
allow the vagina to maintain its normal healthy population of protective
microbial flora.
Ovaries
The ovaries are the female gonads (see [link]). Paired ovals, they are each
about 2 to 3 cm in length, about the size of an almond. The ovaries are
located within the pelvic cavity, and are supported by the mesovarium, an
extension of the peritoneum that connects the ovaries to the broad
ligament. Extending from the mesovarium itself is the suspensory ligament
that contains the ovarian blood and lymph vessels. Finally, the ovary itself
is attached to the uterus via the ovarian ligament.
The ovary comprises an outer covering of cuboidal epithelium called the
ovarian surface epithelium that is superficial to a dense connective tissue
covering called the tunica albuginea. Beneath the tunica albuginea is the
cortex, or outer portion, of the organ. The cortex is composed of a tissue
framework called the ovarian stroma that forms the bulk of the adult ovary.
Oocytes develop within the outer layer of this stroma, each surrounded by
supporting cells. This grouping of an oocyte and its supporting cells is
called a follicle. The growth and development of ovarian follicles will be
described shortly. Beneath the cortex lies the inner ovarian medulla, the site
of blood vessels, lymph vessels, and the nerves of the ovary. You will learn
more about the overall anatomy of the female reproductive system at the
end of this section.
The Ovarian Cycle
The ovarian cycle is a set of predictable changes in a female’s oocytes and
ovarian follicles. During a woman’s reproductive years, it is a roughly 28-
day cycle that can be correlated with, but is not the same as, the menstrual
cycle (discussed shortly). The cycle includes two interrelated processes:
oogenesis (the production of female gametes) and folliculogenesis (the
growth and development of ovarian follicles).
Oogenesis
Gametogenesis in females is called oogenesis. The process begins with the
Ovarian stem cells, or oogonia (({link]). Oogonia are formed during fetal
development, and divide via mitosis, much like spermatogonia in the testis.
Unlike spermatogonia, however, oogonia form primary oocytes in the fetal
ovary prior to birth. These primary oocytes are then arrested in this stage of
meiosis I, only to resume it years later, beginning at puberty and continuing
until the woman is near menopause (the cessation of a woman’s
reproductive functions). The number of primary oocytes present in the
ovaries declines from one to two million in an infant, to approximately
400,000 at puberty, to zero by the end of menopause.
The initiation of ovulation—the release of an oocyte from the ovary—
marks the transition from puberty into reproductive maturity for women.
From then on, throughout a woman’s reproductive years, ovulation occurs
approximately once every 28 days. Just prior to ovulation, a surge of
luteinizing hormone triggers the resumption of meiosis in a primary oocyte.
This initiates the transition from primary to secondary oocyte. However, as
you can see in [link], this cell division does not result in two identical cells.
Instead, the cytoplasm is divided unequally, and one daughter cell is much
larger than the other. This larger cell, the secondary oocyte, eventually
leaves the ovary during ovulation. The smaller cell, called the first polar
body, may or may not complete meiosis and produce second polar bodies;
in either case, it eventually disintegrates. Therefore, even though oogenesis
produces up to four cells, only one survives.
Oogenesis
Te.
— (*) oO By onto
Meiosis arrests in
t prophase |
Before birth
After pube!
in m | 3 Meiosis | resumes
Oocyte meiosis
arrests at Secondary First polar
metaphase II oocyte body
Before sperm penetration
After sperm penetration
4
Oocyte meiosis Second polar
completes bodies
immediately after
sperm penetrates
the oocyte
The unequal cell division of oogenesis produces one to three
polar bodies that later degrade, as well as a single haploid
ovum, which is produced only if there is penetration of the
secondary oocyte by a sperm cell.
How does the diploid secondary oocyte become an ovum—the haploid
female gamete? Meiosis of a secondary oocyte is completed only if a sperm
succeeds in penetrating its barriers. Meiosis II then resumes, producing one
haploid ovum that, at the instant of fertilization by a (haploid) sperm,
becomes the first diploid cell of the new offspring (a zygote). Thus, the
ovum can be thought of as a brief, transitional, haploid stage between the
diploid oocyte and diploid zygote.
The larger amount of cytoplasm contained in the female gamete is used to
supply the developing zygote with nutrients during the period between
fertilization and implantation into the uterus. Interestingly, sperm contribute
only DNA at fertilization —not cytoplasm. Therefore, the cytoplasm and all
of the cytoplasmic organelles in the developing embryo are of maternal
origin. This includes mitochondria, which contain their own DNA.
Scientific research in the 1980s determined that mitochondrial DNA was
maternally inherited, meaning that you can trace your mitochondrial DNA
directly to your mother, her mother, and so on back through your female
ancestors.
Note:
Everyday Connections Feature
Mapping Human History with Mitochondrial DNA
When we talk about human DNA, we’re usually referring to nuclear DNA;
that is, the DNA coiled into chromosomal bundles in the nucleus of our
cells. We inherit half of our nuclear DNA from our father, and half from
our mother. However, mitochondrial DNA (mtDNA) comes only from the
mitochondria in the cytoplasm of the fat ovum we inherit from our mother.
She received her mtDNA from her mother, who got it from her mother, and
so on. Each of our cells contains approximately 1700 mitochondria, with
each mitochondrion packed with mtDNA containing approximately 37
genes.
Mutations (changes) in mtDNA occur spontaneously in a somewhat
organized pattern at regular intervals in human history. By analyzing these
mutational relationships, researchers have been able to determine that we
can all trace our ancestry back to one woman who lived in Africa about
200,000 years ago. Scientists have given this woman the biblical name
Eve, although she is not, of course, the first Homo sapiens female. More
precisely, she is our most recent common ancestor through matrilineal
descent.
This doesn’t mean that everyone’s mtDNA today looks exactly like that of
our ancestral Eve. Because of the spontaneous mutations in mtDNA that
have occurred over the centuries, researchers can map different “branches”
off of the “main trunk” of our mtDNA family tree. Your mtDNA might
have a pattern of mutations that aligns more closely with one branch, and
your neighbor’s may align with another branch. Still, all branches
eventually lead back to Eve.
But what happened to the mtDNA of all of the other Homo sapiens females
who were living at the time of Eve? Researchers explain that, over the
centuries, their female descendants died childless or with only male
children, and thus, their maternal line—and its mtDNA—ended.
Folliculogenesis
Again, ovarian follicles are oocytes and their supporting cells. They grow
and develop in a process called folliculogenesis, which typically leads to
ovulation of one follicle approximately every 28 days, along with death to
multiple other follicles. The death of ovarian follicles is called atresia, and
can occur at any point during follicular development. Recall that, a female
infant at birth will have one to two million oocytes within her ovarian
follicles, and that this number declines throughout life until menopause,
when no follicles remain. As you’ll see next, follicles progress from
primordial, to primary, to secondary and tertiary stages prior to ovulation—
with the oocyte inside the follicle remaining as a primary oocyte until right
before ovulation.
Folliculogenesis begins with follicles in a resting state. These small
primordial follicles are present in newborn females and are the prevailing
follicle type in the adult ovary ([link]). Primordial follicles have only a
single flat layer of support cells, called granulosa cells, that surround the
oocyte, and they can stay in this resting state for years—some until right
before menopause.
After puberty, a few primordial follicles will respond to a recruitment signal
each day, and will join a pool of immature growing follicles called primary
follicles. Primary follicles start with a single layer of granulosa cells, but
the granulosa cells then become active and transition from a flat or
Squamous shape to a rounded, cuboidal shape as they increase in size and
proliferate. As the granulosa cells divide, the follicles—now called
secondary follicles (see [link ])—increase in diameter, adding a new outer
layer of connective tissue, blood vessels, and theca cells—cells that work
with the granulosa cells to produce estrogens.
Within the growing secondary follicle, the primary oocyte now secretes a
thin acellular membrane called the zona pellucida that will play a critical
role in fertilization. A thick fluid, called follicular fluid, that has formed
between the granulosa cells also begins to collect into one large pool, or
antrum. Follicles in which the antrum has become large and fully formed
are considered tertiary follicles (or antral follicles). Several follicles reach
the tertiary stage at the same time, and most of these will undergo atresia.
The one that does not die will continue to grow and develop until ovulation,
when it will expel its secondary oocyte surrounded by several layers of
granulosa cells from the ovary. Keep in mind that most follicles don’t make
it to this point. In fact, roughly 99 percent of the follicles in the ovary will
undergo atresia, which can occur at any stage of folliculogenesis.
Folliculogenesis
(a) Stages of Folliculogenesis
(2) Primordial follicle (@) Primary follicie (8) Secondary follicle
Granulosa cells Oocyte Granulosa cells
“|
‘G) Corpus luteum 6) Ovulating follicle @) Tertiary follicle
pi Granulosa cells
roc
(b) A Secondary Follicle
(a) The maturation of a follicle is shown in a clockwise
direction proceeding from the primordial follicles. FSH
stimulates the growth of a tertiary follicle, and LH stimulates
the production of estrogen by granulosa and theca cells. Once
the follicle is mature, it ruptures and releases the oocyte. Cells
remaining in the follicle then develop into the corpus luteum.
(b) In this electron micrograph of a secondary follicle, the
oocyte, theca cells (thecae folliculi), and developing antrum are
clearly visible. EM x 1100. (Micrograph provided by the
Regents of University of Michigan Medical School © 2012)
Hormonal Control of the Ovarian Cycle
The process of development that we have just described, from primordial
follicle to early tertiary follicle, takes approximately two months in humans.
The final stages of development of a small cohort of tertiary follicles,
ending with ovulation of a secondary oocyte, occur over a course of
approximately 28 days. These changes are regulated by many of the same
hormones that regulate the male reproductive system, including GnRH, LH,
and FSH.
As in men, the hypothalamus produces GnRH, a hormone that signals the
anterior pituitary gland to produce the gonadotropins FSH and LH ([lLink]).
These gonadotropins leave the pituitary and travel through the bloodstream
to the ovaries, where they bind to receptors on the granulosa and theca cells
of the follicles. FSH stimulates the follicles to grow (hence its name of
follicle-stimulating hormone), and the five or six tertiary follicles expand in
diameter. The release of LH also stimulates the granulosa and theca cells of
the follicles to produce the sex steroid hormone estradiol, a type of
estrogen. This phase of the ovarian cycle, when the tertiary follicles are
growing and secreting estrogen, is known as the follicular phase.
The more granulosa and theca cells a follicle has (that is, the larger and
more developed it is), the more estrogen it will produce in response to LH
stimulation. As a result of these large follicles producing large amounts of
estrogen, systemic plasma estrogen concentrations increase. Following a
classic negative feedback loop, the high concentrations of estrogen will
stimulate the hypothalamus and pituitary to reduce the production of GnRH,
LH, and FSH. Because the large tertiary follicles require FSH to grow and
survive at this point, this decline in FSH caused by negative feedback leads
most of them to die (atresia). Typically only one follicle, now called the
dominant follicle, will survive this reduction in FSH, and this follicle will
be the one that releases an oocyte. Scientists have studied many factors that
lead to a particular follicle becoming dominant: size, the number of
granulosa cells, and the number of FSH receptors on those granulosa cells
all contribute to a follicle becoming the one surviving dominant follicle.
Hormonal Regulation of Ovulation
@) Follicular phase @ ovulation
Pituitary hormone Pituitary hormone
effect: LH and FSH effect: LH and FSH
stimulate several —” stimulate maturation cia
follicles to grow. of one of the
growing follicles.
Estradiol
FSH
LH Estradio| Ovarian a Ovarian
hormone hormone
effects: effects:
Boninat ll crowing lo
produces estradiol, continues to produce
Estradio! Which:
estradiol, which:
* Inhibits GnRH, FSH, Estradiol ". stimulates GnRH,
and LH production FSH, and LH
* Causes production
endometrium to Endometrium + LH surge triggers
thicken ovulation
@)Luteal phase
Pituitary hormone
effect: LH stimulates
formation of a corpus
luteum from follicular
tissue left behind after
ovulation.
| GnRH
FSH
LH
Progesterone Ovarlan
hormone
ee
The corpus
luteum secretes
progesterone, which:
¢ Inhibits GnRH, FSH,
Progesterone = and LH production
ae * Maintains the
aaa
corpus luteum degrades,
progesterone declines,
initiating sloughing of
the stratum functionalis
The hypothalamus and pituitary gland regulate the ovarian
cycle and ovulation. GnRH activates the anterior pituitary to
produce LH and FSH, which stimulate the production of
estrogen and progesterone by the ovaries.
When only the one dominant follicle remains in the ovary, it again begins to
secrete estrogen. It produces more estrogen than all of the developing
follicles did together before the negative feedback occurred. It produces so
much estrogen that the normal negative feedback doesn’t occur. Instead,
these extremely high concentrations of systemic plasma estrogen trigger a
regulatory switch in the anterior pituitary that responds by secreting large
amounts of LH and FSH into the bloodstream (see [link]). The positive
feedback loop by which more estrogen triggers release of more LH and
FSH only occurs at this point in the cycle.
It is this large burst of LH (called the LH surge) that leads to ovulation of
the dominant follicle. The LH surge induces many changes in the dominant
follicle, including stimulating the resumption of meiosis of the primary
oocyte to a secondary oocyte. As noted earlier, the polar body that results
from unequal cell division simply degrades. The LH surge also triggers
proteases (enzymes that cleave proteins) to break down structural proteins
in the ovary wall on the surface of the bulging dominant follicle. This
degradation of the wall, combined with pressure from the large, fluid-filled
antrum, results in the expulsion of the oocyte surrounded by granulosa cells
into the peritoneal cavity. This release is ovulation.
In the next section, you will follow the ovulated oocyte as it travels toward
the uterus, but there is one more important event that occurs in the ovarian
cycle. The surge of LH also stimulates a change in the granulosa and theca
cells that remain in the follicle after the oocyte has been ovulated. This
change is called luteinization (recall that the full name of LH is luteinizing
hormone), and it transforms the collapsed follicle into a new endocrine
structure called the corpus luteum, a term meaning “yellowish body” (see
[link]). Instead of estrogen, the luteinized granulosa and theca cells of the
corpus luteum begin to produce large amounts of the sex steroid hormone
progesterone, a hormone that is critical for the establishment and
maintenance of pregnancy. Progesterone triggers negative feedback at the
hypothalamus and pituitary, which keeps GnRH, LH, and FSH secretions
low, so no new dominant follicles develop at this time.
The post-ovulatory phase of progesterone secretion is known as the luteal
phase of the ovarian cycle. If pregnancy does not occur within 10 to 12
days, the corpus luteum will stop secreting progesterone and degrade into
the corpus albicans, a nonfunctional “whitish body” that will disintegrate
in the ovary over a period of several months. During this time of reduced
progesterone secretion, FSH and LH are once again stimulated, and the
follicular phase begins again with a new cohort of early tertiary follicles
beginning to grow and secrete estrogen.
The Uterine Tubes
The uterine tubes (also called fallopian tubes or oviducts) serve as the
conduit of the oocyte from the ovary to the uterus ([link]). Each of the two
uterine tubes is close to, but not directly connected to, the ovary and divided
into sections. The isthmus is the narrow medial end of each uterine tube
that is connected to the uterus. The wide distal infundibulum flares out
with slender, finger-like projections called fimbriae. The middle region of
the tube, called the ampulla, is where fertilization often occurs. The uterine
tubes also have three layers: an outer serosa, a middle smooth muscle layer,
and an inner mucosal layer. In addition to its mucus-secreting cells, the
inner mucosa contains ciliated cells that beat in the direction of the uterus,
producing a current that will be critical to move the oocyte.
Following ovulation, the secondary oocyte surrounded by a few granulosa
cells is released into the peritoneal cavity. The nearby uterine tube, either
left or right, receives the oocyte. Unlike sperm, oocytes lack flagella, and
therefore cannot move on their own. So how do they travel into the uterine
tube and toward the uterus? High concentrations of estrogen that occur
around the time of ovulation induce contractions of the smooth muscle
along the length of the uterine tube. These contractions occur every 4 to 8
seconds, and the result is a coordinated movement that sweeps the surface
of the ovary and the pelvic cavity. Current flowing toward the uterus is
generated by coordinated beating of the cilia that line the outside and lumen
of the length of the uterine tube. These cilia beat more strongly in response
to the high estrogen concentrations that occur around the time of ovulation.
As aresult of these mechanisms, the oocyte—granulosa cell complex is
pulled into the interior of the tube. Once inside, the muscular contractions
and beating cilia move the oocyte slowly toward the uterus. When
fertilization does occur, sperm typically meet the egg while it is still moving
through the ampulla.
Note:
Interactive Link
CHES
—
mess Openstax COLLEGE
cere
Watch this video to observe ovulation and its initiation in response to the
release of FSH and LH from the pituitary gland. What specialized
structures help guide the oocyte from the ovary into the uterine tube?
If the oocyte is successfully fertilized, the resulting zygote will begin to
divide into two cells, then four, and so on, as it makes its way through the
uterine tube and into the uterus. There, it will implant and continue to grow.
If the egg is not fertilized, it will simply degrade—either in the uterine tube
or in the uterus, where it may be shed with the next menstrual period.
Ovaries, Uterine Tubes, and Uterus
Uterine tube (oviduct)
Infundibulum Ampulla Isthmus Fundus Broad
ligament
Edge of
follicle
Fimbriae
Ovarian
cortex and vein
Suspensory
ligament
Tunica albuginea
and vein
Vaginal artery
Vagina
This anterior view shows the relationship of the ovaries, uterine
tubes (oviducts), and uterus. Sperm enter through the vagina,
and fertilization of an ovulated oocyte usually occurs in the
distal uterine tube. From left to right, LM x 400, LM x 20.
(Micrographs provided by the Regents of University of
Michigan Medical School © 2012)
The open-ended structure of the uterine tubes can have significant health
consequences if bacteria or other contagions enter through the vagina and
move through the uterus, into the tubes, and then into the pelvic cavity. If
this is left unchecked, a bacterial infection (sepsis) could quickly become
life-threatening. The spread of an infection in this manner is of special
concern when unskilled practitioners perform abortions in non-sterile
conditions. Sepsis is also associated with sexually transmitted bacterial
infections, especially gonorrhea and chlamydia. These increase a woman’s
risk for pelvic inflammatory disease (PID), infection of the uterine tubes or
other reproductive organs. Even when resolved, PID can leave scar tissue in
the tubes, leading to infertility.
Note:
Interactive Link
Watch this series of videos to look at the movement of the oocyte through
the ovary. The cilia in the uterine tube promote movement of the oocyte.
What would likely occur if the cilia were paralyzed at the time of
ovulation?
The Uterus and Cervix
The uterus is the muscular organ that nourishes and supports the growing
embryo (see [link]). Its average size is approximately 5 cm wide by 7 cm
long (approximately 2 in by 3 in) when a female is not pregnant. It has three
sections. The portion of the uterus superior to the opening of the uterine
tubes is called the fundus. The middle section of the uterus is called the
body of uterus (or corpus). The cervix is the narrow inferior portion of the
uterus that projects into the vagina. The cervix produces mucus secretions
that become thin and stringy under the influence of high systemic plasma
estrogen concentrations, and these secretions can facilitate sperm movement
through the reproductive tract.
Several ligaments maintain the position of the uterus within the
abdominopelvic cavity. The broad ligament is a fold of peritoneum that
serves as a primary support for the uterus, extending laterally from both
sides of the uterus and attaching it to the pelvic wall. The round ligament
attaches to the uterus near the uterine tubes, and extends to the labia majora.
Finally, the uterosacral ligament stabilizes the uterus posteriorly by its
connection from the cervix to the pelvic wall.
The wall of the uterus is made up of three layers. The most superficial layer
is the serous membrane, or perimetrium, which consists of epithelial tissue
that covers the exterior portion of the uterus. The middle layer, or
myometrium, is a thick layer of smooth muscle responsible for uterine
contractions. Most of the uterus is myometrial tissue, and the muscle fibers
run horizontally, vertically, and diagonally, allowing the powerful
contractions that occur during labor and the less powerful contractions (or
cramps) that help to expel menstrual blood during a woman’s period.
Anteriorly directed myometrial contractions also occur near the time of
ovulation, and are thought to possibly facilitate the transport of sperm
through the female reproductive tract.
The innermost layer of the uterus is called the endometrium. The
endometrium contains a connective tissue lining, the lamina propria, which
is covered by epithelial tissue that lines the lumen. Structurally, the
endometrium consists of two layers: the stratum basalis and the stratum
functionalis (the basal and functional layers). The stratum basalis layer is
part of the lamina propria and is adjacent to the myometrium; this layer
does not shed during menses. In contrast, the thicker stratum functionalis
layer contains the glandular portion of the lamina propria and the
endothelial tissue that lines the uterine lumen. It is the stratum functionalis
that grows and thickens in response to increased levels of estrogen and
progesterone. In the luteal phase of the menstrual cycle, special branches
off of the uterine artery called spiral arteries supply the thickened stratum
functionalis. This inner functional layer provides the proper site of
implantation for the fertilized egg, and—should fertilization not occur—it is
only the stratum functionalis layer of the endometrium that sheds during
menstruation.
Recall that during the follicular phase of the ovarian cycle, the tertiary
follicles are growing and secreting estrogen. At the same time, the stratum
functionalis of the endometrium is thickening to prepare for a potential
implantation. The post-ovulatory increase in progesterone, which
characterizes the luteal phase, is key for maintaining a thick stratum
functionalis. As long as a functional corpus luteum is present in the ovary,
the endometrial lining is prepared for implantation. Indeed, if an embryo
implants, signals are sent to the corpus luteum to continue secreting
progesterone to maintain the endometrium, and thus maintain the
pregnancy. If an embryo does not implant, no signal is sent to the corpus
luteum and it degrades, ceasing progesterone production and ending the
luteal phase. Without progesterone, the endometrium thins and, under the
influence of prostaglandins, the spiral arteries of the endometrium constrict
and rupture, preventing oxygenated blood from reaching the endometrial
tissue. As a result, endometrial tissue dies and blood, pieces of the
endometrial tissue, and white blood cells are shed through the vagina during
menstruation, or the menses. The first menses after puberty, called
menarche, can occur either before or after the first ovulation.
The Menstrual Cycle
Now that we have discussed the maturation of the cohort of tertiary follicles
in the ovary, the build-up and then shedding of the endometrial lining in the
uterus, and the function of the uterine tubes and vagina, we can put
everything together to talk about the three phases of the menstrual cycle—
the series of changes in which the uterine lining is shed, rebuilds, and
prepares for implantation.
The timing of the menstrual cycle starts with the first day of menses,
referred to as day one of a woman’s period. Cycle length is determined by
counting the days between the onset of bleeding in two subsequent cycles.
Because the average length of a woman’s menstrual cycle is 28 days, this is
the time period used to identify the timing of events in the cycle. However,
the length of the menstrual cycle varies among women, and even in the
Same woman from one cycle to the next, typically from 21 to 32 days.
Just as the hormones produced by the granulosa and theca cells of the ovary
“drive” the follicular and luteal phases of the ovarian cycle, they also
control the three distinct phases of the menstrual cycle. These are the
menses phase, the proliferative phase, and the secretory phase.
Menses Phase
The menses phase of the menstrual cycle is the phase during which the
lining is shed; that is, the days that the woman menstruates. Although it
averages approximately five days, the menses phase can last from 2 to 7
days, or longer. As shown in [link], the menses phase occurs during the
early days of the follicular phase of the ovarian cycle, when progesterone,
FSH, and LH levels are low. Recall that progesterone concentrations
decline as a result of the degradation of the corpus luteum, marking the end
of the luteal phase. This decline in progesterone triggers the shedding of the
stratum functionalis of the endometrium.
Hormone Levels in Ovarian and Menstrual Cycles
Primordial Primary Secondary
follicles follicles _ follicles
= @Q=©)—_—_—- @
Atresia
Single, selected
om © —o-—— tertiary follicle
i@) ==» © =» © —— &
Atresia
Constant development Selection of one dominant
of early-stage follicles secondary follicle begins
(2 months) each new menstrual cycle
Ovarian cycle phases
Selected tertiary Ovulation Corpus Corpus Degrading
follicle luteum albicans corpus
A g
QO AN 6, B s
0 7 14 21 28
Day of menstrual cycle
Uterine cycle phases
0 ¥ 14 21 28
Day of menstrual cycle
Pituitary Ovulation ——FSH
hormone levels ——LH
5 0 7 14 21 28
2
a Ovarian —— Estrogen
hormone levels === Progesterone
0 rf 14 21 28
Day of menstrual cycle
The correlation of the hormone levels and their effects on
the female reproductive system is shown in this timeline
of the ovarian and menstrual cycles. The menstrual cycle
begins at day one with the start of menses. Ovulation
occurs around day 14 of a 28-day cycle, triggered by the
LH surge.
Proliferative Phase
Once menstrual flow ceases, the endometrium begins to proliferate again,
marking the beginning of the proliferative phase of the menstrual cycle
(see [link]). It occurs when the granulosa and theca cells of the tertiary
follicles begin to produce increased amounts of estrogen. These rising
estrogen concentrations stimulate the endometrial lining to rebuild.
Recall that the high estrogen concentrations will eventually lead to a
decrease in FSH as a result of negative feedback, resulting in atresia of all
but one of the developing tertiary follicles. The switch to positive feedback
—which occurs with the elevated estrogen production from the dominant
follicle—then stimulates the LH surge that will trigger ovulation. In a
typical 28-day menstrual cycle, ovulation occurs on day 14. Ovulation
marks the end of the proliferative phase as well as the end of the follicular
phase.
Secretory Phase
In addition to prompting the LH surge, high estrogen levels increase the
uterine tube contractions that facilitate the pick-up and transfer of the
ovulated oocyte. High estrogen levels also slightly decrease the acidity of
the vagina, making it more hospitable to sperm. In the ovary, the
luteinization of the granulosa cells of the collapsed follicle forms the
progesterone-producing corpus luteum, marking the beginning of the luteal
phase of the ovarian cycle. In the uterus, progesterone from the corpus
luteum begins the secretory phase of the menstrual cycle, in which the
endometrial lining prepares for implantation (see [link]). Over the next 10
to 12 days, the endometrial glands secrete a fluid rich in glycogen. If
fertilization has occurred, this fluid will nourish the ball of cells now
developing from the zygote. At the same time, the spiral arteries develop to
provide blood to the thickened stratum functionalis.
If no pregnancy occurs within approximately 10 to 12 days, the corpus
luteum will degrade into the corpus albicans. Levels of both estrogen and
progesterone will fall, and the endometrium will grow thinner.
Prostaglandins will be secreted that cause constriction of the spiral arteries,
reducing oxygen supply. The endometrial tissue will die, resulting in
menses—or the first day of the next cycle.
Note:
Disorders of the... Feature
Female Reproductive System
Research over many years has confirmed that cervical cancer is most often
caused by a sexually transmitted infection with human papillomavirus
(HPV). There are over 100 related viruses in the HPV family, and the
characteristics of each strain determine the outcome of the infection. In all
cases, the virus enters body cells and uses its own genetic material to take
over the host cell’s metabolic machinery and produce more virus particles.
HPV infections are common in both men and women. Indeed, a recent
study determined that 42.5 percent of females had HPV at the time of
testing. These women ranged in age from 14 to 59 years and differed in
race, ethnicity, and number of sexual partners. Of note, the prevalence of
HPV infection was 53.8 percent among women aged 20 to 24 years, the
age group with the highest infection rate.
HPV strains are classified as high or low risk according to their potential to
cause cancer. Though most HPV infections do not cause disease, the
disruption of normal cellular functions in the low-risk forms of HPV can
cause the male or female human host to develop genital warts. Often, the
body is able to clear an HPV infection by normal immune responses within
2 years. However, the more serious, high-risk infection by certain types of
HPV can result in cancer of the cervix ([link]). Infection with either of the
cancer-causing variants HPV 16 or HPV 18 has been linked to more than
70 percent of all cervical cancer diagnoses. Although even these high-risk
HPV strains can be cleared from the body over time, infections persist in
some individuals. If this happens, the HPV infection can influence the cells
of the cervix to develop precancerous changes.
Risk factors for cervical cancer include having unprotected sex; having
multiple sexual partners; a first sexual experience at a younger age, when
the cells of the cervix are not fully mature; failure to receive the HPV
vaccine; a compromised immune system; and smoking. The risk of
developing cervical cancer is doubled with cigarette smoking.
Development of Cervical Cancer
@) During the G2 @® Cervix celis
checkpoint, a with mutated remains
functional p53 DNA do not healthy
protein detects a divide
DNA mutation
HPV not
present
() p53 protein is With p53 Mutated cervix
deactivated by the deactivated, cervix cells grow
p53 inhibitor cells with mutated uncontrollably
DNA successfully into a tumor
In most cases, cells infected with the HPV virus heal on their
own. In some cases, however, the virus continues to spread and
becomes an invasive cancer.
When the high-risk types of HPV enter a cell, two viral proteins are used to
neutralize proteins that the host cells use as checkpoints in the cell cycle.
The best studied of these proteins is p53. In a normal cell, p53 detects
DNA damage in the cell’s genome and either halts the progression of the
cell cycle—allowing time for DNA repair to occur—or initiates apoptosis.
Both of these processes prevent the accumulation of mutations in a cell’s
genome. High-risk HPV can neutralize p53, keeping the cell in a state in
which fast growth is possible and impairing apoptosis, allowing mutations
to accumulate in the cellular DNA.
The prevalence of cervical cancer in the United States is very low because
of regular screening exams called pap smears. Pap smears sample cells of
the cervix, allowing the detection of abnormal cells. If pre-cancerous cells
are detected, there are several highly effective techniques that are currently
in use to remove them before they pose a danger. However, women in
developing countries often do not have access to regular pap smears. As a
result, these women account for as many as 80 percent of the cases of
cervical cancer worldwide.
In 2006, the first vaccine against the high-risk types of HPV was approved.
There are now two HPV vaccines available: Gardasil® and Cervarix®.
Whereas these vaccines were initially only targeted for women, because
HPV is sexually transmitted, both men and women require vaccination for
this approach to achieve its maximum efficacy. A recent study suggests
that the HPV vaccine has cut the rates of HPV infection by the four
targeted strains at least in half. Unfortunately, the high cost of
manufacturing the vaccine is currently limiting access to many women
worldwide.
The Breasts
Whereas the breasts are located far from the other female reproductive
organs, they are considered accessory organs of the female reproductive
system. The function of the breasts is to supply milk to an infant in a
process called lactation. The external features of the breast include a nipple
surrounded by a pigmented areola ((link]), whose coloration may deepen
during pregnancy. The areola is typically circular and can vary in size from
25 to 100 mm in diameter. The areolar region is characterized by small,
raised areolar glands that secrete lubricating fluid during lactation to protect
the nipple from chafing. When a baby nurses, or draws milk from the
breast, the entire areolar region is taken into the mouth.
Breast milk is produced by the mammary glands, which are modified
sweat glands. The milk itself exits the breast through the nipple via 15 to 20
lactiferous ducts that open on the surface of the nipple. These lactiferous
ducts each extend to a lactiferous sinus that connects to a glandular lobe
within the breast itself that contains groups of milk-secreting cells in
clusters called alveoli (see [link]). The clusters can change in size
depending on the amount of milk in the alveolar lumen. Once milk is made
in the alveoli, stimulated myoepithelial cells that surround the alveoli
contract to push the milk to the lactiferous sinuses. From here, the baby can
draw milk through the lactiferous ducts by suckling. The lobes themselves
are surrounded by fat tissue, which determines the size of the breast; breast
size differs between individuals and does not affect the amount of milk
produced. Supporting the breasts are multiple bands of connective tissue
called suspensory ligaments that connect the breast tissue to the dermis of
the overlying skin.
Anatomy of the Breast
Areolar glands
Lactiferous
Suspensory : sinuses
ligament
Nipple
During lactation, milk moves from the alveoli
through the lactiferous ducts to the nipple.
During the normal hormonal fluctuations in the menstrual cycle, breast
tissue responds to changing levels of estrogen and progesterone, which can
lead to swelling and breast tenderness in some individuals, especially
during the secretory phase. If pregnancy occurs, the increase in hormones
leads to further development of the mammary tissue and enlargement of the
breasts.
Hormonal Birth Control
Birth control pills take advantage of the negative feedback system that
regulates the ovarian and menstrual cycles to stop ovulation and prevent
pregnancy. Typically they work by providing a constant level of both
estrogen and progesterone, which negatively feeds back onto the
hypothalamus and pituitary, thus preventing the release of FSH and LH.
Without FSH, the follicles do not mature, and without the LH surge,
ovulation does not occur. Although the estrogen in birth control pills does
stimulate some thickening of the endometrial wall, it is reduced compared
with a normal cycle and is less likely to support implantation.
Some birth control pills contain 21 active pills containing hormones, and 7
inactive pills (placebos). The decline in hormones during the week that the
woman takes the placebo pills triggers menses, although it is typically
lighter than a normal menstrual flow because of the reduced endometrial
thickening. Newer types of birth control pills have been developed that
deliver low-dose estrogens and progesterone for the entire cycle (these are
meant to be taken 365 days a year), and menses never occurs. While some
women prefer to have the proof of a lack of pregnancy that a monthly
period provides, menstruation every 28 days is not required for health
reasons, and there are no reported adverse effects of not having a menstrual
period in an otherwise healthy individual.
Because birth control pills function by providing constant estrogen and
progesterone levels and disrupting negative feedback, skipping even just
one or two pills at certain points of the cycle (or even being several hours
late taking the pill) can lead to an increase in FSH and LH and result in
ovulation. It is important, therefore, that the woman follow the directions on
the birth control pill package to successfully prevent pregnancy.
Note:
Aging and the... Feature
Female Reproductive System
Female fertility (the ability to conceive) peaks when women are in their
twenties, and is slowly reduced until a women reaches 35 years of age.
After that time, fertility declines more rapidly, until it ends completely at
the end of menopause. Menopause is the cessation of the menstrual cycle
that occurs as a result of the loss of ovarian follicles and the hormones that
they produce. A woman is considered to have completed menopause if she
has not menstruated in a full year. After that point, she is considered
postmenopausal. The average age for this change is consistent worldwide
at between 50 and 52 years of age, but it can normally occur in a woman’s
forties, or later in her fifties. Poor health, including smoking, can lead to
earlier loss of fertility and earlier menopause.
As a woman reaches the age of menopause, depletion of the number of
viable follicles in the ovaries due to atresia affects the hormonal regulation
of the menstrual cycle. During the years leading up to menopause, there is
a decrease in the levels of the hormone inhibin, which normally
participates in a negative feedback loop to the pituitary to control the
production of FSH. The menopausal decrease in inhibin leads to an
increase in FSH. The presence of FSH stimulates more follicles to grow
and secrete estrogen. Because small, secondary follicles also respond to
increases in FSH levels, larger numbers of follicles are stimulated to grow;
however, most undergo atresia and die. Eventually, this process leads to the
depletion of all follicles in the ovaries, and the production of estrogen falls
off dramatically. It is primarily the lack of estrogens that leads to the
symptoms of menopause.
The earliest changes occur during the menopausal transition, often referred
to as peri-menopause, when a women’s cycle becomes irregular but does
not stop entirely. Although the levels of estrogen are still nearly the same
as before the transition, the level of progesterone produced by the corpus
luteum is reduced. This decline in progesterone can lead to abnormal
growth, or hyperplasia, of the endometrium. This condition is a concern
because it increases the risk of developing endometrial cancer. Two
harmless conditions that can develop during the transition are uterine
fibroids, which are benign masses of cells, and irregular bleeding. As
estrogen levels change, other symptoms that occur are hot flashes and night
sweats, trouble sleeping, vaginal dryness, mood swings, difficulty
focusing, and thinning of hair on the head along with the growth of more
hair on the face. Depending on the individual, these symptoms can be
entirely absent, moderate, or severe.
After menopause, lower amounts of estrogens can lead to other changes.
Cardiovascular disease becomes as prevalent in women as in men, possibly
because estrogens reduce the amount of cholesterol in the blood vessels.
When estrogen is lacking, many women find that they suddenly have
problems with high cholesterol and the cardiovascular issues that
accompany it. Osteoporosis is another problem because bone density
decreases rapidly in the first years after menopause. The reduction in bone
density leads to a higher incidence of fractures.
Hormone therapy (HT), which employs medication (synthetic estrogens
and progestins) to increase estrogen and progestin levels, can alleviate
some of the symptoms of menopause. In 2002, the Women’s Health
Initiative began a study to observe women for the long-term outcomes of
hormone replacement therapy over 8.5 years. However, the study was
prematurely terminated after 5.2 years because of evidence of a higher than
normal risk of breast cancer in patients taking estrogen-only HT. The
potential positive effects on cardiovascular disease were also not realized
in the estrogen-only patients. The results of other hormone replacement
studies over the last 50 years, including a 2012 study that followed over
1,000 menopausal women for 10 years, have shown cardiovascular
benefits from estrogen and no increased risk for cancer. Some researchers
believe that the age group tested in the 2002 trial may have been too old to
benefit from the therapy, thus skewing the results. In the meantime, intense
debate and study of the benefits and risks of replacement therapy is
ongoing. Current guidelines approve HT for the reduction of hot flashes or
flushes, but this treatment is generally only considered when women first
start showing signs of menopausal changes, is used in the lowest dose
possible for the shortest time possible (5 years or less), and it is suggested
that women on HT have regular pelvic and breast exams.
Chapter Review
The external female genitalia are collectively called the vulva. The vagina
is the pathway into and out of the uterus. The man’s penis is inserted into
the vagina to deliver sperm, and the baby exits the uterus through the
vagina during childbirth.
The ovaries produce oocytes, the female gametes, in a process called
oogenesis. As with spermatogenesis, meiosis produces the haploid gamete
(in this case, an ovum); however, it is completed only in an oocyte that has
been penetrated by a sperm. In the ovary, an oocyte surrounded by
supporting cells is called a follicle. In folliculogenesis, primordial follicles
develop into primary, secondary, and tertiary follicles. Early tertiary
follicles with their fluid-filled antrum will be stimulated by an increase in
FSH, a gonadotropin produced by the anterior pituitary, to grow in the 28-
day ovarian cycle. Supporting granulosa and theca cells in the growing
follicles produce estrogens, until the level of estrogen in the bloodstream is
high enough that it triggers negative feedback at the hypothalamus and
pituitary. This results in a reduction of FSH and LH, and most tertiary
follicles in the ovary undergo atresia (they die). One follicle, usually the one
with the most FSH receptors, survives this period and is now called the
dominant follicle. The dominant follicle produces more estrogen, triggering
positive feedback and the LH surge that will induce ovulation. Following
ovulation, the granulosa cells of the empty follicle luteinize and transform
into the progesterone-producing corpus luteum. The ovulated oocyte with
its surrounding granulosa cells is picked up by the infundibulum of the
uterine tube, and beating cilia help to transport it through the tube toward
the uterus. Fertilization occurs within the uterine tube, and the final stage of
meiosis is completed.
The uterus has three regions: the fundus, the body, and the cervix. It has
three layers: the outer perimetrium, the muscular myometrium, and the
inner endometrium. The endometrium responds to estrogen released by the
follicles during the menstrual cycle and grows thicker with an increase in
blood vessels in preparation for pregnancy. If the egg is not fertilized, no
signal is sent to extend the life of the corpus luteum, and it degrades,
stopping progesterone production. This decline in progesterone results in
the sloughing of the inner portion of the endometrium in a process called
menses, or menstruation.
The breasts are accessory sexual organs that are utilized after the birth of a
child to produce milk in a process called lactation. Birth control pills
provide constant levels of estrogen and progesterone to negatively feed
back on the hypothalamus and pituitary, and suppress the release of FSH
and LH, which inhibits ovulation and prevents pregnancy.
Interactive Link Questions
Exercise:
Problem:
Watch this video to observe ovulation and its initiation in response to
the release of FSH and LH from the pituitary gland. What specialized
structures help guide the oocyte from the ovary into the uterine tube?
Solution:
The fimbriae sweep the oocyte into the uterine tube.
Exercise:
Problem:
Watch this series of videos to look at the movement of the oocyte
through the ovary. The cilia in the uterine tube promote movement of
the oocyte. What would likely occur if the cilia were paralyzed at the
time of ovulation?
Solution:
The oocyte may not enter the tube and may enter the pelvic cavity.
Review Questions
Exercise:
Problem: What are the female gonads called?
a. oocytes
b. ova
c. oviducts
d. ovaries
Solution:
d
Exercise:
Problem: When do the oogonia undergo mitosis?
a. before birth
b. at puberty
c. at the beginning of each menstrual cycle
d. during fertilization
Solution:
a
Exercise:
Problem: From what structure does the corpus luteum originate?
a. uterine corpus
b. dominant follicle
c. fallopian tube
d. corpus albicans
Solution:
b
Exercise:
Problem:
Where does fertilization of the egg by the sperm typically occur?
a. vagina
b. uterus
c. uterine tube
d. ovary
Solution:
C
Exercise:
Problem: Why do estrogen levels fall after menopause?
a. The ovaries degrade.
b. There are no follicles left to produce estrogen.
c. The pituitary secretes a menopause-specific hormone.
d. The cells of the endometrium degenerate.
Solution:
b
Exercise:
Problem: The vulva includes the
a. lactiferous duct, rugae, and hymen
b. lactiferous duct, endometrium, and bulbourethral glands
c. mons pubis, endometrium, and hymen
d. mons pubis, labia majora, and Bartholin’s glands
Solution:
d
Critical Thinking Questions
Exercise:
Problem:
Follow the path of ejaculated sperm from the vagina to the oocyte.
Include all structures of the female reproductive tract that the sperm
must swim through to reach the egg.
Solution:
The sperm must swim upward in the vagina, through the cervix, and
then through the body of the uterus to one or the other of the two
uterine tubes. Fertilization generally occurs in the uterine tube.
Exercise:
Problem:
Identify some differences between meiosis in men and women.
Solution:
Meiosis in the man results in four viable haploid sperm, whereas
meiosis in the woman results in a secondary oocyte and, upon
completion following fertilization by a sperm, one viable haploid
ovum with abundant cytoplasm and up to three polar bodies with little
cytoplasm that are destined to die.
Exercise:
Problem:
Explain the hormonal regulation of the phases of the menstrual cycle.
Solution:
As aresult of the degradation of the corpus luteum, a decline in
progesterone concentrations triggers the shedding of the endometrial
lining, marking the menses phase of the menstrual cycle. Low
progesterone levels also reduce the negative feedback that had been
occurring at the hypothalamus and pituitary, and result in the release of
GnRH and, subsequently, FSH and LH. FSH stimulates tertiary
follicles to grow and granulosa and theca cells begin to produce
increased amounts of estrogen. High estrogen concentrations stimulate
the endometrial lining to rebuild, marking the proliferative phase of the
menstrual cycle. The high estrogen concentrations will eventually lead
to a decrease in FSH because of negative feedback, resulting in atresia
of all but one of the developing tertiary follicles. The switch to positive
feedback that occurs with elevated estrogen production from the
dominant follicle stimulates the LH surge that will trigger ovulation.
The luteinization of the granulosa cells of the collapsed follicle forms
the progesterone-producing corpus luteum. Progesterone from the
corpus luteum causes the endometrium to prepare for implantation, in
part by secreting nutrient-rich fluid. This marks the secretory phase of
the menstrual cycle. Finally, in a non-fertile cycle, the corpus luteum
will degrade and menses will occur.
Exercise:
Problem:
Endometriosis is a disorder in which endometrial cells implant and
proliferate outside of the uterus—in the uterine tubes, on the ovaries,
or even in the pelvic cavity. Offer a theory as to why endometriosis
increases a woman’s risk of infertility.
Solution:
Endometrial tissue proliferating outside of the endometrium—for
example, in the uterine tubes, on the ovaries, or within the pelvic
cavity—could block the passage of sperm, ovulated oocytes, or a
zygote, thus reducing fertility.
Glossary
alveoli
(of the breast) milk-secreting cells in the mammary gland
ampulla
(of the uterine tube) middle portion of the uterine tube in which
fertilization often occurs
antrum
fluid-filled chamber that characterizes a mature tertiary (antral) follicle
areola
highly pigmented, circular area surrounding the raised nipple and
containing areolar glands that secrete fluid important for lubrication
during suckling
Bartholin’s glands
(also, greater vestibular glands) glands that produce a thick mucus that
maintains moisture in the vulva area; also referred to as the greater
vestibular glands
body of uterus
middle section of the uterus
broad ligament
wide ligament that supports the uterus by attaching laterally to both
sides of the uterus and pelvic wall
cervix
elongate inferior end of the uterus where it connects to the vagina
clitoris
(also, glans clitoris) nerve-rich area of the vulva that contributes to
sexual sensation during intercourse
corpus albicans
nonfunctional structure remaining in the ovarian stroma following
structural and functional regression of the corpus luteum
corpus luteum
transformed follicle after ovulation that secretes progesterone
endometrium
inner lining of the uterus, part of which builds up during the secretory
phase of the menstrual cycle and then sheds with menses
fimbriae
fingerlike projections on the distal uterine tubes
follicle
ovarian structure of one oocyte and surrounding granulosa (and later
theca) cells
folliculogenesis
development of ovarian follicles from primordial to tertiary under the
stimulation of gonadotropins
fundus
(of the uterus) domed portion of the uterus that is superior to the
uterine tubes
granulosa cells
supportive cells in the ovarian follicle that produce estrogen
hymen
membrane that covers part of the opening of the vagina
infundibulum
(of the uterine tube) wide, distal portion of the uterine tube terminating
in fimbriae
isthmus
narrow, medial portion of the uterine tube that joins the uterus
labia majora
hair-covered folds of skin located behind the mons pubis
labia minora
thin, pigmented, hairless flaps of skin located medial and deep to the
labia majora
lactiferous ducts
ducts that connect the mammary glands to the nipple and allow for the
transport of milk
lactiferous sinus
area of milk collection between alveoli and lactiferous duct
mammary glands
glands inside the breast that secrete milk
menarche
first menstruation in a pubertal female
menses
shedding of the inner portion of the endometrium out though the
vagina; also referred to as menstruation
menses phase
phase of the menstrual cycle in which the endometrial lining is shed
menstrual cycle
approximately 28-day cycle of changes in the uterus consisting of a
menses phase, a proliferative phase, and a secretory phase
mons pubis
mound of fatty tissue located at the front of the vulva
myometrium
smooth muscle layer of uterus that allows for uterine contractions
during labor and expulsion of menstrual blood
oocyte
cell that results from the division of the oogonium and undergoes
meiosis I at the LH surge and meiosis II at fertilization to become a
haploid ovum
oogenesis
process by which oogonia divide by mitosis to primary oocytes, which
undergo meiosis to produce the secondary oocyte and, upon
fertilization, the ovum
oogonia
ovarian stem cells that undergo mitosis during female fetal
development to form primary oocytes
ovarian cycle
approximately 28-day cycle of changes in the ovary consisting of a
follicular phase and a luteal phase
ovaries
female gonads that produce oocytes and sex steroid hormones (notably
estrogen and progesterone)
ovulation
release of a secondary oocyte and associated granulosa cells from an
Ovary
ovum
haploid female gamete resulting from completion of meiosis II at
fertilization
perimetrium
outer epithelial layer of uterine wall
polar body
smaller cell produced during the process of meiosis in oogenesis
primary follicles
ovarian follicles with a primary oocyte and one layer of cuboidal
granulosa cells
primordial follicles
least developed ovarian follicles that consist of a single oocyte and a
single layer of flat (squamous) granulosa cells
proliferative phase
phase of the menstrual cycle in which the endometrium proliferates
rugae
(of the vagina) folds of skin in the vagina that allow it to stretch during
intercourse and childbirth
secondary follicles
ovarian follicles with a primary oocyte and multiple layers of
granulosa cells
secretory phase
phase of the menstrual cycle in which the endometrium secretes a
nutrient-rich fluid in preparation for implantation of an embryo
suspensory ligaments
bands of connective tissue that suspend the breast onto the chest wall
by attachment to the overlying dermis
tertiary follicles
(also, antral follicles) ovarian follicles with a primary or secondary
oocyte, multiple layers of granulosa cells, and a fully formed antrum
theca cells
estrogen-producing cells in a maturing ovarian follicle
uterine tubes
(also, fallopian tubes or oviducts) ducts that facilitate transport of an
ovulated oocyte to the uterus
uterus
muscular hollow organ in which a fertilized egg develops into a fetus
vagina
tunnel-like organ that provides access to the uterus for the insertion of
semen and from the uterus for the birth of a baby
vulva
external female genitalia
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